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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. 

Abstract The polar cap arc at 1500 MLT (15MLT‐PCA) has been considered as an auroral signature of the cusp's duskside boundary and been speculated to be caused by lobe reconnection. However, no observational evidence has been provided to support this speculation. Here we report a 15MLT‐PCA event occurred on 29 November 2017 using multi‐instrument observations. During the DMSP observed the 15MLT‐PCA, Cluster, with its footprints at the root of the 15MLT‐PCA, identified two FTEs in the southern hemisphere's lobe region, accompanied by an increase in electron and ion energy from hundreds of eVs to several keVs. AMPERE observed an increase in upward field‐aligned currents associated with the 15MLT‐PCA. SuperDARN observed a single cell convection with an enhancement of sunward plasma flow near the root of 15MLT‐PCA. We suggest that these observations provide the in‐situ observational evidence that the 15MLT‐PCA is generated by a lobe reconnection at the cusp's duskside boundary. 

Abstract Nitric oxide (NO) emission via 5.3 µm wavelength plays dominant role in regulating the thermospheric temperature due to thermostat nature. The response of NO 5.3 mm emission to the negative pressure impulse during November 06–09, 2010 is studied by using Sounding of Atmosphere by Broadband Emission Radiometry (SABER) observations onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite and model simulations. The TIMED/SABER satellite observations demonstrate a significant enhancement in the high latitude region. The Open Geospace General Circulation Model (OpenGGCM), Weimer model simulations and Active Magnetosphere and Planetary Electrodynamics Response Experiment measurements exhibit intensification and equatorward expansion of the field-aligned-currents (FACs) post-negative pressure impulse period due to the expansion of the dayside magnetosphere. The enhanced FACs drive precipitation of low energy particle flux and Joule heating rate affecting whole magnetosphere–ionosphere–thermosphere system. Our study based on electric fields and conductivity derived from the EISCAT Troms$${\o }$$ $ø$radar and TIEGCM simulation suggests that the enhanced Joule heating rate and the particle precipitations prompt the increase in NO cooling emission. 

Abstract During the second recovery phase of the 13–14 March 2022 storm, intense high‐latitude neutral mass density spikes are detected by satellites at ∼500 km in both hemispheres. These density spikes, accurately modeled by the Global Ionospheric Thermosphere Model (GITM), are identified as high‐latitude neutral mass density anomalies (HDAs). The GITM simulation indicates that these HDAs, which extends over the polar region, are important features in high‐latitude neutral density. Furthermore, GITM reveals that these HDAs are manifestations of transpolar traveling atmospheric disturbances triggered on the dawn side. Moreover, GITM also reveals significant interhemispheric asymmetries (IHAs) in the magnitude, propagation speed, and propagation direction of HDAs, which are linked to the IHAs in the distribution and magnitude of Joule heating deposited as well as the thermospheric background conditions. This study presents a dynamic perspective on the IHA of storm‐time high‐latitude neutral density variations that is particularly helpful to the proper interpretation of satellite observations. 

Abstract Specific polar cap auroras, such as 15MLT‐PCA, linked to lobe reconnection due to the influence of the interplanetary magnetic field (IMF) B_ycomponent, were only observed in the summer. Although the variance in ionospheric conductivity between winter and summer has been proposed as a potential explanation for this seasonal dependency, it has also been argued that the differences in lobe reconnection between the winter and summer hemispheres could be the cause. To address this debate, we examined two data periods with similar IMF conditions when the northern hemisphere was in summer and winter, respectively. Using DMSP/SSUSI and AMPERE observations, we detected clear 15MLT‐PCA and associated field‐aligned currents in the summer, but not in the winter. These observations were compared with global MHD simulations from OpenGGCM. Lobe reconnection signatures were identified for both winter and summer in the simulation results. However, a detailed analysis showed that the pattern of lobe reconnection in the winter hemisphere was different from that in the summer. Based on the combined observation and simulation results, we suggest that particular lobe reconnection in summer is critical for generating 15MLT‐PCA, while the winter's reconnection may lead to transient or small‐scale auroral responses that were not easily identified by DMSP/SSUSI observations as a 15MLT‐PCA event. 

Abstract Mid‐latitude auroras are conventionally generated during intense magnetic storms. However, mid‐latitude auroras were observed by naked eyes at Beijing China (39°N, 116°E) unusually during a moderate storm event on 1 December 2023 with the minimum Sym‐H index only −120 nT. This study combines conjugative in‐site and ground‐based observations to analyze the auroras and underlying physical processes. Results indicate that both electron and proton auroras appeared at low latitudes. Electron auroras predominantly arise from low‐energy electron precipitation, but proton auroras may be explained by energetic tens of keV proton precipitation. Pc1/EMIC waves are observed at low latitudes in the ionosphere, potentially accounting for mid‐latitude proton auroras. Downward field‐aligned currents (FACs) are also detected at low latitudes, producing significant magnetic perturbations. This study reveals the underlying ionospheric responses to the mid‐latitude auroras to understand potential reasons for observing aurora at such mid‐latitudes during a moderate storm. 

