Anil Raghav

Quasi‐Homologous Flaring (QHF) activity from active regions on the Sun releases intense electromagnetic radiation in recurrent bursts over extended periods. In this letter, we present the effects of recurrent impulsive extreme ultraviolet (EUV) heating on thermospheric neutral density and the resulting atmospheric drag on satellites based on Swarm‐A measurements. Significant density enhancements up to 25%–30% and 50%–75% sustained for a few days, on both day and night sides, were observed at ∼460 km altitudes during QHF from AR4366 (February 2026) and AR3664 (May 2024), respectively. As a result, Swarm‐A orbital decay rate is increased from 79.43 to 96.28 m/day, and from 19.91 to 42.43 m/day during the AR4366 and AR3664 QHF events, respectively. Repeated EUV heating at intervals shorter than the typical thermospheric relaxation time during QHF flares is responsible for sustained density enhancements and therefore needs to be considered one of the important drivers of satellite drag.

Context . Intervals of enhanced fluctuations are relatively rare within the magnetic cloud region of an interplanetary coronal mass ejection (ICME). Aims . We investigated two such intervals inside an ICME observed by the Wind spacecraft on 8–9 June 2000 and characterized their associated wave populations. Methods . A spectral analysis of magnetic and velocity fluctuations was used to characterize Alfvénicity. We examined the power spectral density (PSD), magnetic helicity, and ellipticity to determine wave properties. Minimum variance analysis (MVA) was used to investigate wave polarization, while wavelet transform coherence (WTC) between density and magnetic fluctuations was employed to assess compressibility. In addition, a linear plasma dispersion solver was used to evaluate the growth and damping rates of the observed wave modes. Results . The ion-scale normalized magnetic helicity reveals a left-handed circularly polarized signature in both intervals. Moreover, the second interval is characterized by an additional right-handed population at higher frequencies. All these fluctuations propagate in a quasi-parallel direction to the local magnetic field. Therefore, the left-handed fluctuations are consistent with Alfvén ion-cyclotron (AIC) waves, while the right-handed fluctuations are consistent with fast magnetosonic-whistler (FM/W) waves. Conclusions . The ICME plasma accesses resonance conditions supporting multiple ion-scale wave modes. Although the observed fluctuations exhibit characteristics consistent with AIC-like and FM/W-like modes, the enhanced wave activity near ion scales is unlikely to originate from locally growing instabilities under the observed bulk plasma conditions. Instead, these fluctuations are more plausibly convected from regions outside the local plasma environment.

Stream interaction regions (SIRs) are key large-scale structures in the solar wind that regulate the transport of mass, momentum, and energy across the heliosphere. We examine the thermodynamic behavior of SIR plasmas by estimating the effective polytropic index ( γ ) using upstream measurements from MAVEN near Mars (∼1.5 au). Quasi-static intervals are identified using the Bernoulli integral, and correlation coefficients are applied to ensure a valid polytropic relation between thermal pressure and proton density. Our earlier analysis of 25 SIR events observed by Parker Solar Probe showed a broad distribution of γ , spanning subadiabatic to strongly superadiabatic regimes. Observations near 1 au indicate nearly adiabatic behavior. In contrast, the present analysis of 185 SIR events at Mars reveals a predominantly subadiabatic response. These results show a clear radial evolution of γ : strongly nonadiabatic with superadiabatic dominance close to the Sun, near-adiabatic at 1 au, and subadiabatic at ∼1.5 au. This trend indicates that the effective polytropic index is not constant but reflects the interplay of turbulent heating, solar wind expansion, and evolving particle distributions. The results place new constraints on the thermodynamic treatment of SIRs and highlight the role of nonadiabatic processes in structuring the solar wind.

Corotating interaction regions (CIRs), formed by the interaction of fast and slow solar wind streams, are characterized by strong compression, enhanced turbulence, and elevated proton temperatures. We investigate how temperature anisotropy and kinetic instabilities regulate proton temperatures using 463 Earth-directed CIRs observed by the Wind spacecraft during 1995–2009 at 1 au. The plasma distribution in ( β _{p ∥} , R _p ) space, where R p = T p ⊥ T p ∥ , is largely constrained by the mirror-mode (MM) and oblique firehose (OFH) instabilities. Proton temperatures are enhanced near the MM boundary for β _{p ∥} < 1 and near the OFH boundary for β _{p ∥} > 1. The enhancement near the MM boundary is dominated by perpendicular proton temperature ( T _{p ⊥} ), whereas that near the OFH boundary is dominated by parallel proton temperature ( T _{p ∥} ). Intervals unstable to proton cyclotron modes show temperatures up to 5.2–6.5 times higher than stable plasma, while parallel firehose-unstable intervals exhibit enhancements of ∼2–3 times, indicating distinct preferential heating channels. Temperature enhancements are strongest for low- β _{p ∥} plasma, whereas high- β _{p ∥} intervals exhibit limited heating despite elevated magnetic fluctuations and compressibility. An anticorrelation between proton temperature and collisional age indicates that hotter plasma is less collisional and remains closer to instability thresholds. Magnetic fluctuation power decreases weakly with increasing collisional age, suggesting stronger wave activity in collisionally young plasma. These results demonstrate that proton heating in CIRs is anisotropic and self-regulated by kinetic instabilities, with plasma beta, turbulence, compressibility, and collisionality jointly governing CIR thermodynamics.

