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Abstract A geomagnetic storm, one of the largest in this solar cycle, was launched on 10 May 2024, producing spectacular auroral displays that could be observed across the continental United States (US) at middle and low latitudes. In this study, we focus on a brief 20‐min interval during the peak of the storm when the Sym‐H index dropped to −500 nT, and the auroral activity specified by the AL and AU indices was elevated. During this interval, the Blackstone (BKS) Super Dual Auroral Radar Network (SuperDARN) radar, observed strong ionospheric backscatter blanketing the near‐ranges across its field‐of‐view. Upon analyzing the elevation and virtual height characteristics of this backscatter we find that: (a) the BKS radar observed F‐region backscatter at unusually close ranges (750 km), and (b) this backscatter was observed over a broad range of elevation angles, including unusual very high ones. It is not physically realistic that all the radio waves, launched over a broad range of elevation angles, refract to become perpendicular to the B‐field. We therefore interpret that a sizable portion of this backscatter is produced by irregularities that are not field‐aligned. These observations show that plasma irregularities generated during strong geomagnetic storms can produce strong and unusual High Frequency (HF) radar backscatter, and significantly impact their operations. Finally, we suggest that the high‐aspect angle backscatter was most likely associated with the non‐linear decay of gradient‐drift modes that had been excited unusually strongly during the event. 

Abstract Solar flares are a rapid increase in solar irradiance, specifically in X‐ray and Extreme Ultraviolet spectra, which enhances the ionization in the dayside ionosphere and creates Sudden Ionospheric Disturbances (SIDs). SIDs are known to create space weather impacts on traveling high frequency (HF: 3–30 MHz) radio waves, by disrupting the communication channels. In this study, we examine ionospheric scatters at dawn terminator, which stems from a severe X9.3 flare on 6 September 2017 peaked at 12:02 UT, utilizing SuperDARN HF coherent scatter radars and Global Navigation Satellite System (GNSS) Total Electron Content (TEC) observations. Specifically, we are interested in the transients in the ionospheric electrodynamics at the sub‐auroral latitude near the terminator stemming from the flare effect. Observations suggest that flare‐induced density gradient likely favors the formation of gradient‐drift instability near the dawn terminator, leading to the irregularities observed by the SuperDARN radars with line‐of‐sight (LoS) Doppler velocity reaching nearly 300 m/s. The flare amplifies the eastward TEC gradient near the dawn terminator by approximately 2–3 times compared to a geomagnetically quiet and non‐flare day. The observed irregularities, attributed to flare‐driven instabilities, exhibit a velocity consistent with the equatorial return flow of ionospheric Hall convection. In contrast to prior studies indicating decreased cross‐polar‐cap potential and associated ionospheric convection flow, our findings show the flare is followed by an increase in localized electric field near the dawn terminator, as depicted in radar LoS velocity. 

Abstract Recent observations show very near‐Earth reconnection (∼8–13R_E) could efficiently power the ring current during the main phase of geomagnetic storms, but whether the recovery phase might be contributed remains unclear. During the recovery phase of the May 2024 major geomagnetic storm, intense auroral brightening and geomagnetic disturbances were observed at midnight, indicative of particle injections. Current wedges observed by mid‐latitude ground magnetometers around midnight suggest dipolarizing flux bundles (DFBs). The latitude of the auroral brightening was clearly lower than usual, suggesting near‐Earth reconnection (NERX) was closer to Earth than during substorms (∼20–30R_E). GOES‐18 at midnight detected magnetic field and plasma signatures consistent with DFBs, following an extremely thin current sheet likely compressed by strong upstream dynamic pressure. These results indicate NERX could have been close enough for resultant DFBs to penetrate geosynchronous orbit and contribute to the ring current during the recovery phase. This scenario deserves further examination in future. 

