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Large-scale observations of a subauroral polarization stream by midlatitude SuperDARN radars: Instantaneous longitudinal velocity variations

Clausen, L. B. N., J. B. H. Baker, J. M. Ruohoniemi, R. A. Greenwald, E. G. Thomas, S. G. Shepherd, E. R. Talaat, W. A. Bristow, Y. Zheng, A. J. Coster, and S. Sazykini (2012), Large-scale observations of a subauroral polarization stream by midlatitude SuperDARN radars: Instantaneous longitudinal velocity variations, J. Geophys. Res., 117, A05306, doi:10.1029/2011JA017232.

Published in Journal of Geophysical Research, 2012

We present simultaneous measurements of flow velocities inside a subauroral polarization stream (SAPS) made by six midlatitude high-frequency SuperDARN radars. The instantaneous observations cover three hours of universal time and six hours of magnetic local time (MLT). From velocity variations across the field-of-view of the radars we infer the local 2D flow direction at three different longitudes. We find that the local flow direction inside the SAPS channel is remarkably constant over the course of the event. The flow speed, however, shows significant temporal and spatial variations. After correcting for the radar look direction we are able to accurately determine the dependence of the SAPS velocity on magnetic local time. We find that the SAPS velocity variation with magnetic local time is best described by an exponential function. The average velocity at 00 MLT was 1.2 km/s and it decreased with a spatial e-folding scale of two hours of MLT toward the dawn sector. We speculate that the longitudinal distribution of pressure gradients in the ring current is responsible for this dependence and find these observations in good agreement with results from ring current models. Using TEC measurements we find that the high westward velocities of the SAPS are - as expected - located in a region of low TEC values, indicating low ionospheric conductivities.

An examination of interhemispheric conjugacy in a sub-auroral polarization stream

Kunduri, B. S. R., J. B. H. Baker, J. M. Ruohoniemi, L. B. N. Clausen, A. Grocott, E. G. Thomas, M. P. Freeman, and E. R. Talaat (2012), An examination of interhemispheric conjugacy in a sub-auroral polarization stream, J. Geophys. Res., 117, A08225, doi:10.1029/2012JA017784.

Published in Journal of Geophysical Research, 2012

During geomagnetically disturbed conditions the midlatitude ionosphere is subject to intense poleward directed electric fields in the dusk-midnight sector. These electric fields lead to the generation of a latitudinally narrow westward directed flow channel in the subauroral region called a subauroral polarization stream (SAPS). If the magnetic field lines are treated as equipotentials, electrodynamic events such as SAPS are expected to occur simultaneously at magnetically conjugate locations with similar features. In this paper we present simultaneous observations of a SAPS event in both hemispheres made by midlatitude SuperDARN radars with conjugate fields-of-view. We analyze the relation between the geomagnetic conditions and the characteristics of the channels such as latitudinal location, electric field, total potential variations across the channels, and Pedersen current. The results suggest a strong correlation between the strength of the ring current and the latitudinal location of the channel. An inter-hemispheric comparison of the characteristics of the channel indicates that the potential variations across the channels are similar while the electric fields, Pedersen currents and latitudinal widths of the channel exhibit differences that are consistent with equal potential variations. We attribute these differences to seasonal differences in ionospheric conductivity between the hemispheres and magnetic distortion effects in the inner magnetosphere.

Direct observations of the role of convection electric field in the formation of a polar tongue of ionization from storm enhanced density

Thomas, E. G., J. B. H. Baker, J. M. Ruohoniemi, L. B. N. Clausen, A. J. Coster, J. C. Foster, and P. J. Erickson (2013), Direct observations of the role of convection electric field in the formation of a polar tongue of ionization from storm enhanced density, J. Geophys. Res. Space Physics, 118, 1180-1189, doi:10.1002/jgra.50116.

Published in Journal of Geophysical Research Space Physics, 2013

We examine the relationship of convection electric fields to the formation of a polar cap tongue of ionization (TOI) from midlatitude plumes of storm enhanced density (SED). Observations from the geomagnetic storm on 26–27 September 2011 are presented for two distinct SED events. During an hour-long period of geomagnetic activity driven by a coronal mass ejection, a channel of high-density F region plasma was transported from the dayside subauroral ionosphere and into the polar cap by enhanced convection electric fields extending to middle latitudes. This TOI feature was associated with enhanced HF backscatter, indicating that it was the seat of active formation of small-scale irregularities. After the solar wind interplanetary magnetic field conditions quieted and the dayside convection electric fields retreated to higher latitudes, an SED plume was observed extending to, but not entering, the dayside cusp region. This prominent feature in the distribution of total electron content (TEC) persisted for several hours and elongated in magnetic local time with the rotation of the Earth. No ionospheric scatter from SuperDARN radars was observed within this SED region. The source mechanism (enhanced electric fields) previously drawing the plasma from midlatitudes and into the polar cap as a TOI was no longer active, resulting in a fossil feature. We thus demonstrate the controlling role exercised by the convection electric field in generating a TOI from midlatitude SED.

Direct Observations of the Evolution of Polar Cap Ionization Patches

Zhang, Q.-H., B.-C. Zhang, M. Lockwood, H.-Q. Hu, J. Moen, J. M. Ruohoniemi, E. G. Thomas, S.-R. Zhang, H.-G. Yang, R.-Y. Liu, K. A. McWilliams, and J. B. H. Baker (2013), Direct Observations of the Evolution of Polar Cap Ionization Patches, Science, 339, 1597-1600, doi:10.1126/science.1231487.

Published in Science, 2013

Patches of ionization are common in the polar ionosphere, where their motion and associated density gradients give variable disturbances to high-frequency (HF) radio communications, over-the-horizon radar location errors, and disruption and errors to satellite navigation and communication. Their formation and evolution are poorly understood, particularly under disturbed space weather conditions. We report direct observations of the full evolution of patches during a geomagnetic storm, including formation, polar cap entry, transpolar evolution, polar cap exit, and sunward return flow. Our observations show that modulation of nightside reconnection in the substorm cycle of the magnetosphere helps form the gaps between patches where steady convection would give a “tongue” of ionization (TOI).

GPS phase scintillation and proxy index at high latitudes during a moderate geomagnetic storm

Prikryl, P., R. Ghoddousi-Fard, B. S. R. Kunduri, E. G. Thomas, A. J. Coster, P. T. Jayachandran, E. Spanswick, and D. W. Danskin (2013), GPS phase scintillation and proxy index at high latitudes during a moderate geomagnetic storm, Ann. Geophys., 31, 805-816, doi:10.5194/angeo-31-805-2013.

Published in Annales Geophysicae, 2013

The amplitude and phase scintillation indices are customarily obtained by specialised GPS Ionospheric Scintillation and TEC Monitors (GISTMs) from L1 signal recorded at the rate of 50 Hz. The scintillation indices S4 and σΦ are stored in real time from an array of high-rate scintillation receivers of the Canadian High Arctic Ionospheric Network (CHAIN). Ionospheric phase scintillation was observed at high latitudes during a moderate geomagnetic storm (Dst = −61 nT) that was caused by a moderate solar wind plasma stream compounded with the impact of two coronal mass ejections. The most intense phase scintillation (σΦ ~ 1 rad) occurred in the cusp and the polar cap where it was co-located with a strong ionospheric convection, an extended tongue of ionisation and dense polar cap patches that were observed with ionosondes and HF radars. At sub-auroral latitudes, a sub-auroral polarisation stream that was observed by mid-latitude radars was associated with weak scintillation (defined arbitrarily as σΦ < 0.5 rad). In the auroral zone, moderate scintillation coincided with auroral breakups observed by an all-sky imager, a riometer and a magnetometer in Yellowknife. To overcome the limited geographic coverage by GISTMs other GNSS data sampled at 1 Hz can be used to obtain scintillation proxy indices. In this study, a phase scintillation proxy index (delta phase rate, DPR) is obtained from 1-Hz data from CHAIN and other GPS receivers. The 50-Hz and 1-Hz phase scintillation indices are correlated. The percentage occurrences of σΦ > 0.1 rad and DPR > 2 mm s−1, both mapped as a function of magnetic latitude and magnetic local time, are very similar.

Multi-instrument observations of SED during 24-25 October 2011 storm: Implications for SED formation processes

Zou, S., A. J. Ridley, M. B. Moldwin, M. J. Nicolls, A. J. Coster, E. G. Thomas, and J. M. Ruohoniemi (2013), Multi-instrument observations of SED during 24-25 October 2011 storm: Implications for SED formation processes, J. Geophys. Res. Space Physics, 118, 7798-7809, doi:10.1002/2013JA018860.

Published in Journal of Geophysical Research Space Physics, 2013

We present multiple instrument observations of a storm-enhanced density (SED) during the 24–25 October 2011 intense geomagnetic storm. Formation and the subsequent evolution of the SED and the midlatitude trough are revealed by global GPS vertical total electron content maps. In addition, we present high time resolution Poker Flat Incoherent Scatter Radar (PFISR) observations of ionospheric profiles within the SED. We divided the SED observed by PFISR into two parts. Both parts are characterized by elevated ionospheric peak height (hmF2) and total electron content, compared to quiet time values. However, the two parts of the SED have different characteristics in the electron temperature (Te), the F region peak density (NmF2), and convection flows. The first part of the SED is associated with enhanced Te in the lower F region and reduced Te in the upper F region and is collocated with northward convection flows. The NmF2 was lower than quiet time values. The second part of the SED is associated with significantly increased NmF2, elevated Te at all altitudes and is located near the equatorward boundary of large northwestward flows. Based on these observations, we suggest that the mechanisms responsible for the formation of the two parts of the SED may be different. The first part is due to equatorward expansion of the convection pattern and the projection of northward convection flows in the vertical direction, which lifts the ionospheric plasma to higher altitudes and thus reduces the loss rate of plasma recombination. The second part is more complicated. Besides equatorward expansion of the convection pattern and large upward flows, evidences of other mechanisms, including horizontal advection due to fast flows, energetic particle precipitation, and enhanced thermospheric wind in the topside ionosphere, are also present. Estimates show that contribution from precipitating energetic protons is at most ~10% of the total F region density. The thermospheric wind also plays a minor role in this case.

