Using recent in-situ Arase satellite observations during Solar Cycle 25, between March 2017 and December 2023, we perform a comprehensive statistical study of the inner magnetospheric ion (H + , He + , and O + ) distributions as a function of geomagnetic conditions and solar activity. The analysis employed the intercalibration method to combine two ion datasets (LEPi and MEPi) onboard the Arase satellite, covering an energy range of 0.03–187 keV/q. In this study, we found distinct ion populations on energy: (1) plasmaspheric population (E < 30 eV) at L < 5; (2) suprathermal population at energies of several tens of eV to several keV at L > 5 for H + and L < 5 for He + and O + ; (3) ring current population (E = 1 keV – several tens of keV) with the ion nose structures; and (4) high-energy ring current particles (E > 30 keV) with symmetric MLT distributions. Ion fluxes and partial number densities exhibited significant enhancements with increasing Kp levels and solar activity. The O + fraction increased dramatically during periods of elevated Kp and enhanced solar activity, becoming comparable to the H + contribution within the plasmasphere. From our observations, we suggest that ion behavior in the inner magnetosphere is strongly influenced by geomagnetic disturbance levels and solar activity. This ion behavior can provide the favorable conditions for understanding the major driver of various magnetospheric plasma waves.

Using Arase satellite observations, this study provides a comprehensive statistical analysis of ions (H+, He+, O+) and electron contributions to the total ring current pressure during storms with two different drivers. The results demonstrate the effect of different solar wind drivers on the composition, energy distribution, and spatial characteristics of the ring current. Using 32 CIR- and 30 Interplanetary Coronal Mass Ejection (ICME)-driven storms, we characterize the ring current pressure evolution during the prestorm, main, early-recovery, and late-recovery storm phases as a function of magnetic local time and L-shell. In CIR-driven storms, H+ ions are the dominant (similar to 70%) contributor to the total ring current pressure during main/early recovery phases and increasing to similar to 80% during late recovery. In contrast, the O+ pressure (E = 20-50 keV) response is significantly stronger in ICME-driven storms contributing similar to 40% to the overall pressure during the main/early recovery phases and even dominate (similar to 53%) in certain MLT sectors. Additionally, ICME-driven storms tend to have peak pressure at lower L-shells (L approximate to 3-4), while CIR-driven storms show pressure peaks at slightly higher L-shells (L approximate to 4-5). Interestingly, electron pressure also plays a notable role in specific MLT sectors, contributing similar to 18% (03-09 MLT) during the main phase of CIR-driven storms and similar to 11% (21-03 MLT) during ICME-driven storms. The results highlight that the storm time electron pressure plays a crucial role in the ring current buildup. Another noteworthy feature of this study is that Arase's fine-energy resolution and broad coverage enable a detailed investigation of energy-dependent ring current dynamics.

In the polar middle and upper atmosphere, nitric oxide (NO) is produced in large amounts by both solar EUV and X-ray radiation and energetic particle precipitation, and its chemical loss is driven by photodissociation. As a result, polar atmospheric NO has a clear seasonal variability and a solar cycle dependency which have been measured by satellite-based instruments. On shorter timescales, NO response to magnetospheric electron precipitation has been shown to take place on a day-to-day basis. Despite recent studies using observations and simulations, it remains challenging to understand NO daily distribution in the mesosphere-lower thermosphere during geomagnetic storms and to separate contributions of electron forcing and atmospheric chemistry and dynamics. This is due to the uncertainties existing in the available electron flux observations, differences in representation of NO chemistry in models, and differences between NO observations from satellite instruments. In this paper, we use mesospheric-lower-thermospheric NO column density data measured with a millimeter-wave spectroscopic radiometer at the Syowa station in Antarctica. In the period 2012-2017, we study both the long-term and short-term variability of NO. Comparisons are made with results from the Whole Atmosphere Community Climate Model to understand the shortcomings of current electron forcing in models and how the representation of the NO variability can be improved in simulations. We find that, qualitatively, the simulated year-to-year and day-to-day variability of NO is in agreement with the observations. On the other hand, there is up to a factor of 2 underestimation of the NO column density in wintertime. Also, the model captures only 27 % of the range of observed daily NO values. The observed day-to-day variability has a good correlation with three different geomagnetic indices, indicating the importance of electron forcing in atmospheric NO production. Using electron flux measurements from the Arase satellite, we demonstrate their potential in atmospheric research. Our results call for improved representation of electron forcing in simulations to capture the observed day-to-day variability.