Abstract Magnetopause reconnection is the dominant mechanism for transporting solar wind energy and momentum into the magnetosphere‐ionosphere system. Magnetopause reconnection can occur along X‐lines of variable extent in the direction perpendicular to the reconnection plane. Identifying the spatial extent of X‐lines using satellite observations has critical limitations. However, we can infer the azimuthal extent of the X‐lines by probing the ionospheric signature of reconnection, the antisunward flow channels across the ionospheric Open‐Closed Field Line Boundary (OCB). We study 39 dayside magnetopause reconnection events using conjugate in situ and ionospheric observations to investigate the variability and controlling factors of the spatial extent of reconnection. We use spacecraft data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) to identify in situ reconnection events. The width of the antisunward flow channels across the OCB is measured using the concurrent measurements from Super Dual Auroral Radar Network (SuperDARN). Also, the X‐line lengths are estimated by tracing the magnetic field lines from the ionospheric flow boundaries to the magnetopause. The solar wind driving conditions upstream of the bow shock are studied using solar wind monitors located at the L1 point. Results show that the magnetopause reconnection X‐lines can extend from a few Earth Radii (RE) to at least 22 RE in the GSM‐Y direction. Furthermore, the magnetopause reconnection tends to be spatially limited during high solar wind speed conditions. 

Abstract 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. 

Abstract Ultra low frequency (ULF; 1 mHz ‐ several Hz) waves are key to energy transport within the geospace system, yet their contribution to Joule heating in the upper atmosphere remains poorly quantified. This study statistically examines Joule heating associated with ionospheric ULF perturbations using Super Dual Auroral Radar Network (SuperDARN) data spanning middle to polar latitudes. Our analysis utilizes high‐time‐resolution measurements from SuperDARN high‐frequency coherent scatter radars operating in a special mode, sampling three “camping beams” approximately every 18 s. We focus on ULF perturbations within the Pc5 frequency range (1.6–6.7 mHz), estimating Joule heating rates from ionospheric electric fields derived from SuperDARN data and height‐integrated Pedersen conductance from empirical models. The analysis includes statistical characterization of Pc5 wave occurrence, electric fields, Joule heating rates, and azimuthal wave numbers. Our results reveal enhanced electric fields and Joule heating rates in the morning and pre‐midnight sectors, even though Pc5 wave occurrences peak in the afternoon. Joule heating is more pronounced in the high‐latitude morning sector during northward interplanetary magnetic field conditions, attributed to local time asymmetry in Pedersen conductance and Pc5 waves driven by Kelvin‐Helmholtz instability. Pc5 waves observed by multiple camping beams predominantly propagate westward at low azimuthal wave numbers , while high‐m waves propagate mainly eastward. Although Joule heating estimates may be underestimated due to assumptions about empirical conductance models and the underestimation of electric fields resulting from SuperDARN line‐of‐sight velocity measurements, these findings offer valuable insights into ULF wave‐related energy dissipation in the geospace system. 

Abstract The duskside and dawnside subauroral polarization streams (SAPS) refer to high‐velocity westward and eastward plasma flows located equatorward of the auroral oval. While extensive research has focused on the duskside SAPS, the simultaneous evolution of both dawnside and duskside SAPS remains unreported. In this study, for the first time, we investigated the simultaneous evolution of duskside and dawnside SAPS using multiple Super Dual Auroral Radar Network radars during an intense storm. Observations indicate that the duskside SAPS exhibits a wider magnetic local time extension (∼7 MLT) and longer duration (∼1 hr) than the dawnside SAPS. Furthermore, the duskside SAPS resides within low‐density mid‐latitude troughs, whereas the dawnside SAPS is not located within the trough. The dawnside SAPS exhibits significantly higher electron density but comparable velocity to the duskside SAPS. These findings highlight the distinct evolution of dawnside and duskside SAPS, providing new insights into the electrodynamic processes of subauroral ionosphere and magnetosphere coupling. 