Abstract Enhancement of currents in Earth's ionosphere adversely impacts systems and technologies, and one example of extreme enhancement is supersubstorms. Despite the name, whether a supersubstorm is a substorm remains an open question, because studies suggest that unlike substorms, supersubstorms sometimes affect all local times including the dayside. The spectacular May 2024 storm contains signatures of two supersubstorms that occurred successively in time with similar magnitude and duration, and we explore the nature of them by examining the morphology of the auroral electrojet, the corresponding disturbances in the magnetosphere, and the solar wind driving conditions. The results show that the two events exhibit distinctly different features. The first event was characterized by a locally intensified electrojet followed by a rapid expansion in latitude and local time. Auroral observations showed poleward expansion of auroras (or aurorae), and geosynchronous observations showed thickening of the plasma sheet, magnetic field dipolarization, and energetic particle injections. The second event was characterized by an instantaneous intensification of the electrojet over broad latitude and local time. Auroras did not expand but brightened simultaneously across the sky. Radar and LEO observations showed enhancement of the ionospheric electric field. Therefore, the first event is a substorm, whereas the second event is enhancement of general magnetospheric convection driven by a solar wind pressure increase. These results illustrate that the so‐called supersubstorms have more than one type of driver, and that internal instability in the magnetotail and external driving of the solar wind are equally important in driving extreme auroral electrojet activity. 

Abstract Two interacting high‐speed solar wind streams (HSSs) and associated stream interaction regions (SIR) caused a moderate geomagnetic storm during 14–20 March 2016. The spatio‐temporal evolution of the total electron content (TEC) during the storm is studied by using Global Navigation Satellite System (GNSS) data. The moderate storm caused significant and long‐lasting changes on TEC within the polar cap (70–90 MLAT), at auroral and sub‐auroral latitudes (60–70 MLAT), and at mid‐latitudes (40–60 MLAT). A 25%–50% depletion in TEC was observed for six days in the day, dusk and dawn sectors in the polar cap region and in the day and dusk sectors at the auroral and sub‐auroral latitudes. Sub‐auroral polarization streams observed by the Defense Meteorological Satellite Program satellite contributed to the sub‐auroral dusk TEC decreases. At mid‐latitudes, TEC depletion was observed in all local time sectors 21 hr after the storm onset. It is suggested that ion‐neutral frictional heating causes the TEC depletions, which is further supported by the observed spatial correlation between TEC depletions and O/N₂decreases at mid‐latitudes observed by TIMED/GUVI. The storm induced a prolonged positive phase at mid‐latitudes lasting 9 hr. In the polar cap, enhancements of TEC up to 200% were caused by polar cap patches. TEC increases were the dominant feature in the night and morning sectors within the auroral oval because of particle precipitation and resulted up to regionally averaged 6 TECU (200%) increases. 

Abstract Geomagnetic disturbances (GMDs) are rapid fluctuations in the strength and direction of the magnetic field near the surface of the Earth which can cause electric currents to be induced in the ground. The geomagnetically induced currents (GICs) can cause damage to pipelines and power grids. A detection algorithm has been developed to identify rapid changes in 10 s averaged magnetometer data. This higher resolution data is important in capturing the most rapid changes associated with extreme GIC events. The algorithm has been used on an array of ground‐based magnetometers from SuperMAG data from 2010 to 2022, creating a new list of global GMDs. Data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) is used to place the observed GMDs in the context of the global pattern of magnetosphere‐ionosphere field‐aligned currents (FACs). A dawn sector population of GMDs is found to lie near the boundary between the region 1 and region 2 FACs, while a pre‐midnight sector population is found to occur poleward of the FAC boundary on region 1 upward FACs. It is also shown that the latitude of the GMDs expands with the FAC boundary and their occurrence peaks around 77° magnetic latitude. 

Abstract Interplanetary (IP) shock is one of the most common phenomena that controls the shape and size of the magnetosphere. It affects the whole magnetosphere‐ionosphere‐thermosphere (MIT) system. We utilized the NO 5.3 m radiative emission, as observed by SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) onboard NASA's TIMED (Thermosphere Ionosphere Mesosphere Energetics Dynamics) satellite, to investigate its response to fast forward shock during 26 January 2017. The high latitude NO emission exhibits a strong enhancement (three times with respect to pre‐event value) during IP shock within 5 hr of onset. We analyzed both the energy dissipation sources and subsequent chemical mechanisms. The Field‐Aligned‐Current observations from Active Magnetosphere and Planetary Response Experiment (AMPERE), EISCAT measurements of Pederson conductivity and the defense Meteorological Satellite Program (DMSP F18) calculated hemispheric power demonstrate a strong intensification. The low energy particle precipitation from DMSP F18 spacecraft shows an early enhancement for energy less than 1 keV. The particle flux of higher energy responds later which remained elevated for longer duration. The thermospheric density and temperature also experience significant variation during IP shock. The NO molecule and temperature displayed an early enhancement. NO density increased by an order of magnitude with respect to the pre‐event value. About 20 increase is noticed in the temperature variation. The atomic oxygen and atomic nitrogen illustrate an early depletion during IP event. The enhanced response of NO cooling to IP shock can be attributed to the combined effects of energy input and subsequent chemical mechanisms.