On the generation/decay of the storm-enhanced density plumes: Role of the convection flow and field-aligned ion flow

Zou, S., M. B. Moldwin, A. J. Ridley, M. J. Nicolls, A. J. Coster, E. G. Thomas, and J. M. Ruohoniemi (2014), On the generation/decay of the storm-enhanced density plumes: Role of the convection flow and field-aligned ion flow, J. Geophys. Res. Space Physics, 119, 8543-8559, doi:10.1002/2014JA020408.

Published in Journal of Geophysical Research Space Physics, 2014

Storm-enhanced density (SED) plumes are prominent ionospheric electron density increases at the dayside middle and high latitudes. The generation and decay mechanisms of the plumes are still not clear. We present observations of SED plumes during six storms between 2010 and 2013 and comprehensively analyze the associated ionospheric parameters within the plumes, including vertical ion flow, field-aligned ion flow and flux, plasma temperature, and field-aligned currents, obtained from multiple instruments, including GPS total electron content (TEC), Poker Flat Incoherent Scatter Radar (PFISR), Super Dual Auroral Radar Network, and Active Magnetosphere and Planetary Electrodynamics Response Experiment. The TEC increase within the SED plumes at the PFISR site can be 1.4–5.5 times their quiet time value. The plumes are usually associated with northwestward E × B flows ranging from a couple of hundred m s−1 to > 1 km s−1. Upward vertical flows due to the projection of these E × B drifts are mainly responsible for lifting the plasma in sunlit regions to higher altitude and thus leading to plume density enhancement. The upward vertical flows near the poleward part of the plumes are more persistent, while those near the equatorward part are more patchy. In addition, the plumes can be collocated with either upward or downward field-aligned currents (FACs) but are usually observed equatorward of the peak of the Region 1 upward FAC, suggesting that the northwestward flows collocated with plumes can be either subauroral or auroral flows. Furthermore, during the decay phase of the plume, large downward ion flows, as large as ~200 m s−1, and downward fluxes, as large as 1014 m−2 s−1, are often observed within the plumes. In our study of six storms, enhanced ambipolar diffusion due to an elevated pressure gradient is able to explain two of the four large downward flow/flux cases, but this mechanism is not sufficient for the other two cases where the flows are of larger magnitude. For the latter two cases, enhanced poleward thermospheric wind is suggested to be another mechanism for pushing the plasma downward along the field line. These downward flows should be an important mechanism for the decay of the SED plumes.

GPS phase scintillation at high latitudes during geomagnetic storms of 7-17 March 2012 - Part 1: The North American sector

Prikryl, P., R. Ghoddousi-Fard, E. G. Thomas, J. M. Ruohoniemi, S. G. Shepherd, P. T. Jayachandran, D. W. Danskin, E. Spanswick, Y. Zhang, Y. Jiao, and Y. T. Morton (2015), GPS phase scintillation at high latitudes during geomagnetic storms of 7-17 March 2012 - Part 1: The North American sector, Ann. Geophys., 33, 636-656, doi:10.5194/angeo-33-637-2015.

Published in Annales Geophysicae, 2015

The interval of geomagnetic storms of 7–17 March 2012 was selected at the Climate and Weather of the Sun-Earth System (CAWSES) II Workshop for group study of space weather effects during the ascending phase of solar cycle 24 (Tsurutani et al., 2014). The high-latitude ionospheric response to a series of storms is studied using arrays of GPS receivers, HF radars, ionosondes, riometers, magnetometers, and auroral imagers focusing on GPS phase scintillation. Four geomagnetic storms showed varied responses to solar wind conditions characterized by the interplanetary magnetic field (IMF) and solar wind dynamic pressure. As a function of magnetic latitude and magnetic local time, regions of enhanced scintillation are identified in the context of coupling processes between the solar wind and the magnetosphere–ionosphere system. Large southward IMF and high solar wind dynamic pressure resulted in the strongest scintillation in the nightside auroral oval. Scintillation occurrence was correlated with ground magnetic field perturbations and riometer absorption enhancements, and collocated with mapped auroral emission. During periods of southward IMF, scintillation was also collocated with ionospheric convection in the expanded dawn and dusk cells, with the antisunward convection in the polar cap and with a tongue of ionization fractured into patches. In contrast, large northward IMF combined with a strong solar wind dynamic pressure pulse was followed by scintillation caused by transpolar arcs in the polar cap.

GPS phase scintillation at high latitudes during geomagnetic storms of 7-17 March 2012 - Part 2: Interhemispheric comparison

Prikryl, P., R. Ghoddousi-Fard, L. Spogli, C. N. Mitchell, G. Li, B. Ning, P. J. Cilliers, V. Sreeja, M. Aquino, M. Terkildsen, P. T. Jayachandran, Y. Jiao, Y. T. Morton, J. M. Ruohoniemi, E. G. Thomas, Y. Zhang, A. T. Weatherwax, L. Alfonsi, G. De Franceschi, and V. Romano (2015), GPS phase scintillation at high latitudes during geomagnetic storms of 7-17 March 2012 - Part 2: Interhemispheric comparison, Ann. Geophys., 33, 657-670, doi:10.5194/angeo-33-657-2015.

Published in Annales Geophysicae, 2015

During the ascending phase of solar cycle 24, a series of interplanetary coronal mass ejections (ICMEs) in the period 7–17 March 2012 caused geomagnetic storms that strongly affected high-latitude ionosphere in the Northern and Southern Hemisphere. GPS phase scintillation was observed at northern and southern high latitudes by arrays of GPS ionospheric scintillation and TEC monitors (GISTMs) and geodetic-quality GPS receivers sampling at 1 Hz. Mapped as a function of magnetic latitude and magnetic local time (MLT), the scintillation was observed in the ionospheric cusp, the tongue of ionization fragmented into patches, sun-aligned arcs in the polar cap, and nightside auroral oval and subauroral latitudes. Complementing a companion paper (Prikryl et al., 2015a) that focuses on the high-latitude ionospheric response to variable solar wind in the North American sector, interhemispheric comparison reveals commonalities as well as differences and asymmetries between the northern and southern high latitudes, as a consequence of the coupling between the solar wind and magnetosphere. The interhemispheric asymmetries are caused by the dawn–dusk component of the interplanetary magnetic field controlling the MLT of the cusp entry of the storm-enhanced density plasma into the polar cap and the orientation relative to the noon–midnight meridian of the tongue of ionization.

Dense plasma and Kelvin-Helmholtz waves at Earth's dayside magnetopause

Walsh, B. M., E. G. Thomas, K.-J. Hwang, J. B. H. Baker, J. M. Ruohoniemi, and J. W. Bonnell (2015), Dense plasma and Kelvin-Helmholtz waves at Earth's dayside magnetopause, J. Geophys. Res. Space Physics, 120, 5560-5573, doi:10.1002/2015JA021014.

Published in Journal of Geophysical Research Space Physics, 2015

Spacecraft observations of boundary waves at the dayside terrestrial magnetopause and their ground-based signatures are presented. Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft measured boundary waves at the magnetopause while ground-based HF radar measured corresponding signatures in the ionosphere indicating a large-scale response and tailward propagating waves. The properties of the oscillations are consistent with linear phase Kelvin-Helmholtz waves along the magnetopause boundary. During this time period multiple THEMIS spacecraft also measured a plasmaspheric plume contacting the local magnetopause and mass loading the boundary. Previous work has demonstrated that increasing the density at the magnetopause can lower the efficiency of reconnection. Extending this further, present observations suggest that a plume can modulate instability processes such as the Kelvin-Helmholtz instability and allow them to form closer to the subsolar point along the magnetopause than without a plume. The current THEMIS observations from 21 September 2010 are consistent with a theory which predicts that increasing the density at the boundary will lower the Kelvin-Helmholtz threshold and allow waves to form for a lower velocity shear.

Formation of polar ionospheric tongue of ionization during minor geomagnetic disturbed conditions

Liu, J., T. Nakamura, L. Liu, W. Wang, N. Balan, T. Nishiyama, M. R. hairston, and E. G. Thomas (2015), Formation of polar ionospheric tongue of ionization during minor geomagnetic disturbed conditions, J. Geophys. Res. Space Physics, 120, 6860-6873, doi:10.1002/2015JA021393.

Published in Journal of Geophysical Research Space Physics, 2015

Previous investigations of ionospheric storm-enhanced density (SED) and tongue of ionization (TOI) focused mostly on the behavior of TOI during intense geomagnetic storms. Little attention has been paid to the spatial and temporal variations of TOI during weak to moderate geomagnetic disturbed conditions. In this paper we investigate the source and development of TOI during a moderate geomagnetic storm on 14 October 2012. Multi-instrumental observations including GPS total electron content (TEC), Defense Meteorological Satellite Program (DMSP) in situ measured total ion concentration and ion drift velocity, SuperDARN measured polar ion convection patterns, and electron density profiles from the Poker Flat Incoherent Scatter Radar (PFISR) have been utilized in the current analysis. GPS TEC maps show salient TOI structures persisting for about 5 h over high latitudes of North America on 14 October 2012 in the later recovery phase of the storm when the magnitudes of IMF By and Bz were less than 5 nT. The PFISR electron density profiles indicate that the extra ionization for TEC enhancements mainly occurred in the topside ionosphere with no obvious changes in the bottomside ionosphere and vertical plasma drifts. Additionally, there were no signatures of penetration electric fields in the equatorial electrojet data and upward ion drifts at high latitudes. At the same time, strong subauroral polarization streams with ion drift speeds exceeding 2.5 km/s carried sunward fluxes and migrated toward lower latitudes for about 5° based on the DMSP cross-track drift measurements. Based on those measurements, we postulate that the combined effects of initial build-up of ionization at midlatitudes through daytime production of ionization and equatorward (or less poleward than normal daytime) neutral wind reducing downward diffusion along the inclined filed lines, and an expanded polar ion convection pattern and its associated horizontal plasma transport are important in the formation of the TOI.