Recent simulation studies using the RAM-SCB model showed that proton precipitation contributes significantly to the total energy flux deposited into the subauroral ionosphere thereby affecting the magnetosphere-ionosphere coupling. In this study, we use the BATS-R-US + RAM-SCB model to understand the evolution of ElectroMagnetic Ion Cyclotron (EMIC) waves in the inner magnetosphere, their correspondence to the proton precipitation into the subauroral ionosphere, and to assess the performance of the model in reproducing the EMIC wave-particle interactions. During the 27 May 2017 storm, Arase and RBSP-A satellites observed typical signatures of EMIC waves in the inner magnetosphere. Within this interval, Defense Meteorological Satellite Program (DMSP) and National Oceanic and Atmospheric Administration (NOAA)/MetOp satellites observed significant proton precipitation in the dusk-midnight sector. Simulation results show that H- and He-band EMIC waves are excited within regions of strong temperature anisotropy near the plasmapause. The simulated growth rates of EMIC waves show a similar trend to that of the EMIC wave power observed by the Arase and RBSP-A satellites, suggesting that the model can reproduce the EMIC wave activity qualitatively. The simulated H-band waves in the dusk sector are stronger than He-band waves possibly due to the presence of excess protons in the boundary conditions obtained from the BATS-R-US code. The precipitating proton fluxes reproduced by the simulation with EMIC waves are found to agree reasonably well with the DMSP and NOAA/MetOp satellite observations. It is suggested that EMIC wave scattering of ring current ions can account for proton precipitation observed by the DMSP and MetOp satellites during the 27 May 2017 storm. During geomagnetic storms, plasma waves are generated in the Earth's magnetosphere. Among these waves, ElectroMagnetic Ion Cyclotron (EMIC) waves can scatter protons from the ring current, causing them to precipitate into the subauroral ionosphere. Such precipitation not only affects the midlatitude ionosphere but also impacts the dynamics of the magnetosphere. Understanding the origin of magnetospheric plasma waves and how they interact with the magnetospheric populations, along with their subsequent impact on the ionosphere, is crucial for predicting space weather accurately. In our study, we combined ground and satellite observations with simulations using the BATS-R-US + RAM-SCB to investigate EMIC wave-particle interactions in the inner magnetosphere and the resulting proton precipitation during the 27 May 2017 storm. We found that EMIC waves were excited in the dusk-midnight sector during the storm's main phase, within the regions of strong temperature anisotropy. The simulations reproduced the proton precipitation observed in the dusk-midnight sector by the Defense Meteorological Satellite Program /National Oceanic and Atmospheric Administration MetOP satellites fairly well. The model qualitatively captured the growth of the EMIC waves during the storm and showed that the EMIC waves, by scattering the ring current, were responsible for the proton precipitation into the dusk-midnight sector during the storm. ElectroMagnetic Ion Cyclotron (EMIC) wave activity and proton precipitation were observed simultaneously in the dusk-midnight sector during the 27 May 2017 storm The BATS-R-US + RAM-SCB model can capture the EMIC wave growth during the storm qualitatively The EMIC wave scattering of ring current ions can account for the proton precipitation in the dusk-midnight sector during the storm

