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We incorporate field line curvature (FLC) scattering into a kinetic ring current model two‐way coupled with a global magnetospheric MHD model to investigate its role in SYM‐H prediction and global magnetospheric dynamics. FLC scattering reduces plasma pressure in regions where the ion gyroradius is comparable to the local FLC radius. This pressure reduction weakens magnetic tension, resulting in a more dipolar magnetic field configuration that, in turn, suppresses further FLC scattering. This self‐regulating feedback loop prevents excessive FLC scattering and mitigates the overestimation of ring current decay often found in models using empirical magnetic fields. We demonstrate that incorporating FLC scattering improves agreement with observed SYM‐H throughout the entire storm period. These findings highlight the importance of self‐consistent coupling between plasma dynamics and magnetic field configuration, offering enhanced predictive capability for space weather modeling.

Energetic particle deep penetration into low L‐shells (L < 4) impacts the dynamics of the radiation belts and ring current. Previous studies reported that electrons penetrate more frequently, deeply, and faster than protons of similar energies, but underlying mechanisms are unclear. In this study, we compare heavy‐ion behavior with electrons and protons to further identify the underlying mechanisms. Using Van Allen Probes data, we show that electron deep penetration occurs most frequently and deeply, followed by O+ ions, then He+ ions, and finally protons. Most particle deep penetrations occur within several hours. Superposed epoch analysis shows that prior to deep penetration, electrons have the steepest phase space density radial gradients, followed by heavy ions and then protons for the same μ and K. Our study suggests that a combination of two or more mechanisms, such as convection electric field and plasma wave‐induced scattering, may be needed to fully explain particle deep penetration.

This study analyzes electromagnetic power density E⋅j$\\mathbf{E}\\cdot \\mathbf{j}$in the ring current region using MMS data and multi‐point analysis method. For the first time, we statistically examine the large‐scale distribution characteristics of E⋅j$\\left\\langle \\mathbf{E}\\cdot \\mathbf{j}\\right\\rangle $(〈〉$\\langle \\rangle $ represents statistical results) within the magnetic equatorial plane, revealing a stably particles acceleration region E⋅j>0$\\left\\langle \\mathbf{E}\\cdot \\mathbf{j}\\right\\rangle > 0$on the dawn side and a dynamo region E⋅j<0$\\left\\langle \\mathbf{E}\\cdot \\mathbf{j}\\right\\rangle < 0$on the dusk side. Meanwhile, the corresponding distribution of current density and net charge density are also calculated. In the predawn sector near the acceleration region, a significant accumulation of net negative charges was observed. The dusk‐side dynamo region coincides with the net positive charge accumulation. The statistical analysis of electromagnetic power and net charge accumulation implies that the electrostatic field may play an important role in the energy variation of drifting particles, providing new insights into the evolution and dynamics of the ring current.

During the May 2024 storm, the minimum Dst index was approximately −412 nT, marking the largest geomagnetic storm of the past decade. This event caused the inner edge of the ring current to penetrate deeply into the inner magnetosphere during the main phase of the storm. We present observations of high‐frequency electromagnetic ion cyclotron (HF EMIC) wave activity during this intense geomagnetic storm using data from the Arase satellite. Arase observations showed that HF EMIC waves with frequencies of 5–36 Hz at L ∼ 2, occurred mainly during the main and early‐recovery phases. The minimum resonance energy of energetic protons and relativistic electrons associated with HF EMIC waves suggests their potential to cause the loss of relativistic electrons in the low L‐shell region. Our observations provide new insights into the generation of EMIC waves and the dynamics of energetic particles at low L‐shells in the inner magnetosphere. Plain Language Summary Intense geomagnetic storms can transport energetic particles into the deep inner magnetosphere, generating various magnetospheric plasma waves in regions close to the Earth. Electromagnetic ion cyclotron (EMIC) waves play an important role in controlling the dynamics of charged particles in the inner magnetosphere. In particular, EMIC waves can cause the loss of relativistic electrons and ring current ions. In this study, we present observations of high‐frequency (HF) EMIC waves during the May 2024 storm using data from the Arase satellite. Remarkably, these HF EMIC waves were detected in the deep inner magnetosphere during the main and early‐recovery phases of the storm and exhibited unusually high frequencies. The minimum resonance energy of HF EMIC waves suggests their capability to cause the loss of energetic protons and relativistic electrons in the low L‐shell region. The present results provide new insights into the generation processes and wave‐particle interactions of HF EMIC waves in the deep inner magnetosphere. Key Points High‐frequency (HF) EMIC waves were observed at regions close to Earth during a severe geomagnetic storm Most of HF EMIC waves occurred during the main and early‐recovery phase of the storm with the highest wave frequency HF EMIC waves at low L‐shells have the potential to cause loss of relativistic electrons in the slot regions of the radiation belt