Observations of storm time midlatitude ion-neutral coupling using SuperDARN radars and NATION Fabry-Perot interferometers

Joshi, P. P., J. B. H. Baker, J. M. Ruohoniemi, J. J. Makela, D. J. Fisher, B. J. Harding, N. A. Frissell, and E. G. Thomas (2015), Observations of storm time midlatitude ion-neutral coupling using SuperDARN radars and NATION Fabry-Perot interferometers, J. Geophys. Res. Space Physics, 120, 8989-9003, doi:10.1002/2015JA021475.

Published in Journal of Geophysical Research Space Physics, 2015

Ion drag is known to play an important role in driving neutral thermosphere circulation at auroral latitudes, especially during the main phase of geomagnetic storms. During the recovery phase, the neutrals are known to drive the ions and generate ionospheric electric fields and currents via the disturbance dynamo mechanism. At midlatitudes, the precise interplay between ions and neutrals is less understood largely because of the paucity of measurements that have been available. In this work, we investigate ion-neutral coupling at middle latitudes using colocated ion drift velocity measurements obtained from Super Dual Auroral Radar Network radars and neutral wind velocity and temperature measurements obtained from the North American Thermosphere Ionosphere Observing Network (NATION) Fabry-Perot interferometers. We examine one recent storm period on 2–3 October 2013 during both the main phase and late recovery phase. By using ion-neutral momentum exchange theory and a time-lagged correlation analysis, we analyze the coupling time scales and dominant driving mechanisms. We observe that during the main phase the neutrals respond to the ion convection on a time scale of ∼84 min which is significantly faster than what would be expected from local ion drag momentum forcing alone. This suggests that other storm time influences are important for driving the neutrals during the main phase, such as Joule heating. During the late recovery phase, the neutrals are observed to drive the ion convection without any significant time delay, consistent with the so-called “neutral fly wheel effect” or disturbance dynamo persisting well into the late recovery phase.

Multi-instrument, high-resolution imaging of polar cap patch transportation

Thomas, E. G., K. Hosokawa, J. Sakai, J. B. H. Baker, J. M. Ruohoniemi, S. Taguchi, K. Shiokawa, Y. Otsuka, A. J. Coster, J.-P. St.-Maurice, and K. A. McWilliams (2015), Multi-instrument, high-resolution imaging of polar cap patch transportation, Radio Sci., 50, 904-915, doi:10.1002/2015RS005672.

Published in Radio Science, 2015

Transionospheric radio signals in the high-latitude polar cap are susceptible to degradation when encountering sharp electron density gradients associated with discrete plasma structures, or patches. Multi-instrument measurements of polar cap patches are examined during a geomagnetic storm interval on 22 January 2012. For the first time, we monitor the transportation of patches with high spatial and temporal resolution across the polar cap for 1–2 h using a combination of GPS total electron content (TEC), all-sky airglow imagers (ASIs), and Super Dual Auroral Radar Network (SuperDARN) HF radar backscatter. Simultaneous measurements from these data sets allow for continuous tracking of patch location, horizontal extent, and velocity despite adverse observational conditions for the primary technique (e.g., sunlit regions in the ASI data). Spatial collocation between patch-like features in relatively coarse but global GPS TEC measurements and those mapped by high-resolution ASI data was very good, indicating that GPS TEC can be applied to track patches continuously as they are transported across the polar cap. In contrast to previous observations of cigar-shaped patches formed under weakly disturbed conditions, the relatively narrow dawn-dusk extent of patches in the present interval (500–800 km) suggests association with a longitudinally confined plasma source region, such as storm-enhanced density (SED) plume. SuperDARN observations show that the backscatter power enhancements corresponded to the optical patches, and for the first time we demonstrate that the motion of the optical patches was consistent with background plasma convection velocities.

GPS Phase Scintillation at High Latitudes during Two Geomagnetic Storms

Prikryl, P., R. Ghoddousi-Fard, J. M. Ruohoniemi, and E. G. Thomas (2015), GPS Phase Scintillation at High Latitudes during Two Geomagnetic Storms, in Auroral Dynamics and Space Weather (eds Y. Zhang and L. J. Paxton), John Wiley & Sons, Inc, Hoboken, NJ, doi:10.1002/9781118978719.ch15.

Published in Auroral Dynamics and Space Weather, 2015

The GPS phase scintillation at high latitudes is a consequence of coupling between variable solar wind and the magnetosphere-ionosphere (M-I) system resulting in structured and dynamic ionosphere. Intense GPS phase scintillation at high latitudes was observed during two geomagnetic storms that were caused by impacts of coronal mass ejections on November 1, 2011 and March 17, 2013. This chapter focuses on GPS phase scintillation during the two storms. Ionospheric regions of enhanced scintillation are identified in the context of coupling processes between solar wind and the M-I system. The occurrence of scintillation as a function of magnetic latitude and magnetic local time is strongly controlled by the interplanetary magnetic field orientation. A link is suggested between scintillation occurrence in the polar cap and the nightside auroral oval that is consistent with recently discovered relationships between polar cap patches and substorms, and between enhanced polar cap flows and poleward boundary intensifications.

The geomagnetic storm time response of GPS total electron content in the North American sector

Thomas, E. G., J. B. H. Baker, J. M. Ruohoniemi, A. J. Coster, and S.-R. Zhang (2016), The geomagnetic storm time response of GPS total electron content in the North American sector, J. Geophys. Res. Space Physics, 121, 1744-1759, doi:10.1002/2015JA022182.

Published in Journal of Geophysical Research Space Physics, 2016

Over the last two decades, maps of GPS total electron content (TEC) have improved our understanding of the large perturbations in ionospheric electron density which occur during geomagnetic storms. However, previous regional and global studies of ionospheric storms have performed only a limited separation of storm time, local time, longitudinal, and seasonal effects. Using 13 years of GPS TEC data, we present a complete statistical characterization of the ionospheric response to geomagnetic storms for midlatitudes in the North American sector where dense ground receiver coverage is available. The rapid onset of a positive phase is observed across much of the dayside and evening ionosphere followed by a longer-lasting negative phase across all latitudes and local times. Our results show clear seasonal variations in the storm time TEC, such that summer events tend to be dominated by the negative storm response while winter events exhibit a stronger initial positive phase with minimal negative storm effects. We find no discernable difference between spring and fall equinox events with both being equivalent to the average storm time response across all seasons. We also identify a prominent magnetic declination effect such that stronger dayside positive storm effects are observed in regions of negative declination (i.e., eastern North America). On the nightside, asymmetries in the TEC response are observed near the auroral oval and midlatitude trough which may be attributed to thermospheric zonal winds pushing plasma upward/downward along field lines of opposite declination.

Earth's ion upflow associated with polar cap patches: Global and in situ observations

Zhang, Q.-H., M. Lockwood, R. Heelis, M. Hairston, J. Liang, I. McCrea, B.-C. Zhang, J. Moen, S.-R. Zhang, Y. Zhang, J. M. Ruohoniemi, M. Lester, E. G. Thomas, R.-Y. Liu, M. W. Dunlop, Y. Liu, and Y.-Z. Ma (2016), Earth's ion upflow associated with polar cap patches: Global and in situ observations, Geophys. Res. Lett., 43, 1845-1853, doi:10.1002/2016GL067897.

Published in Geophysical Research Letters, 2016

We report simultaneous global monitoring of a patch of ionization and in situ observation of ion upflow at the center of the polar cap region during a geomagnetic storm. Our observations indicate strong fluxes of upwelling O+ ions originating from frictional heating produced by rapid antisunward flow of the plasma patch. The statistical results from the crossings of the central polar cap region by Defense Meteorological Satellite Program F16–F18 from 2010 to 2013 confirm that the field-aligned flow can turn upward when rapid antisunward flows appear, with consequent significant frictional heating of the ions, which overcomes the gravity effect. We suggest that such rapidly moving patches can provide an important source of upwelling ions in a region where downward flows are usually expected. These observations give new insight into the processes of ionosphere-magnetosphere coupling.

Polar cap patch transportation beyond the classic scenario

Zhang, Q.-H., J. Moen, M. Lockwood, I. McCrea, B.-C. Zhang, K. A. McWilliams, Q.-G. Zong, S.-R. Zhang, J. M. Ruohoniemi, E. G. Thomas, M. W. Dunlop, R.-Y. Liu, H. Yang, H.-Q. Hu, and M. Lester (2016), Polar cap patch transportation beyond the classic scenario, J. Geophys. Res. Space Physics, 121, 9063-9074, doi:10.1002/2016JA022443.

Published in Journal of Geophysical Research Space Physics, 2016

We report the continuous monitoring of a polar cap patch, encompassing its creation, and a subsequent evolution that differs from the classic behavior. The patch was formed from the storm-enhanced density plume, by segmentation associated with a subauroral polarization stream generated by a substorm. Its initial antisunward motion was halted due to a rapidly changing of interplanetary magnetic field (IMF) conditions from strong southward to strong eastward with weaker northward components, and the patch subsequently very slowly evolved behind the duskside of a lobe reverse convection cell in afternoon sectors, associated with high-latitude lobe reconnection, much of it fading rapidly due to an enhancement of the ionization recombination rate. This differs from the classic scenario where polar cap patches are transported across the polar cap along the streamlines of twin-cell convection pattern from day to night. This observation provides us new important insights into patch formation and control by the IMF, which has to be taken into account in F region transport models and space weather forecasts.

Solar Cycle 24 Observations of Storm Enhanced Density and the Tongue of Ionization

Coster, A. J., P. J. Erickson, J. C. Foster, E. G. Thomas, J. M. Ruohoniemi, and J. B. H. Baker (2016), Solar Cycle 24 Observations of Storm Enhanced Density and the Tongue of Ionization, in Ionospheric Space Weather: Longitude and Hemispheric Dependencies and Lower Atmosphere Forcing (eds T. Fuller-Rowell, E. Yizengaw, P. H. Doherty, and S. Basu), John Wiley & Sons, Inc, Hoboken, NJ, doi:10.1002/9781118929216.ch6.