Abstract The weakly ionized plasma in the Earth's ionosphere is controlled by a complex interplay between solar and magnetospheric inputs from above, atmospheric processes from below, and plasma electrodynamics from within. This interaction results in ionosphere structuring and variability that pose major challenges for accurate ionosphere prediction for global navigation satellite system (GNSS) related applications and space weather research. The ionospheric structuring and variability are often probed using the total electron content (TEC) and its relative perturbations (dTEC). Among dTEC variations observed at high latitudes, a unique modulation pattern has been linked to magnetospheric ultra‐low‐frequency (ULF) waves, yet its underlying mechanisms remain unclear. Here using magnetically conjugate observations from the THEMIS spacecraft and a ground‐based GPS receiver at Fairbanks, Alaska, we provide direct evidence that these dTEC modulations are driven by magnetospheric electron precipitation induced by ULF‐modulated whistler‐mode waves. We observed peak‐to‐peak dTEC amplitudes reaching 0.5 TECU (1 TECU is equal to electrons/) with modulations spanning scales of 5–100 km. The cross‐correlation between our modeled and observed dTEC reached 0.8 during the conjugacy period but decreased outside of it. The spectra of whistler‐mode waves and dTEC also matched closely at ULF frequencies during the conjugacy period but diverged outside of it. Our findings elucidate the high‐latitude dTEC generation from magnetospheric wave‐induced precipitation, addressing a significant gap in current physics‐based dTEC modeling. Theses results thus improve ionospheric dTEC prediction and enhance our understanding of magnetosphere‐ionosphere coupling via ULF waves. 

Abstract Joule heating is a major energy sink in the solar wind‐magnetosphere‐ionosphere system and modeling it is key to understanding the impact of space weather on the neutral atmosphere. Ion drifts and neutral wind velocities are key parameters when modeling Joule heating, however there is limited validation of the modeled ion and neutral velocities at mid‐latitudes. We use the Blackstone Super Dual Auroral Radar Network radar and the Michigan North American Thermosphere Ionosphere Observing Network Fabry‐Perot interferometer to obtain the local nightside ion and neutral velocities at ∼40° geographic latitude during the nighttime of 16 July 2014. Despite being a geomagnetically quiet period, we observe significant sub‐auroral ion flows in excess of 200 ms⁻¹. We calculate an enhancement to the local Joule heating rate due to these ion flows and find that the neutrals impart a significant increase or decrease to the total Joule heating rate of >75% depending on their direction. We compare our observations to outputs from the Thermosphere Ionosphere Electrodynamic General Circulation Model (TIEGCM). At such a low geomagnetic activity however, TIEGCM was not able to model significant sub‐auroral ion flows and any resulting Joule heating enhancements equivalent to our observations. We found that the neutral winds were the primary contributor to the Joule heating rates modeled by TIEGCM rather than the ions as suggested by our observations. 

Abstract The mid‐latitude ionospheric trough (MLIT), an anomaly in the ionosphere's F layer caused by various mechanisms, affects radio wave propagation. In this study, we investigated the morphology and oscillations of the MLIT using global Global Positioning System total electron content map data between 1 January 2018, and 31 December 2020. The MLIT position varies longitudinally, reaching its farthest equatorward at 60W and its farthest poleward at 30E. The MLIT occurrence rates peak during the winter and equinoxes and dip in summer, while seasonal variations in MLIT position vary across longitude bands. Heightened geomagnetic activities, quantified by the SME6 index, promote MLIT occurrence, especially during pre‐midnight hours in summer and equinoxes, and shift the MLIT equatorward, particularly during midnight and post‐midnight hours. The MLIT position shows clear local time variation, with a gradual decrease before midnight, stabilization afterward, and a minor resurgence around dawn. Wavelet analysis reveals three distinct periodic components in the MLIT position: 27, 13.5, and 9, with the 27‐day period being the most persistent. Cross‐wavelet and wavelet coherence analyses suggest that solar wind (SW) velocity variations precede changes in the MLIT position. The main factors responsible for the equatorward movement of MLIT are the electric fields in high‐speed SW that enhance the ionospheric convection pattern, and the intensified geomagnetic activities induced by interplanetary shocks.