The offset between Earth's magnetic and rotational axes introduces a diurnal dependence in the high‐latitude EUV exposure of the northern hemisphere (NH) and southern hemisphere (SH). This variation raises the question: Does the Universal Time (UT) of geomagnetic storm onset impact its geospace consequences? To address this question, we used the Multiscale Atmosphere‐Geospace Environment (MAGE) model to simulate the 10 October 2024, geomagnetic storm—the year's second strongest (SYM‐H minimum of −346 nT). Since the storm occurred near equinox, we did not expect, but found, significant interhemispheric asymmetries in magnetosphere‐ionosphere‐thermosphere (M‐I‐T) coupling parameters such as the cross‐polar cap potential, hemispherically integrated field‐aligned current, and hemispheric power of electron precipitation. Controlled simulations show that interplanetary magnetic field (IMF) and solar wind have negligible effects on these asymmetries, whereas the EUV variation arising from the diurnal dipole tilt produces noticeable interhemispheric differences. Coincidentally, during this storm, IMF B _z turned southward when the SH was tilted toward the Sun, and it maintained this orientation for 12 hr. A controlled simulation with storm onset shifted by 12 hr exhibits a substantial reduction in interhemispheric asymmetry. Differences in integrated Joule heating power and the SML/SMU indices also occurred with the shifted onset, underscoring the importance of UT in stormtime magnetosphere‐ionosphere coupling.

Interactions of solar wind dynamic pressure (SWDP) discontinuities with Earth's magnetosphere cause geomagnetic Sudden Commencements (SCs). Typically, positive/negative SCs occur at low latitudes due to enhancements/reductions in SWDP. However, anomalous dawn‐side SCs of opposite polarity were recently reported during the 10 May 2024, superstorm [Nilam et al., 2025, https://doi.org/10.1029/2025GL117032 ]. This study examines SC responses to positive and negative pressure discontinuities on the May 10 and October 10 storms under similar storm phases and local times. Both events consistently revealed anomalous dawn‐side low latitude SCs opposite to those at other longitudes. We suggest the main impulse of the Disturbance Polar (DP) field extending equatorward as the most likely source. Under highly compressed background magnetosphere conditions, field‐aligned currents associated with DP fields can shift to lower L‐shells, producing such anomalous SCs at dawn‐side low latitudes. These findings provide new insights into dawn‐side magnetosphere–ionosphere coupling during intense storms.

We investigated a comprehensive sample of 474 interplanetary coronal mass ejection (ICME) events observed by the Wind spacecraft to examine the Alfvénic characteristics within their magnetic cloud regions. The analysis focuses on identifying inward and outward Alfvénic fluctuations of the magnetic and velocity field components. Our results reveal that approximately 74.68% of ICME magnetic cloud regions are non-Alfvénic, while 25.32% exhibit clear Alfvénic signatures, predominantly characterized by outward fluctuations. Furthermore, the temperature distribution shows a substantial enhancement in Alfvénic regions compared to non-Alfvénic ones, with proton temperatures elevated up to ∼33 eV. These findings suggest that Alfvénic fluctuations may play a significant role in the localized plasma heating within ICME structures and provide potential constraints on their broader contribution to space plasma environments.

A planar magnetic structure refers to a compressed and ordered region in interplanetary space where magnetic field vectors are predominantly confined to a plane. Due to enhanced plasma parameters associated with them, they play a significant role in space weather dynamics. To examine their geoeffectiveness, we investigate the impact of planar and non-planar sheaths of Interplanetary Coronal Mass Ejections on the outer radiation belt electrons, using in-situ particle data from NASA’s Van Allen Probes mission. A superposed epoch analysis on electron flux data revealed a significant reduction in electron flux during planar sheath passages as compared to non-planar sheaths, possibly driven by stronger magnetospheric compression resulting in intensifying magnetopause shadowing. Notably, electron flux levels exhibit a significant post-passage increase, surpassing pre-passage levels three days later. For the first time, the efforts taken to highlight the interplay between planar sheath-driven magnetospheric compression and radiation belt dynamics offer new insights for improving space weather forecasting.