Published in Ionospheric Space Weather: Longitude and Hemispheric Dependencies and Lower Atmosphere Forcing, 2016

Storm-enhanced density (SED) plumes form in the ionosphere and plasmasphere at midlatitudes during geomagnetically disturbed conditions. In some cases, ionospheric plasma in the storm-enhanced density (SED) plume becomes entrained in expanded polar cap convection patterns and enters the polar cap through the cusp region to form the tongue of ionization (TOI). The recent addition of several GPS receivers in the Antarctic has led to improvements in TEC spatial coverage in the Southern Hemisphere. In addition, new tools have been developed for merging Super Dual Auroral Network (SuperDARN) observations of decameter scale irregularities with GPS-derived total electron content (TEC) data. This paper will review these new merged data sets and will present a selection of SED and TOI observations from the years 2009, 2012, and 2013 during the current solar cycle 24. The particular SED/TOI observational cases presented represent characteristic longitudinal, hemispherical, and seasonal differences observed during both quiet (2009) and moderate (2012, 2013) solar-flux conditions. The denser observational coverage of GPS receivers during this current solar cycle, especially in Antarctica, allows for better comparisons of SED features in the two hemispheres, and allows conclusions to be drawn about key TEC longitude, hemisphere, and seasonal patterns.

GPS phase scintillation at high latitudes during the geomagnetic storm of March 17-18, 2015

Prikryl, P., R. Ghoddousi-Fard, J. M. Weygand, A. Viljanen, M. Connors, D. W. Danskin, P. T. Jayachandran, K. S. Jacobsen, Y. L. Andalsvik, E. G. Thomas, J. M. Ruohoniemi, T. Durgonics, K. Oksavik, E. Spanswick, M. Aquino, and V. Sreeja (2016), GPS phase scintillation at high latitudes during the geomagnetic storm of March 17-18, 2015, J. Geophys. Res. Space Physics, 121, 10,448-10,465, doi:10.1002/2016JA023171.

Published in Journal of Geophysical Research Space Physics, 2016

The geomagnetic storm of 17–18 March 2015 was caused by the impacts of a coronal mass ejection and a high-speed plasma stream from a coronal hole. The high-latitude ionosphere dynamics is studied using arrays of ground-based instruments including GPS receivers, HF radars, ionosondes, riometers, and magnetometers. The phase scintillation index is computed for signals sampled at a rate of up to 100 Hz by specialized GPS scintillation receivers supplemented by the phase scintillation proxy index obtained from geodetic-quality GPS data sampled at 1 Hz. In the context of solar wind coupling to the magnetosphere-ionosphere system, it is shown that GPS phase scintillation is primarily enhanced in the cusp, the tongue of ionization that is broken into patches drawn into the polar cap from the dayside storm-enhanced plasma density, and in the auroral oval. In this paper we examine the relation between the scintillation and auroral electrojet currents observed by arrays of ground-based magnetometers as well as energetic particle precipitation observed by the DMSP satellites. Equivalent ionospheric currents are obtained from ground magnetometer data using the spherical elementary currents systems technique that has been applied over the ground magnetometer networks in North America and North Europe. The GPS phase scintillation is mapped to the poleward side of strong westward electrojet and to the edge of the eastward electrojet region. Also, the scintillation was generally collocated with fluxes of energetic electron precipitation observed by DMSP satellites with the exception of a period of pulsating aurora when only very weak currents were observed.

PFISR observation of intense ion upflow fluxes associated with an SED during the 1 June 2013 geomagnetic storm

Zou, S., A. J. Ridley, X. Jia, E. Boyd, M. J. Nicolls, A. J. Coster, E. G. Thomas, and J. M. Ruohoniemi (2017), PFISR observation of intense ion upflow fluxes associated with an SED during the 1 June 2013 geomagnetic storm, J. Geophys. Res. Space Physics, 122, 2589-2604, doi:10.1002/2016JA023697.

Published in Journal of Geophysical Research Space Physics, 2017

The Earth’s ionosphere plays an important role in supplying plasma into the magnetosphere through ion upflow/outflow, particularly during periods of strong solar wind driving. An intense ion upflow flux event during the 1 June 2013 storm has been studied using observations from multiple instruments. When the open-closed field line boundary (OCB) moved into the Poker Flat incoherent scatter radar (PFISR) field of view, divergent ion fluxes were observed by PFISR with intense upflow fluxes reaching ~1.9 × 1014 m−2 s−1 at ~600 km altitude. Both ion and electron temperatures increased significantly within the ion upflow, and thus, this event has been classified as a type 2 upflow. We discuss factors contributing to the high electron density and intense ion upflow fluxes, including plasma temperature effect and preconditioning by storm-enhanced density (SED). Our analysis shows that the significantly enhanced electron temperature due to soft electron precipitation in the cusp can reduce the dissociative recombination rate of molecular ions above ~400 km and contributed to the density increase. In addition, this intense ion upflow flux event is preconditioned by the lifted F region ionosphere due to northwestward convection flows in the SED plume. During this event, the OCB and cusp were detected by DMSP between 15 and 16 magnetic local times, unusually duskward. Results from a global magnetohydrodynamics simulation using the Space Weather Modeling Framework have been used to provide a global context for this event. This case study provides a more comprehensive mechanism for the generation of intense ion upflow fluxes observed in association with SEDs.

Statistical characterization of the large-scale structure of the subauroral polarization stream

Kunduri, B. S. R., J. B. H. Baker, J. M. Ruohoniemi, E. G. Thomas, S. G. Shepherd, and K. T. Sterne (2017), Statistical characterization of the large-scale structure of the subauroral polarization stream, J. Geophys. Res. Space Physics, 122, 6035-6048, doi:10.1002/2017JA024131.

Published in Journal of Geophysical Research Space Physics, 2017

The subauroral polarization streams (SAPS) are latitudinally narrow regions of westward directed flows observed equatorward of the evening sector auroral oval. Previous studies have shown that SAPS generally occur during geomagnetically disturbed conditions and exhibit a strong dependence on geomagnetic activity. In this paper, we present the first comprehensive statistical study of SAPS using measurements from the U.S. midlatitude Super Dual Auroral Radar Network (SuperDARN) radars. The study period spans January 2011 to December 2014, and the results show that SuperDARN radars observe SAPS over a broad range of activity levels spanning storm time and nonstorm conditions. During relatively quiet conditions (−10 nT <Dst< 10 nT) SAPS occur 15% of the time and tend to be localized to the midnight sector and centered above 60° magnetic latitude. As the activity level increases, the peak SAPS location shifts equatorward and duskward. During moderately disturbed conditions (−75 nT <Dst <− 50 nT) SAPS occur 87% of the time and tend to be centered at 20 magnetic local time (MLT) and below 60° magnetic latitude. This behavior has been encoded into a new empirical model which uses Dst as input to estimate the probability of SAPS occurrence at a given magnetic latitude and MLT. Similar to some previous studies, the variation of SAPS speed with MLT is found to be nearly linear at low to moderate levels of geomagnetic activity but becomes increasingly nonlinear near dusk sector as geomagnetic activity increases. We interpret this behavior as indicative of active ionosphere-thermosphere feedback playing an important role in modulating SAPS speeds.

Ionospheric F-region response to the 26 September 2011 geomagnetic storm in the Antarctic American and Australian sectors

Correia, E., L. Spogli, L. Alfonsi, C. Cesaroni, A. M. Gulisano, E. G. Thomas, R. F. H. Ramirez, and A. A. Rodel (2017), Ionospheric F-region response to the 26 September 2011 geomagnetic storm in the Antarctic American and Australian sectors, Ann. Geophys., 35, 1113-1129, doi:10.5194/angeo-35-1113-2017.

Published in Annales Geophysicae, 2017

The ionospheric response at middle and high latitudes in the Antarctica American and Australian sectors to the 26–27 September 2011 moderately intense geomagnetic storm was investigated using instruments including an ionosonde, riometer, and GNSS receivers. The multi-instrument observations permitted us to characterize the ionospheric storm-enhanced density (SED) and tongues of ionization (TOIs) as a function of storm time and location, considering the effect of prompt penetration electric fields (PPEFs). During the main phase of the geomagnetic storm, dayside SEDs were observed at middle latitudes, and in the nightside only density depletions were observed from middle to high latitudes. Both the increase and decrease in ionospheric density at middle latitudes can be attributed to a combination of processes, including the PPEF effect just after the storm onset, dominated by disturbance dynamo processes during the evolution of the main phase. Two SEDs–TOIs were identified in the Southern Hemisphere, but only the first episode had a counterpart in the Northern Hemisphere. This difference can be explained by the interhemispheric asymmetry caused by the high-latitude coupling between solar wind and the magnetosphere, which drives the dawn-to-dusk component of the interplanetary magnetic field. The formation of polar TOI is a function of the SED plume location that might be near the dayside cusp from which it can enter the polar cap, which was the case in the Southern Hemisphere. Strong GNSS scintillations were observed at stations collocated with SED plumes at middle latitudes and cusp on the dayside and at polar cap TOIs on the nightside.

Statistical patterns of ionospheric convection derived from mid-latitude, high-latitude, and polar SuperDARN HF radar observations

Thomas, E. G., and S. G. Shepherd (2018), Statistical patterns of ionospheric convection derived from mid-latitude, high-latitude, and polar SuperDARN HF radar observations, J. Geophys. Res. Space Physics, 123, 3196-3216, doi:10.1002/2018JA025280.

Published in Journal of Geophysical Research Space Physics, 2018

Over the last decade, the Super Dual Auroral Radar Network (SuperDARN) has undergone a dramatic expansion in the Northern Hemisphere with the addition of more than a dozen radars offering improved coverage at mid-latitudes (50°–60° magnetic latitude) and in the polar cap (80°–90° magnetic latitude). In this study, we derive a statistical model of ionospheric convection (TS18) using line-of-sight velocity measurements from the complete network of mid-latitude, high-latitude, and polar radars for the years 2010–2016. These climatological patterns are organized by solar wind, interplanetary magnetic field (IMF), and dipole tilt angle conditions. We find that for weak solar wind driving conditions the TS18 model patterns are largely similar to the average patterns obtained using high-latitude radar data only. For stronger solar wind driving the inclusion of mid-latitude radar data at the equatorward extent of the ionospheric convection can increase the measured cross-polar cap potential (ΦPC) by as much as 40%. We also derive an alternative model organized by the Kp index to better characterize the statistical convection under a range of magnetic activity conditions. These Kp patterns exhibit similar IMF By dependencies as the TS18 model results and demonstrate a linear increase in ΦPC with increasing Kp for a given IMF orientation. Overall, the mid-latitude radars provide a better specification of the flows within the nightside Harang reversal region for moderate to strong solar wind driving or geomagnetic activity, while the polar radars improve the quality of velocity measurements in the deep polar cap under all conditions.