&lt;p&gt;Geomagnetic storms are the main component of space weather. Enhancement of the ring current is a typical feature of the geomagnetic storm and a global decrease in the &lt;em&gt;H&lt;/em&gt; component of the geomagnetic field is observed during the main phase of the geomagnetic storm. &amp;#160;The ring current represents a diamagnetic current driven by the plasma pressure in the inner magnetosphere. The plasma pressure is mainly dominated by protons in an energy range of a few to a few hundred keVs during quiet times. The O&lt;sup&gt;+&lt;/sup&gt; contribution is also important, and sometimes dominates more than H&lt;sup&gt;+&lt;/sup&gt; during intense geomagnetic storms. However, electron contribution to the ring current is not studied well. Recently, we showed that the electron pressure also contributes to the depression of ground magnetic field during the November 2017 CIR-driven storm by comparing &lt;strong&gt;R&lt;/strong&gt;ing current&amp;#160;&lt;strong&gt;A&lt;/strong&gt;tmosphere interactions&amp;#160;&lt;strong&gt;M&lt;/strong&gt;odel with&amp;#160;&lt;strong&gt;S&lt;/strong&gt;elf&amp;#160;&lt;strong&gt;C&lt;/strong&gt;onsistent magnetic field (RAM-SCB) simulation, Arase in-situ plasma/particle data, and ground-based magnetometer data [Kumar et al., 2021]. Arase satellite observed 26 geomagnetic storms driven by Corotating Interaction Regions (CIR) during 2017-2021. In this study, we examine statistically the spatial and temporal distribution of ions (H&lt;sup&gt;+&lt;/sup&gt;, He&lt;sup&gt;+&lt;/sup&gt;, O&lt;sup&gt;+&lt;/sup&gt;) and electrons pressure as a function of magnetic local time, L shell and wide range of energies during prestorm, main phase, early recovery and late recovery phase for 26 CIR storms using in situ plasma/particle data obtained by Arase. The results indicate that the electrons (20-50 keV) contribution to the ring current pressure is non-negligible.&lt;/p&gt;

Abstract The total energy transfer from the solar wind to the magnetosphere is governed by the reconnection rate at the magnetosphere edges as the Z‐component of interplanetary magnetic field (IMF Bz) turns southward. The geomagnetic storm on 21–22 January 2005 is considered to be anomalous as the SYM‐H index that signifies the strength of ring current, decreases and had a sustained trough value of −101 nT lasting more than 6 hr under northward IMF Bz conditions. In this work, the standard WINDMI model is utilized to estimate the growth and decay of magnetospheric currents by using several solar wind‐magnetosphere coupling functions. However, it is found that the WINDMI model driven by any of these coupling functions is not fully able to explain the decrease of SYM‐H under northward IMF Bz. A dense plasma sheet along with signatures of a highly stretched magnetosphere was observed during this storm. The SYM‐H variations during the entire duration of the storm were only reproduced after modifying the WINDMI model to account for the effects of the dense plasma sheet. The limitations of directly driven models relying purely on the solar wind parameters and not accounting for the state of the magnetosphere are highlighted by this work.

Using Arase observations of the inner magnetosphere during 26 CIR‐driven geomagnetic storms with minimum Sym‐H between −33 and −86 nT, we investigated ring current pressure development of ions (H + , He + , O + ) and electron during prestorm, main, early recovery and late recovery phases as a function of L‐shell and magnetic local time. It is found that during the main and early recovery phase of the storms the ion pressure is asymmetric in the inner magnetosphere, leading to a strong partial ring current. The ion pressure becomes symmetric during the late recovery phase. H + ions with energies of ∼20–50 keV and ∼50–100 keV contribute more to the ring current pressure during the main phase and early/late recovery phase, respectively. O + ions with energies of ∼10–20 keV contribute significantly during main and early recovery phase. These are consistent with previous studies. The electron pressure was found to be asymmetric during the main, early recovery and late recovery phase. The electron pressure peaks from midnight to the dawn sector. Electrons with energy of <50 keV contribute to the ring current pressure during the main and early recovery phase of the storms. Overall, the electron contribution to the total ring current is found to be ∼11% during the main and early recovery phases. However, the electron contribution is found to be significant (∼22%) in the 03–09 MLT sector during the main and early recovery phase. The results indicate an important role of electrons in the ring current build up.

Geomagnetic storms are the main component of space weather and are driven by coronal mass ejections (CMEs) or corotating interaction regions (CIRs). During the main phase of geomagnetic storms, the ring current enhances and a global decrease in the H component of the geomagnetic field is observed. The storm time distribution of ring current ions and electrons in the inner magnetosphere depend strongly on their transport in evolutions of electric and magnetic fields along with acceleration and loss. Recently, we showed that the electron pressure contributes to the depression of ground magnetic field during the storm time by comparing Ring current Atmosphere interactions Model with Self Consistent magnetic field (RAM-SCB) simulation, Arase in-situ plasma/particle data, and ground-based magnetometer data [Kumar et al., 2021]. In this study, we compare the contribution of electron pressure to the ring current during selected CIR and CME geomagnetic storms using ground observations and the self-consistent inner magnetosphere model: RAM-SCB. The previous results show that the ions are the major contributor (~ 90 %) to the total ring current and the electron contributes ~10 % to the ring current pressure in the post-midnight to dawn sector where electrons flux is higher compared to ions flux. As CIR and CME storms have different origins, we will discuss expected differences in the contribution of electron pressure to the ring current.

