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1. Association of Equatorward Extended Auroral Streamers With Overshielding Conditions at Equatorial Latitudes: First Observations

   https://doi.org/10.1029/2023JA031726  

   Yadav, Sneha; Lyons, Larry. R.; Nishimura, Yukitoshi; Weygand, James M.; Liu, Jiang; Zhang, Shun‐Rong; Tian, Sheng; Zou, Ying; Donovan, Eric F. (November 2023, Journal of Geophysical Research: Space Physics)

   Abstract We report the first observations of the association between equatorward extending streamers and overshielding using the THEMIS all‐sky imagers and ground magnetometers. Because auroral streamers indicate plasma sheet flow bursts, these observations uncover the effect of flow bursts on overshielding. Results show that, in general, bright equatorward extended streamers were associated with an increase in equatorial electrojet (EEJ) on the nightside and a decrease in the dayside EEJ, indicating a striking correspondence between auroral streamers and overshielding conditions. Thus, the driving of overshielding at equatorial latitudes can be identified via bright equatorward extended streamers, indicating that flow bursts are an alternate means to discern the earthward injections that increase the region 2 field aligned currents and associated overshielding electric fields. Repetitive auroral streamers were associated with repetitive overshielding, resulting in a stepwise development of the dayside and nightside EEJ. The stepwise intensifications were also observed in the midlatitude positive bay and Pi2 pulsations. Our results could explain the occurrence of overshielding conditions at equatorial latitudes during substorms and nonsubstorm times without a northward turning of IMF‐Bz. As seen through streamers, the localized current structures (wedgelets) associated with flow bursts giving injection that leads to overshielding is titled northeast‐to‐southwest. Our results add a new element to the understanding of high‐to‐low latitude electrodynamical coupling by demonstrating the association between bright equatorward extended auroral streamers and enhanced shielding electric fields caused by earthward injections associated with flow bursts. 

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2. Evidence of Substorm‐Driven Penetration Electric Field Contributions to Low‐Latitude Phenomena: Enhanced Upward Drifts, Plasma Bubble Development and Severe Scintillation

   https://doi.org/10.1029/2024SW004297  

   Sousasantos, J; Rodrigues, F S; Fejer, B G; Eastes, R W; Moraes, A O (March 2025, Space Weather)

   Abstract In this work, it is demonstrated that substorm‐driven penetration electric fields can efficiently enhance the upward plasma transport, favoring the development and structuring of plasma irregularities and the occurrence of scintillation on L‐band signals. While most previous studies focus on investigating penetration electric fields during intense geomagnetic storms, here, the period used (April 01–05, 2020) was under very mild geomagnetic activity (−27 nT SYM‐H 6 nT), so that interplanetary and disturbance dynamo contributions are minimized. This period comprised the same seasonal and solar flux conditions, while undergoing multiple short‐lived substorms, making it well‐suited to evaluate unequivocally: (a) to what extent substorm‐driven penetration electric fields alter electrodynamical processes over low latitudes, and (b) how effective they are in contributing to the structuring of the early nighttime ionosphere and the subsequent occurrence of severe scintillation on L‐band signals. Ground‐based and space‐based multi‐instrument data sets were used. The results show that, even under weak geomagnetic activity, substorm‐driven penetration electric fields—despite being subtle and short‐lived—play a decisive role, enhancing the upward drifts, favoring the development of equatorial plasma bubbles and severe scintillation. The findings indicate that substorms with onsets coinciding with early nighttime are more impactful. This decisive contribution is more likely to be identified during late spring and early fall in the northern hemisphere (or vice versa in the southern hemisphere), when the prereversal vertical drifts are moderate—neither too small nor too large—and may have direct impacts on the day‐to‐day variability of equatorial plasma bubbles. 

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3. Four‐Peak Structure on Equatorial Ionization Anomaly Crests During the May 2024 Geomagnetic Storms

   https://doi.org/10.1029/2025SW004542  

   Pu, Weixing; Yang, Yanyan; Yan, Rui; Zhang, Shun‐Rong; Xiong, Chao; Battiston, Roberto; Zhima, Zeren; Mirko, Piersanti; Huang, He; D’Angelo, Giulia; et al (December 2025, Space Weather)