Polar traveling ionospheric disturbances inferred with the B-spline method and associated scintillations in the Southern Hemisphere

Priyadarshi, S., Q.-H. Zhang, E. G. Thomas, L. Spogli, and C. Cesaroni (2018), Polar traveling ionospheric disturbances inferred with the B-spline method and associated scintillations in the Southern Hemisphere, Adv. Space Res., 62, 3249-3266, doi:10.1016/j.asr.2018.08.015.

Published in Advances in Space Research, 2018

A new method for analyzing travelling ionospheric disturbances (TIDs) is developed by using two B-spline basis functions of degree 4 on the Total Electron Content (TEC) data from the ground-based Global Positioning System (GPS) receivers. This method enhances the spatial resolution to about 0.1° (geographic latitude) × 0.1° (geographic longitude), which is useful in studying all scales (small, medium and large) TIDs. Using this method, we investigated TIDs and their associated scintillation on 18–19 July 2013 at Southern Hemisphere and found phase scintillation is more sensitive than amplitude scintillation to the TIDs at South Pole. To see the full impact of TIDs on scintillation, we have used a proxy phase scintillation index, calculated using geodetic GPS receivers over Antarctica. We have verified the presence of TIDs during these two days by using a Global Navigation Satellite System (GNSS)-TEC single station approach and SuperDARN slant range signals. Our results show the TEC fluctuations are associated with ionospheric scintillation. The shape of TIDs, their elongation and flattening along/across the geographic latitude/longitude, seems to be related to the magnitude and occurrence of ionospheric scintillations. Magnetospheric particle precipitation boost TEC gradients and generate stronger amplitude scintillation, however, large-scale plasma irregularities cause overall enhancement in magnitude of the phase scintillation index. Due to the high turbulence in the polar ionosphere, TIDs change their shapes quite quickly and/or may disappear in the background ionosphere. B-spline TIDs analysis method is very useful in identifying the visible as well as hidden TIDs parts in the polar ionosphere. For the first time, ionospheric scintillation has been investigated in the vicinity of TIDs at high- latitude in the southern hemisphere. Further, the presented B-spline TIDs analysis method is unique and simple in itself as it uses GPS receiver processed TEC data as the primary input. Our results show that at polar latitude it is not necessary that TIDs always appear near the high TEC regions. Usefulness of the B-spline TIDs detection method has been demonstrated in analyzing TIDs at all geographic locations and different solar activity conditions by comparing B-spline TIDs method produced results with the previous case studies.

Review of the accomplishments of mid-latitude Super Dual Auroral Radar Network (SuperDARN) HF radars

Nishitani, N., J. M. Ruohoniemi, M. Lester, J. B. H. Baker, A. V. Koustov, S. G. Shepherd, G. Chisham, T. Hori, E. G. Thomas, R. A. Makarevich, A. Marchaudon, P. Ponomarenko, J. A. Wild, S. E. Milan, W. A. Bristow, J. Devlin, E. Miller, R. A. Greenwald, T. Ogawa, and T. Kikuchi (2019), Review of the accomplishments of mid-latitude Super Dual Auroral Radar Network (SuperDARN) HF radars, Prog Earth Planet Sci, 6:27, doi:10.1186/s40645-019-0270-5.

Published in Progress in Earth and Planetary Science, 2019

The Super Dual Auroral Radar Network (SuperDARN) is a network of high-frequency (HF) radars located in the high- and mid-latitude regions of both hemispheres that is operated under international cooperation. The network was originally designed for monitoring the dynamics of the ionosphere and upper atmosphere in the high-latitude regions. However, over the last approximately 15 years, SuperDARN has expanded into the mid-latitude regions. With radar coverage that now extends continuously from auroral to sub-auroral and mid-latitudes, a wide variety of new scientific findings have been obtained. In this paper, the background of mid-latitude SuperDARN is presented at first. Then, the accomplishments made with mid-latitude SuperDARN radars are reviewed in five specified scientific and technical areas: convection, ionospheric irregularities, HF propagation analysis, ion-neutral interactions, and magnetohydrodynamic (MHD) waves. Finally, the present status of mid-latitude SuperDARN is updated and directions for future research are discussed.

Morphological study on the ionospheric variability at Bharati, a polar cusp station in the southern hemisphere

Shreedevi, P. R., R. K. Choudhary, Y. Yu, and E. G. Thomas (2019), Morphological study on the ionospheric variability at Bharati, a polar cusp station in the southern hemisphere, J. Atmos. Sol.-Terr. Phys., 193, 105058, doi:10.1016/j.jastp.2019.105058.

Published in Journal of Atmospheric and Solar-Terrestrial Physics, 2019

Morphological features of the quiet/disturbed time variations in the Total Electron Content (TEC) at the polar cusp station Bharati (76.69°S MLAT) during a period of 5 years starting from February 2013 to December 2017 has been studied using GPS TEC measurements. The TEC at Bharati follows a diurnal pattern with its peak appearing close to local noon/magnetic noon during the summer/winter months. A nighttime enhancement in the TEC is seen around the magnetic midnight during winter. The plasma density at Bharati also exhibits semi-annual variation and a strong dependence on solar activity. A comparison of the IRI 2016 model derived TEC and the GPS TEC at Bharati shows significant differences with large underestimation of TEC especially during the nighttime period of the winter months. A two fold difference in magnitude between the GPS and modeled TEC is also observed in the summer months of the high solar activity period of 2013–2015. The response of the TEC to geomagnetic storms is found to depend on the onset time of the storm. We show that the morphological features in the temporal evolution of the plasma density at Bharati vary as the location of Bharati changes from being inside the polar cap, to the auroral region, and to the polar cusp in quick succession in a day. Our results highlight the fact that the dynamic nature of the location of Bharati with respect to the position of the polar cap plays an important role in deciding the plasma distribution at the polar cusp station.

Separation and quantification of ionospheric convection sources: 2. The dipole tilt angle influence on reverse convection cells during northward IMF

Reistad, J. P., K. M. Laundal, N. Østgaard, A. Ohma, E. G. Thomas, S. Haaland, K. Oksavik, and S. E. Milan (2019), Separation and quantification of ionospheric convection sources: 2. The dipole tilt angle influence on reverse convection cells during northward IMF, J. Geophys. Res. Space Physics, 124, 6182-6194, doi:10.1029/2019JA026641.

Published in Journal of Geophysical Research Space Physics, 2019

This paper investigates the influence of Earth’s dipole tilt angle on the reverse convection cells (sometimes referred to as lobe cells) in the Northern Hemisphere ionosphere during northward IMF, which we relate to high-latitude reconnection. Super Dual Auroral Radar Network plasma drift observations in 2010–2016 are used to quantify the ionospheric convection. A novel technique based on Spherical Elementary Convection Systems (SECS) that was presented in our companion paper (Reistad et al., 2019, https://doi.org/10.1029/2019JA026634) is used to isolate and quantify the reverse convection cells. We find that the dipole tilt angle has a linear influence on the reverse cell potential. In the Northern Hemisphere the reverse cell potential is typically two times higher in summer than in winter. This change is interpreted as the change in interplanetary magnetic field-lobe reconnection rate due to the orientation of the dipole tilt. Hence, the dipole tilt influence on reverse ionospheric convection can be a significant modification of the more known influence from vswBz. These results could be adopted by the scientific community as key input parameters for lobe reconnection coupling functions.

Spatial variation in the responses of the surface external and induced magnetic field to the solar wind

Shore, R. M., M. P. Freeman, J. C. Coxon, E. G. Thomas, J. W. Gjerloev, and N. Olsen (2019), Spatial variation in the responses of the surface external and induced magnetic field to the solar wind, J. Geophys. Res. Space Physics, 124, 6195-6211, doi:10.1029/2019JA026543.

Published in Journal of Geophysical Research Space Physics, 2019

We analyze the spatial variation in the response of the surface geomagnetic field (or the equivalent ionospheric current) to variations in the solar wind. Specifically, we regress a reanalysis of surface external and induced magnetic field (SEIMF) variations onto measurements of the solar wind. The regression is performed in monthly sets, independently for 559 regularly spaced locations covering the entire northern polar region above 50° magnetic latitude. At each location, we find the lag applied to the solar wind data that maximizes the correlation with the SEIMF. The resulting spatial maps of these independent lags and regression coefficients provide a model of the localized SEIMF response to variations in the solar wind, which we call “Spatial Information from Distributed Exogenous Regression.” We find that the lag and regression coefficients vary systematically with ionospheric region, season, and solar wind driver. In the polar cap region the SEIMF is best described by the By component of the interplanetary magnetic field (50–75% of total variance explained) at a lag ∼20–25 min. Conversely, in the auroral zone the SEIMF is best described by the solar wind ϵ function (60–80% of total variance explained), with a lag that varies with season and magnetic local time (MLT), from ∼15–20 min for dayside and afternoon MLT (except in Oct–Dec) to typically 30–40 min for nightside and morning MLT and even longer (60–65 min) around midnight MLT.

Polar cap patch prediction in the expanding contracting polar cap paradigm

Fæhn Follestad, A., L. B. N. Clausen, E. G. Thomas, Y. Jin, and A. J. Coster (2019), Polar cap patch prediction in the expanding contracting polar cap paradigm, Space Weather, 17, 1570-1583, doi:10.1029/2019SW002276.