Large-amplitude geomagnetically induced currents (GICs) are the natural consequences of the solar–terrestrial connection triggered by solar eruptions. The threat of severe damage of power grids due to the GICs is a major concern, in particular, at high latitudes, but is not well understood as for low-latitude power grids. The purpose of this study is to evaluate the lower limit of the GICs that could flow in the Japanese power grid against a Carrington-class severe magnetic storm. On the basis of the geomagnetic disturbances (GMDs) observed at Colaba, India, during the Carrington event in 1859, we calculated the geoelectric disturbances (GEDs) by a convolution theory, and calculated GICs flowing through transformers at 3 substations in the Japanese extra-high-voltage (500-kV) power grid by a linear combination of the GEDs. The estimated GEDs could reach ~ 2.5 V/km at Kakioka, and the GICs could reach, at least, 89 ± 30 A near the storm maximum. These values are several times larger than those estimated for the 13–14 March 1989 storm (in which power blackout occurred in Canada), and the 29–31 October 2003 storm (in which power blackout occurred in Sweden). The GICs estimated here are the lower limits, and there is a probability of stronger GICs at other substations. The method introduced here will be immediately applicable for benchmark evaluation of low-latitude GICs against the Carrington-class magnetic storms if one assumes electrical parameters, such as resistance of transmission lines, with sufficient accuracy.

Understanding the physical processes that control the dynamics of energetic particles in the inner magnetosphere is important for both space‐borne and ground‐based assets essential to the modern society. The storm time distribution of ring current particles in the inner magnetosphere depends strongly on their transport in the evolving electric and magnetic fields along with particle acceleration and loss. In this study, we investigated the ring current particle variations using observations and simulations. We compared the ion (H + , He + , and O + ) and electron flux and plasma pressure variations from Arase observations with the self‐consistent inner magnetosphere model: Ring current Atmosphere interactions Model with Self Consistent magnetic field (RAM‐SCB) during the 7–8 November 2017 geomagnetic storm. We investigated the contribution of the different species (ions and electrons) to the magnetic field deformation observed at ground magnetic stations (09°–45° MLat) using RAM‐SCB simulations. The results show that the ions are the major contributor with ∼88% and electrons contribute ∼12% to the total ring current pressure. It is also found that the electron contribution is non‐negligible (∼18%) to the ring current in dawn‐side during the main phase of the storm. Thus, the electron contribution to the storm time ring current is important and should not be neglected.

In this paper, we have evaluated the role of interplanetary magnetic field (IMF) By on the asymmetry of the ring current during the main phase of geomagnetic storms. The mean H variations have been calculated using 31 ground magnetic stations over magnetic latitudes of 09–45° following the methodology of Li et al. (2011, https://doi.org/10.1029/2011JA016886). Further, the magnetic local time (MLT) variations in the H component at these stations w.r.t. the mean H were investigated for three cases of geomagnetic storms with varying southward IMF Bz and IMF By conditions. Significant ring current asymmetries were observed during the main phase of geomagnetic storms. The primary role of IMF Bz on the asymmetry of the ring current is observed from these cases. More importantly, the investigation brings out for the first time, the additional role of IMF By in influencing the MLT distribution of ring current observed at ground magnetic stations. Under southward IMF Bz conditions, it is shown based on SuperDARN and AMPERE data that IMF By can alter the MLT distribution of ring current under suitable conditions. The timescales of IMF By also play very important role in determining the asymmetry in the ring current. Under steady convection state, IMF By can rotate the convection cells based on its polarity, which in turn can change the MLT distribution of ring current observed by low‐latitude ground stations. This investigation, thus, brings out the important role of IMF By on the asymmetric MLT distribution of ring current under southward IMF Bz.