   Abstract This study investigates the four‐peak electron density structure in the Equatorial Ionization Anomaly (EIA) during the intense geomagnetic storms of 10–20 May 2024, utilizing dayside data at ∼500 km from the China Seismo‐Electromagnetic Satellite (CSES‐01). Observations revealed distinct four peaks in latitudinal profiles of electron density, symmetrically distributed across both hemispheres. Particularly, the poleward crests can expand beyond ±30° quasi‐dipole latitude during the 10 May superstorm. To reveal the potential driver of such unique EIA structure, statistical analysis is conducted on 35 CSES‐01 orbits which identified the four‐peak phenomenon. The enhanced equatorial electrojet activity, inferred from the scalar magnetic field residuals, indicates that the four‐peak morphology is likely related to the prompt penetration electric field (PPEF). Superposed epoch analysis also highlighted that intensification of the mean SuperMAG |SML| index preceded the onset of four‐peak structures by ∼20 min, whereas the mean eastward equatorial electric field, derived from an empirical model of the interplanetary electric field driven penetration, exhibits no statistically significant enhancement, implying that substorm‐associated PPEF may be the primary driver of this type of EIA structure. This study advances understanding of EIA variability under extreme space weather conditions and the findings underscore the critical role of magnetosphere‐ionosphere coupling via substorm‐driven electric fields in reshaping storm‐time ionospheric plasma distribution. 

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4. Solar Wind‐Magnetosphere Coupling During High‐Intensity Long‐Duration Continuous AE Activity (HILDCAA)

   https://doi.org/10.1029/2023JA032027  

   Milan, S E; Mooney, M K; Bower, G; Fleetham, A L; Vines, S K; Gjerloev, J (November 2023, Journal of Geophysical Research: Space Physics)

   Abstract High‐Intensity Long‐Duration Continuous AE Activity (HILDCAA) intervals are driven by High Speed solar wind Streams (HSSs) during which the rapidly‐varying interplanetary magnetic field (IMF) produces high but intermittent dayside reconnection rates. This results in several days of large, quasi‐periodic enhancements in the auroral electrojet (AE) index. There has been debate over whether the enhancements in AE are produced by substorms or whether HILDCAAs represent a distinct class of magnetospheric dynamics. We investigate 16 HILDCAA events using the expanding/contracting polar cap model as a framework to understand the magnetospheric dynamics occurring during HSSs. Each HILDCAA onset shows variations in open magnetic flux, dayside and nightside reconnection rates, the cross‐polar cap potential, and AL that are characteristic of substorms. The enhancements in AE are produced by activity in the pre‐midnight sector, which is the typical substorm onset region. The periodicities present in the intermittent IMF determine the exact nature of the activity, producing a range of behaviors from a sequence of isolated substorms, through substorms which merge into one‐another, to almost continuous geomagnetic activity. The magnitude of magnetic fluctuations,dB/dt, in the pre‐midnight sector during HSSs is sufficient to produce a significant risk of Geomagnetically Induced Currents. 

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5. “Polar” Substorms During Slow Solar Wind

   https://doi.org/10.1029/2024JA033555  

   Despirak, I V; Kleimenova, N G; Lubchich, A A; Setsko, P V; Malysheva, L M (February 2025, Journal of Geophysical Research: Space Physics)

   Abstract “Polar” substorms are identified as substorm‐like disturbances that are exclusively observed at high geomagnetic latitudes (>70° MLAT) and are absent at lower latitudes. Although “polar” substorms typically occur during periods of quiet geomagnetic activity, it is still unclear whether they can develop under extremely quiet conditions when geoeffective space weather parameters are exceptionally low. Utilizing data from the IMAGE network across the Svalbard archipelago within the longitudinal sector of (∼108–114 Mlong), we examined 92 “extremely quiet geomagnetic” intervals from 2010 to 2020, which were associated with intervals of extremely slow solar wind (ESSWs,V < 300 km/s). We discovered that “polar” substorms can occur during ESSWs, but only with the presence of a negative Bz component. A total of 32 such events were identified from 17 ESSW intervals (∼19% of all ESSW intervals). We found that “polar” substorms during ESSWs display the primary characteristics of ordinary substorms, including the accompaniment of Pi1B geomagnetic pulsations, positive subauroral or mid‐latitude magnetic bays, a poleward shift of the westward electrojet, and auroral activity during their expansion phase. Additionally, it was found that the majority of “polar” substorm events during ESSWs (∼82%) were isolated substorms, developing solely in the pre‐midnight sector without disturbances in other longitudinal sectors. Several “polar” substorm events have been examined in detail. 

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