Published in Space Weather, 2019

Space weather can cause serious disturbances of global navigation satellite systems (GNSS) used for positioning and navigation purposes. This paper describes a new method to forecast space weather disturbances on GNSS at high latitudes, in which we describe the formation and propagation of polar cap patches and predict their arrival at the nightside auroral oval. The space weather prediction model builds on the expanding/contracting polar cap (ECPC) paradigm and total electron content (TEC) observations from the Global Positioning System (GPS) network. The input parameter is satellite observations of the interplanetary magnetic field at the first Lagrange point. To validate our prediction model, we perform a case study in which we compare the results from our prediction model to observations from the GPS TEC data from the MIT’s Madrigal database, convection data from Super Dual Auroral radar network, and scintillation data from Svalbard. Our results show that the ECPC paradigm describes the polar cap patch motion well and can be used to predict scintillations of GPS signals at high latitudes.

AMPERE polar cap boundaries

Burrell, A. G., G. Chisham, S. E. Milan, L. Kilcommons, Y.-J. Chen, E. G. Thomas, and B. Anderson (2020), AMPERE polar cap boundaries, Ann. Geophys., 38, 481-490, doi:10.5194/angeo-38-481-2020.

Published in Annales Geophysicae, 2020

This study presents a new set of open–closed magnetic field line boundaries (OCBs) obtained from Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) magnetic perturbation data. AMPERE observations of field-aligned currents (FACs) are used to determine the location of the boundary between the Region 1 (R1) and Region 2 (R2) FAC systems. This current boundary is thought to typically lie a few degrees equatorward of the OCB, making it a good candidate for obtaining OCB locations. The AMPERE R1–R2 boundaries are compared to the Defense Meteorological Satellite Program Special Sensor J (DMSP SSJ) electron energy flux boundaries to test this hypothesis and determine the best estimate of the systematic offset between the R1–R2 boundary and the OCB as a function of magnetic local time. These calibrated boundaries, as well as OCBs obtained from the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) observations, are validated using simultaneous observations of the convection reversal boundary measured by DMSP. The validation shows that the OCBs from IMAGE and AMPERE may be used together in statistical studies, providing the basis of a long-term data set that can be used to separate observations originating inside and outside of the polar cap.

An improved estimation of SuperDARN Heppner-Maynard Boundaries using AMPERE data

Fogg, A. R., M. Lester, T. K.Yeoman, A. G. Burrell, S. M. Imber, S. E. Milan, E. G. Thomas, H. Sangha, and B. J. Anderson (2020), An improved estimation of SuperDARN Heppner-Maynard Boundaries using AMPERE data, J. Geophys. Res. Space Physics, 125, e2019JA027218, doi:10.1029/2019JA027218.

Published in Journal of Geophysical Research Space Physics, 2020

Super Dual Auroral Radar Network (SuperDARN) ionospheric convection maps are a powerful tool for the study of solar wind-magnetosphere-ionosphere interactions. SuperDARN data have high temporal (approximately minutes) and spatial (∼45 km) resolution, meaning that the convection can be mapped on fine time scales that show more detail than the large-scale changes in the pattern. The Heppner-Maynard boundary (HMB) defines the low-latitude limit of the convection region, and its identification is an essential component of the standard SuperDARN convection mapping technique. However, the estimation of the latitude of this boundary is dependent on ionospheric scatter availability. Consequentially it is susceptible to nonphysical variations as areas of scatter in different latitude and local time regions appear and disappear, often due to changing propagation conditions. In this paper, the HMB is compared to an independent field-aligned current data set from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). A linear trend is found between the HMB and the boundary between the AMPERE Region 1 and Region 2 field-aligned currents in the Northern Hemisphere, at both solar minimum and solar maximum. The use of this trend and the AMPERE current data set to predict the latitude position of the HMB is found to improve the interpretation of the SuperDARN measurements in convection mapping.

Bistatic observations with SuperDARN HF radars: First results

Shepherd, S. G., K. T. Sterne, E. G. Thomas, J. M. Ruohoniemi, J. B. H. Baker, R. T. Parris, T. R. Pedersen, and J. M. Holmes (2020), Bistatic observations with SuperDARN HF radars: First results, Radio Sci., 55, e2020RS007121, doi:10.1029/2020RS007121.

Published in Radio Science, 2020

Super Dual Auroral Radar Network (SuperDARN) radars operate in a coordinated but monostatic configuration whereby high-frequency (HF) signals scattered from ionospheric density irregularities or from the surface of the Earth return to the transmitting radar where Doppler parameters are then acquired. A bistatic arrangement has been developed for SuperDARN radars in which HF signals transmitted from one radar are received and analyzed by another radar that is separated by a large distance (>1,000 km). This new capability was developed and tested on radars located in Oregon and Kansas. Numerous 3-day bistatic campaigns have been conducted over a period extending from September 2019 through March 2020. During these campaigns three distinct bistatic propagation modes have been identified including a direct mode in which signals are transmitted and received through the radar side lobes. Direct mode signals propagate along the great-circle arc connecting the two bistatic radar sites, reflecting from the ionosphere at both E region and F region altitudes. Two additional modes are observed in which HF signals transmitted from one radar scatter from either ionospheric density irregularities or from the surface of the Earth before propagating to the bistatic receiving radar. Ray tracing simulations performed for examples of each mode show good agreement with observations and confirm our understanding of these three bistatic propagation modes. Bistatic campaigns continue to be scheduled in order to improve technical aspects of this new capability, to further explore the physical processes involved in the propagation and scattering of HF bistatic signals and to expand the coverage of ionospheric effects that is possible with SuperDARN.

Geomagnetic storm induced plasma density enhancements in the southern polar ionospheric region: a comparative study using St. Patrick's day storms of 2013 and 2015

Shreedevi, P. R., R. K. Choudhary, S. V. Thampi, S. Yadav, T. K. Pant, Y. Yu, R. M. McGranaghan, E. G. Thomas, A. Bhardwaj, and A. K. Sinha (2020), Geomagnetic storm induced plasma density enhancements in the southern polar ionospheric region: a comparative study using St. Patrick's day storms of 2013 and 2015, Space Weather, 18, e2019SW002383, doi:10.1029/2019SW002383.

Published in Space Weather, 2020

The occurrence of St. Patrick’s Day (17 March) geomagnetic storms during two different years (2013 and 2015) with similar solar flux levels but varying storm intensity provided an opportunity to compare and contrast the responses of the ionosphere-thermosphere (IT) system to different levels of geomagnetic activity. The evolution of positive ionospheric storms at the southern polar stations Bharati (76.6°S MLAT) and Davis (76.2°S MLAT) and its causative connection to the solar wind driving mechanisms during these storms has been investigated in this paper. During the main phase of both the storms, significant enhancements in TEC and phase scintillation were observed in the magnetic noon/ midnight period at Bharati and Davis. The TEC in the midnight sector on 17 March 2015 was significantly higher compared to that on 17 March 2013, in line with the storm intensity. The TEC enhancements during both the storm events are associated with the formation of the storm-enhanced densities (SEDs)/tongue of ionization (TOI). The strong and sustained magnetopause erosion led to the prevalence of stronger storm time electric fields (prompt penetration electric field (PPEF)/subauroral polarization streams (SAPS)) for long duration on 17 March 2015. This combined with the action of neutral winds at midlatitudes favored the formation of higher plasma densities in the regions of SED formation on this day. The same was weaker during the 17 March 2013 storm due to the fast fluctuating nature of interplanetary magnetic field (IMF) Bz. This study shows that the duration and extent of magnetopause erosion play an important role in the spatiotemporal evolution of the plasma density distribution in the high-midlatitude ionosphere.

Comparison of interferometer calibration techniques for improved SuperDARN elevation angles

Chisham, G., A. G. Burrell, A. Marchaudon, S. G. Shepherd, E. G. Thomas, and P. V. Ponomarenko (2021), Comparison of interferometer calibration techniques for improved SuperDARN elevation angles, Polar Science, 28, 100638, doi:10.1016/j.polar.2021.100638.

Published in Polar Science, 2021

The high frequency radars in the Super Dual Auroral Radar Network (SuperDARN) estimate the elevation angles of returned backscatter using interferometric techniques. These elevation angles allow the ground range to the scattering point to be estimated, which is crucial for the accurate geolocation of ionospheric measurements. For elevation angles to be accurately estimated, it is important to calibrate the interferometer measurements by determining the difference in the signal time delays caused by the difference in the electrical path lengths from the main array and the interferometer array to the point at which the signals are correlated. This time delay is known as tdiff. Several methods have been proposed to estimate tdiff using historical observations; these methods are summarised in this paper. Comparisons of the tdiff estimates from the different calibration methods are presented and sources of uncertainty discussed. The effect of errors in the estimated tdiff value on the accuracy of geolocation is evaluated and discussed. The paper concludes with a series of recommendations for both scientific SuperDARN data users and SuperDARN radar operators.

Editorial: Special issue: “SuperDARN / Studies of Geospace Dynamics - Today and Future”

Yukimatu, A. S., A. Grocott, E. G. Thomas, T. Nagatsuma, N. Nishitani, K. Hosokawa, and M. Watanabe (2021), Editorial: Special issue: "SuperDARN / Studies of Geospace Dynamics - Today and Future", Polar Science, 28, 100690, doi:10.1016/j.polar.2021.100690.

Published in Polar Science, 2021

This Polar Science special issue, “SuperDARN/Studies of Geospace Dynamics - Today and Future,” originated from an international SuperDARN (Super Dual Auroral Radar Network) annual workshop held in Japan in June 2019, and is focused on studies of geospace dynamics particularly related to SuperDARN. Its purpose is to overview recent wide and active research, new scientific results and future perspectives mainly through, but not limited to, the scientific papers presented at the workshop. This special issue is an opportunity to commemorate a quarter century since the establishment of SuperDARN in 1995 and to contribute to the further development of geospace sciences and relevant technology. Thirteen valuable papers have been published covering a wide variety of scientific and technical topics.

Quantifying the lobe reconnection rate during dominant IMF By periods and different dipole tilt orientations

Reistad, J. P., K. M. Laundal, N. Østgaard, A. Ohma, A. G. Burrell, S. M. Hatch, S. Haaland, and E. G. Thomas (2021), Quantifying the lobe reconnection rate during dominant IMF By periods and different dipole tilt orientations, J. Geophys. Res. Space Physics, 126, e2021JA029742, doi:10.1029/2021JA029742.