During the geomagnetic storm periods, the convection electric field penetrates promptly to equatorial latitudes, causing an eastward electric field in the dayside ionosphere. Occasionally, the polarity of the dayside ionospheric electric field is inverted when the Interplanetary Magnetic Field (IMF) turns northward. In this paper, interesting observations of the strong westward electric field in the day side equatorial latitudes are presented, as evidenced by strong Counter Electrojet (CEJ) at Indian and Japanese sectors under the steady southward IMF Bz. The westward electric field perturbations are quite large with CEJ amplitude of ∼−120 nT over the Indian sector (14–15 December 2006) and ∼−220 nT for Japanese sector (7–8 November 2004). The plausible mechanisms for the observed overshielding electric fields under steady southward IMF Bz have been investigated in light of the possible role of substorm activity. The clear signatures of substorm were observed at geosynchronous particle flux measurements from LANL (Los Alamos National Laboratory) satellite and associated with sudden decrease in AL index. The observed variations of asymmetric ring current shows the enhancement of Partial Ring Current (PRC) at the dusk sector further supporting the substorm onset during that period which will probably enhance the overshielding due to increased Region 2 Field-Aligned Currents (R2 FACs). The observations of such significantly large amplitudes of CEJ associated with the substorm related overshielding events are sparse and the results bring out the important role of substorm onset and the development of PRC in accordance with the R2 FACs during intense magnetic storms which alter the day time equatorial electric field perturbations.

The source region and propagation mechanism of low latitude whistlers (Geomag. lat. <30 degrees) have puzzled scientific community for last many decades. In view of recent reports, there is consensus on the source region of low latitude whistlers in the vicinity of the conjugate point. But the plausible conditions of ionospheric medium through which they travel are still uncertain. In addition to that, the whistlers in daytime are never observed at geomagnetic latitudes less than 20 degrees. Here, for the first time, we present a rare observations of whistlers during sunlit hours from a very low-latitude station Allahabad (Geomag. Lat: 16.79 degrees N, L = 1.08) in India on 04 February 2011. More than 90 whistlers are recorded during 1200-1300 UT during which the whole propagation path from lightning source region to whistler observation site is under sunlit. The favorable factors that facilitated the whistlers prior to the sunset are investigated in terms of source lightning characteristics, geomagnetic and background ionospheric medium conditions. The whistler activity period was found to be geomagnetically quiet. However, a significant suppression in ionospheric total electron content (TEC) compared to its quiet day average is found. This shows that background ionospheric conditions may play a key role in low latitude whistler propagation. This study reveals that whistlers can occur under sunlit hours at latitudes as low as L = 1.08 when the source lightning and ionospheric medium characteristics are optimally favorable. (C) 2017 COSPAR. Published by Elsevier Ltd. All rights reserved.

The Carrington storm (1859 September 1/2) is one of the largest magnetic storms ever observed, and it caused global auroral displays in low-latitude areas, together with a series of multiple magnetic storms from 1859 August 28 to September 4. In this study, we revisit contemporary auroral observation records to extract information on their elevation angle, color, and direction to investigate this stormy interval in detail. We first examine the equatorward boundary of the “auroral emission with multiple colors” based on descriptions of elevation angle and color. We find that their locations were 36.°5 ILAT on August 28/29 and 32.°7 ILAT on September 1/2, suggesting that trapped electrons moved to, at least, L ∼ 1.55 and L ∼ 1.41, respectively. The equatorward boundary of “purely red emission” was likely located at 30.°8 ILAT on September 1/2. If the “purely red emission” was a stable auroral red arc, it would suggest that trapped protons moved to, at least, L ∼ 1.36. This reconstruction with observed auroral emission regions provides conservative estimations of magnetic storm intensities. We compare the auroral records with magnetic observations. We confirm that multiple magnetic storms occurred during this stormy interval, and that the equatorward expansion of the auroral oval is consistent with the timing of magnetic disturbances. It is possible that the August 28/29 interplanetary coronal mass ejections (ICMEs) cleared out the interplanetary medium, making the ICME for the Carrington storm on September 1/2 more geoeffective.