Published in Journal of Geophysical Research Space Physics, 2021

Lobe reconnection is usually thought to play an important role in geospace dynamics only when the Interplanetary Magnetic Field (IMF) is mainly northward. This is because the most common and unambiguous signature of lobe reconnection is the strong sunward convection in the polar cap ionosphere observed during these conditions. During more typical conditions, when the IMF is mainly oriented in a dawn-dusk direction, plasma flows initiated by dayside and lobe reconnection both map to high-latitude ionospheric locations in close proximity to each other on the dayside. This makes the distinction of the source of the observed dayside polar cap convection ambiguous, as the flow magnitude and direction are similar from the two topologically different source regions. We here overcome this challenge by normalizing the ionospheric convection observed by the Super Dual Aurora Radar Network (SuperDARN) to the polar cap boundary, inferred from simultaneous observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). This new method enable us to separate and quantify the relative contribution of both lobe reconnection and dayside/nightside (Dungey cycle) reconnection during periods of dominating IMF By. Our main findings are twofold. First, the lobe reconnection rate can typically account for 20% of the Dungey cycle flux transport during local summer when IMF By is dominating and IMF Bz ≥ 0. Second, the dayside convection relative to the open/closed boundary is vastly different in local summer versus local winter, as defined by the dipole tilt angle.

Super Dual Auroral Radar Network expansion and its influence on the derived ionospheric convection pattern

Walach, M.-T., A. Grocott, F. Staples, and E. G. Thomas (2022), Super Dual Auroral Radar Network expansion and its influence on the derived ionospheric convection pattern, J. Geophys. Res. Space Physics, 127, e2021JA029559, doi:10.1029/2021JA029559.

Published in Journal of Geophysical Research Space Physics, 2022

The Super Dual Auroral Radar Network (SuperDARN) was built to study ionospheric convection and has in recent years been expanded geographically. Alongside software developments, this has resulted in many different versions of the convection maps data set being available. Using data from 2012 to 2018, we produce five different versions of the widely used convection maps, using limited backscatter ranges, background models and the exclusion/inclusion of data from specific radar groups such as the StormDARN radars. This enables us to simulate how much information was missing from older SuperDARN research. We study changes in the Heppner-Maynard boundary (HMB), the cross polar cap potential (CPCP), the number of backscatter echoes (n) and the χ2/n statistic which is a measure of the global agreement between the measured and fitted velocities. We find that the CPCP is reduced when the PolarDARN radars are introduced, but then increases again when the StormDARN radars are added. When the background model is changed from the RG96 model, to the most recent TS18 model, the CPCP tends to decrease for lower values, but tends to increase for higher values. When comparing to geomagnetic indices, we find that there is on average a linear relationship between the HMB and the geomagnetic indices, as well as n, which breaks when the HMB is located at latitudes below ∼50° due to the low observational density. Whilst n is important in constraining the maps (maps with n > 400 data points are unlikely to differ), it is insufficient as the sole measure of quality.

Virtual height characteristics of ionospheric and ground scatter observed by mid-latitude SuperDARN HF radars

Thomas, E. G., and S. G. Shepherd (2022), Virtual height characteristics of ionospheric and ground scatter observed by mid-latitude SuperDARN HF radars, Radio Sci., 57, e2022RS007429, doi:10.1029/2022RS007429.

Published in Radio Science, 2022

Propagation of high-frequency (HF) radio signals is strongly dependent on the ionospheric electron density structure along a communications link. The ground-based, HF space weather radars of the Super Dual Auroral Radar Network (SuperDARN) utilize the ionospheric refraction of transmitted signals to monitor the global circulation of E- and F-region plasma irregularities. Previous studies have assessed the propagation characteristics of backscatter echoes from ionospheric irregularities in the auroral and polar regions of the Earth’s ionosphere. By default, the geographic location of these echoes are found using empirical models which estimate the virtual backscattering height from the measured range along the radar signal path. However, the performance of these virtual height models has not yet been evaluated for mid-latitude SuperDARN radar observations or for ground scatter (GS) propagation modes. In this study, we derive a virtual height model suitable for mid-latitude SuperDARN observations using 5 years of data from the Christmas Valley East and West radars. This empirical model can be applied to both ionospheric and GS observations and provides an improved estimate of the ground range to the backscatter location compared to existing high-latitude virtual height models. We also identify a region of overlapping half-hop F-region ionospheric scatter and one-hop E-region GS where the measured radar parameters (e.g., velocity, spectral width, elevation angle) are insufficient to discriminate between the two scatter types. Further studies are required to determine whether these backscatter echoes of ambiguous origin are observed by other mid-latitude SuperDARN radars and their potential impact on scatter classification schemes.

Ionospheric boundaries derived from auroral images

Chisham, G., A. G. Burrell, E. G. Thomas, and Y.-J. Chen (2022), Ionospheric boundaries derived from auroral images, J. Geophys. Res. Space Physics, 127, e2022JA030622, doi:10.1029/2022JA030622.

Published in Journal of Geophysical Research Space Physics, 2022

This paper presents updated methods for locating the Poleward and Equatorward Auroral Luminosity Boundaries (PALB and EALB) directly from IMAGE Far UltraViolet (FUV) images of the Northern Hemisphere auroral oval. Separate boundaries are determined from images measured at different FUV wavelengths. In addition, new methods for indirectly estimating the Open-Closed magnetic field line Boundary (OCB) and the Equatorward Precipitation Boundary (EPB) locations are presented; these new boundaries are derived from a combination of the auroral luminosity boundary estimates with statistical latitudinal offsets derived from comparisons with low-altitude spacecraft Particle Precipitation Boundaries (PPBs). Subsequently, we derive new circle model fits for all these boundary data sets, as well as new quality control criteria for these model fits. The suitability of circle fits for each of the data sets is discussed, and the OCB and PALB circle fits are validated against the Convection Reversal Boundary (CRB), as measured by low-altitude in situ spacecraft. All the new boundary data sets, covering the epoch May 2000 to October 2002, are freely available online.

Traveling ionospheric disturbances in the vicinity of storm enhanced density at midlatitudes

Zhang, S.-R., Y. Nishimura, P. J. Erickson, E. Aa, H. Kil, Y. Deng, E. G. Thomas, W. Rideout, A. J. Coster, R. Kerr, and J. Vierinen (2022), Traveling ionospheric disturbances in the vicinity of storm enhanced density at midlatitudes, J. Geophys. Res. Space Physics, 127, e2022JA030429, doi:10.1029/2022JA030429.

Published in Journal of Geophysical Research Space Physics, 2022

This study provides first storm time observations of the westward-propagating medium-scale traveling ionospheric disturbances (MSTIDs), particularly, associated with characteristic subauroral storm time features, storm-enhanced density (SED), subauroral polarization stream (SAPS), and enhanced thermospheric westward winds over the continental US. In the four recent (2017–2019) geomagnetic storm cases examined in this study (i.e., 2018-08-25/26, 2017-09-07/08, 2017-05-27/28, and 2016-02-02/03 with minimum SYM-H index −206, −146, −142, and −58 nT, respectively), MSTIDs were observed from dusk-to-midnight local times predominately during the intervals of interplanetary magnetic field (IMF) Bz stably southward. Multiple wavefronts of the TIDs were elongated NW-SE, 2°–3° longitude apart, and southwestward propagated at a range of zonal phase speeds between 100 and 300 m/s. These TIDs initiated in the northeastern US and intensified or developed in the central US with either the coincident SED structure (especially the SED basis region) or concurrent small electron density patches adjacent to the SED. Observations also indicate coincident intense storm time electric fields associated with the magnetosphere–ionosphere–thermosphere coupling electrodynamics at subauroral latitudes (such as SAPS) as well as enhanced thermospheric westward winds. We speculate that these electric fields trigger plasma instability (with large growth rates) and MSTIDs. These electrified MSTIDs propagated westward along with the background westward ion flow which resulted from the disturbance westward wind dynamo and/or SAPS.

An examination of SuperDARN backscatter modes using machine learning guided by ray-tracing

Kunduri, B. S. R., J. B. H. Baker, J. M. Ruohoniemi, E. G. Thomas, and S. G. Shepherd (2022), An examination of SuperDARN backscatter modes using machine learning guided by ray-tracing, Space Weather, 20, e2022SW003130, doi:10.1029/2022SW003130.

Published in Journal of Geophysical Research Space Physics, 2022

The Super Dual Auroral Radar Network (SuperDARN) is a network of High Frequency (HF) radars that are typically used for monitoring plasma convection in the Earth’s ionosphere. A majority of SuperDARN backscatter can broadly be divided into three categories: (a) ionospheric scatter due to reflections from plasma irregularities in the E and F regions of the ionosphere, (b) ground scatter caused by reflections from the ground/sea surface following reflection in the ionosphere, and (c) backscatter from meteor trails left by meteoroids as they enter the Earth’s atmosphere. Due to the complex nature of HF propagation and mid-latitude electrodynamics, it is often not straightforward to distinguish between different modes of backscatter observed by SuperDARN. In this study, we present a new two-stage machine learning algorithm for identifying different backscatter modes in SuperDARN data. In the first stage, a neural network that “mimics” ray-tracing is used to predict the probability of ionospheric and ground scatter occurring at a given location along with parameters like the elevation angles, reflection heights etc. The inputs to the network include parameters that control HF propagation, such as signal frequency, season, UT time, and geomagnetic activity levels. In the second stage, the output probabilities from the neural network and actual SuperDARN data are clustered together to determine the category of the backscatter. Our model can distinguish between meteor scatter, 1/2 hop E-/F-region ionospheric as well as ground/sea scatter. We validate our model by comparing predicted elevation angles with those measured at a SuperDARN radar.

Multi-instrument observations of polar cap patches and traveling ionospheric disturbances generated by solar wind Alfvén waves coupling to the dayside magnetosphere

Prikryl, P., R. G. Gillies, D. R. Themens, J. M. Weygand, E. G. Thomas, and S. Chakraborty (2022), Multi-instrument observations of polar cap patches and traveling ionospheric disturbances generated by solar wind Alfven waves coupling to the dayside magnetosphere, Ann. Geophys., 40, 619-639, doi:10.5194/angeo-40-619-2022.

Published in Annales Geophysicae, 2022

During minor to moderate geomagnetic storms, caused by corotating interaction regions (CIRs) at the leading edge of high-speed streams (HSSs), solar wind Alfvén waves modulated the magnetic reconnection at the dayside magnetopause. The Resolute Bay Incoherent Scatter Radars (RISR-C and RISR-N), measuring plasma parameters in the cusp and polar cap, observed ionospheric signatures of flux transfer events (FTEs) that resulted in the formation of polar cap patches. The patches were observed as they moved over the RISR, and the Canadian High-Arctic Ionospheric Network (CHAIN) ionosondes and GPS receivers. The coupling process modulated the ionospheric convection and the intensity of ionospheric currents, including the auroral electrojets. The horizontal equivalent ionospheric currents (EICs) are estimated from ground-based magnetometer data using an inversion technique. Pulses of ionospheric currents that are a source of Joule heating in the lower thermosphere launched atmospheric gravity waves, causing traveling ionospheric disturbances (TIDs) that propagated equatorward. The TIDs were observed in the SuperDual Auroral Radar Network (SuperDARN) high-frequency (HF) radar ground scatter and the detrended total electron content (TEC) measured by globally distributed Global Navigation Satellite System (GNSS) receivers.

Dusk-dawn asymmetries in SuperDARN convection maps

Walach, M.-T., A. Grocott, E. G. Thomas, and F. Staples (2022), Dusk-dawn asymmetries in SuperDARN convection maps, J. Geophys. Res. Space Physics, 127, e2022JA030906, doi:10.1029/2022JA030906.

Published in Journal of Geophysical Research Space Physics, 2022

The Super Dual Auroral Radar Network (SuperDARN) is a collection of radars built to study ionospheric convection. We use a 7-year archive of SuperDARN convection maps, processed in 3 different ways, to build a statistical understanding of dusk-dawn asymmetries in the convection patterns. We find that the data set processing alone can introduce a bias which manifests itself in dusk-dawn asymmetries. We find that the solar wind clock angle affects the balance in the strength of the convection cells. We further find that the location of the positive potential foci is most likely observed at latitudes of 78° for long periods (>300 min) of southward interplanetary magnetic field (IMF), as opposed to 74° for short periods (<20 min) of steady IMF. For long steady dawnward IMF the median is also at 78°. For long steady periods of duskward IMF, the positive potential foci tends to be at lower latitudes than the negative potential and vice versa during dawnward IMF. For long periods of steady Northward IMF, the positive and negative cells can swap sides in the convection pattern. We find that they move from ∼0–9 MLT to 15 MLT or ∼15–23 MLT to 10 MLT, which reduces asymmetry in the average convection cell locations for Northward IMF. We also investigate the width of the region in which the convection returns to the dayside, the return flow width. Asymmetries in this are not obvious, until we select by solar wind conditions, when the return flow region is widest for the negative convection cell during Southward IMF.

Multi-frequency SuperDARN interferometer calibration

Thomas, E. G., S. G. Shepherd, and G. Chisham (2024), Multi-frequency SuperDARN interferometer calibration, Radio Sci., 59, e2024RS007957, doi:10.1029/2024RS007957.

Published in Radio Science, 2024

The ground-based, high-frequency radars of the Super Dual Auroral Radar Network (SuperDARN) observe backscatter from ionospheric field-aligned plasma irregularities and features on the Earth’s surface out to ranges of several thousand kilometers via over-the-horizon propagation of transmitted radio waves. Interferometric techniques can be applied to the received signals at the primary and secondary antenna arrays to measure the vertical angle of arrival, or elevation angle, for more accurate geolocation of SuperDARN observations. However, the calibration of SuperDARN interferometer measurements remains challenging for several reasons, including a 2pi phase ambiguity when solving for the time delay correction factor needed to account for differences in the electrical path lengths between signals received at the two antenna arrays. We present a new technique using multi-frequency ionospheric and ground backscatter observations for the calibration of SuperDARN interferometer data, and demonstrate its application to both historical and recent data.

HF radar observations and modeling of the impact of the 8 April 2024 total solar eclipse on the ionosphere-thermosphere system

Kunduri, B. S. R., J. B. H. Baker, J. M. Ruohoniemi, E. G. Thomas, J. D. Huba, D. J. Emmons, D. R. Themens, K. T. Sterne, G. Farinas Perez, W. A. Bristow, S. G. Shepherd, J. M. Holmes, E. V. Dao, A. T. Chartier, G. W. Perry, and K. Pandey (2024), HF radar observations and modeling of the impact of the 8 April 2024 total solar eclipse on the ionosphere-thermosphere system, Geophys. Res. Lett., 51, e2024GL112484, doi:10.1029/2024GL112484.

Published in Geophysical Research Letters, 2024

The path of totality of the 8 April 2024 solar eclipse traversed the fields-of-view of four US SuperDARN radars. This rare scenario provided an excellent opportunity to monitor the large-scale ionospheric response to the eclipse. In this study, we present observations made by the Blackstone (BKS) SuperDARN radar and a Digisonde during the eclipse. Two striking effects were observed by the BKS radar: (a) the Doppler velocities associated with ground scatter coalesced into a pattern clearly organized by the line of totality, with a reversal in sign across this line, and, (b) a delay of ~45 min between time of maximum obscuration and maximum effect on the skip distance. The skip distance estimated using a SAMI3 simulation of the eclipse did not however capture the asymmetric time-delay. These observations suggest that the neutral atmosphere plays an important role in controlling ionospheric plasma dynamics, which were missing in SAMI3 simulations.

Multi-frequency SuperDARN HF radar observations of the ionospheric response to the October 2023 annular solar eclipse

Thomas, E. G., S. G. Shepherd, B. S. R. Kunduri, and D. R. Themens (2024), Multi-frequency SuperDARN HF radar observations of the ionospheric response to the October 2023 annular solar eclipse, Geophys. Res. Lett., 51, e2024GL112450, doi:10.1029/2024GL112450.

Published in Geophysical Research Letters, 2024

An annular solar eclipse was visible on 14 October 2023 from 15:00–21:00 UT as its path traveled across North, Central, and South America. In this letter, we present the first multi-frequency Super Dual Auroral Radar Network (SuperDARN) observations of the bottomside ionospheric response to a solar eclipse using a novel experimental mode designed for the October 2023 annular eclipse. We compare our results from the mid-latitude Christmas Valley East radar with measurements of the vertical electron density profile from the nearby Boulder Digisonde, finding the changes in 1- and 2-hop ground scatter skip distance are well correlated with the F2-layer density response, which lags the peak obscuration by ~30 min. Changes in the line-of-sight Doppler shifts are better aligned with the time derivative of eclipse obscuration.

Observation and simulation studies of ionospheric F-region in the South American and Antarctic sectors in the intense geomagnetic storm of August 2018

de Abreu, A. J., E. Correia, O. F. Jonah, K. Venkatesh, E. G. Thomas, R. de Jesus, M. Roberto, J. R. Abalde, and P. R. Fagundes (2025), Observation and simulation studies of ionospheric F-region in the South American and Antarctic sectors during the intense space weather event of August 2018, J. Atmos. Sol.-Terr. Phys., 266, 106394, doi:10.1016/j.jastp.2024.106394.

Published in Journal of Atmospheric and Solar-Terrestrial Physics, 2025

In this investigation, we present and discuss the ionospheric F region observations in the equatorial, low-, mid-, and near high-latitude regions in the South American and Antarctic sectors during the intense geomagnetic storm that occurred on 25–27 August 2018. The geomagnetic storm reached a minimum Dst of −175 nT at ∼0700 UT on 26 August. We present the variations of vertical total electron content (VTEC) from a chain of almost 200 GPS stations, covering the South American and Antarctic sectors. A comparison with model simulations from the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIE-GCM) is realized. The results obtained show that during the main phase of the storm, a southward Bz component of the interplanetary magnetic field (IMF) and an eastward prompt penetration electric field (PPEF) can be observed, but they had no significant impact on the ionospheric plasma. A long recovery phase a predominance of positive phase is observed during daytime. The observations show the effects of an unusual case of multiple PPEF, occurred on 26 August, and effects of thermospheric winds disturbances, occurred on 27 August, resulting in increased VTEC values on both days. The TIE-GCM model reproduces the VTEC increases during the main and recovery phases from mid-latitudes to the equatorial region, but it underestimates the observed values near high-latitudes.

Magnetosheath control of the cross polar cap potential: Correcting for measurement uncertainty in space weather

O'Brien, C., B. M. Walsh, and E. G. Thomas (2025), Magnetosheath control of the cross polar cap potential: Correcting for measurement uncertainty in space weather, J. Geophys. Res. Space Physics, 130, e2024JA033468, doi:10.1029/2024JA033468.

Published in Journal of Geophysical Research Space Physics, 2025

This study quantifies the variation and apparent saturation of the cross polar cap potential (phiPC) with respect to the motional electric field (EM) in the solar wind and magnetosheath. The electric potential across the polar cap (phiPC) is often observed to respond linearly to solar wind driving during weak driving and nonlinearly during strong driving, with phiPC eventually responding less and less to increased driving. This effect is called “polar cap potential saturation” and has been observed in many studies that correlate some measure of solar wind driving (typically the motional electric field EM) with phiPC. In this study, it is shown that measurement error in the solar wind driver creates a nonlinear bias in its variation relative to phiPC. This is accomplished by associating a decade of radar measurements of phiPC with simultaneous probabilistic predictions of EM in the solar wind just upstream of the bow shock and in the magnetosheath. After correcting for the bias in the solar wind and magnetosheath EM caused by measurement uncertainty, the extent of saturation between phiPC and the corrected solar wind EM is reduced. More importantly, phiPC saturates with respect to the solar wind EM but not the magnetosheath EM. This effect is caused by the magnetosheath EM being reduced relative to the solar wind EM when the solar wind Alfven Mach number is low. These findings both support the theory that the phiPC saturation is due to the magnetosheath flow being magnetically dominated for large solar wind magnetic field magnitudes.

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