Magnetopause shadowing (MPS) effect could drive a concurrent dropout of radiation belt electrons and ring current protons. However, its relative role in the dropout of both plasma populations has not been well quantified. In this work, we study the simultaneous dropout of MeV electrons and 100s keV protons during an intense geomagnetic storm in May 2017. A radial diffusion model with an event‐specific last closed drift shell is used to simulate the MPS loss of both populations. The model well captures the fast shadowing loss of both populations at L* > 4.6, while the loss at L* < 4.6, possibly due to the electromagnetic ion cyclotron wave scattering, is not captured. The observed butterfly pitch angle distributions of electron fluxes in the initial loss phase are well reproduced by the model. The initial proton losses at low pitch angles are underestimated, potentially also contributed by other mechanisms such as field line curvature scattering. Plain Language Summary Magnetopause shadowing, due to the solar wind compression of the magnetopause combined with outward radial diffusion driven by ultra low frequency waves, is known to be one of the major loss mechanisms for both radiation belt electrons and ring current protons. However, the role of MPS in driving the simultaneous dropout of both populations has not been well quantified. In this study, for the first time, we quantitatively model the fast shadowing loss of radiation belt electrons and ring current protons during a geomagnetic storm event using a radial diffusion model with event‐specific inputs. The results indicate that MPS can efficiently capture the concurrent fast depletion of both populations at high L*. Key Points A radial diffusion model with event‐specific last closed drift shell is used to simulate the concurrent dropout of electrons and protons due to magnetopause shadowing The model captures the fast shadowing loss of both populations at high L* but not the loss at low L* possibly from electromagnetic ion cyclotron wave scattering The model reproduces the butterfly PAD of electrons in the initial loss phase but underestimates the loss of protons at low pitch angles

Using the Arase and Van Allen Probes satellite observations, we investigate the nonlinear electromagnetic ion cyclotron (EMIC) rising‐tone (RT) emissions with an increase of the solar wind dynamic pressure in the dayside magnetosphere. We find that EMIC RT emissions are accompanied by the extended dayside uniform zone (DUZ) over |MLAT| < 25° due to the dayside magnetospheric compression by an increase in Pdyn. Using the observed plasma and magnetic field data, we modeled the threshold amplitude for the nonlinear EMIC waves and compared it with the observation. The small gradient of the ambient magnetic field strongly contributes to the reduction in the threshold amplitude of nonlinear wave growth compared to other parameters. When the threshold amplitude falls to comparable level of pre‐existing EMIC waves, EMIC RT emissions are immediately triggered, suggesting direct evidence that the DUZ is the preferred condition to cause the nonlinear EMIC RT emission in the dayside magnetosphere. Plain Language Summary Electromagnetic ion cyclotron (EMIC) waves play an important role in controlling the dynamics of charged particles in the inner magnetosphere. Especially, nonlinear EMIC rising‐tone (RT) emissions can cause the rapid loss of relativistic electrons and ring current ions. Here, we present direct evidence demonstrating that the distortion of the dayside magnetic field causes nonlinear EMIC RT emission in response to the intensification of the solar wind dynamic pressure. Remarkably, these nonlinear EMIC waves are generated through a reduction in the threshold wave amplitudes by the distortion of the magnetic fields, even in the absence of any significant change in the pre‐existing EMIC wave amplitude. The present result provides new insights into a triggering process of nonlinear plasma waves in the magnetosphere. Key Points Electromagnetic ion cyclotron (EMIC) waves with rising‐tone (RT) elements were observed in the dayside magnetosphere during an increase in the solar wind dynamic pressure Increasing solar wind dynamic pressure extends the dayside uniform zone of the magnetic field to higher magnetic latitudes The uniform zone leads to the reduction of the nonlinear threshold wave amplitude, which triggers nonlinear EMIC RT emissions

One of the definitions of space weather describes it as the time-varying space environment that may be hazardous to technological systems in space and/or on the ground and/or endanger human health or life. The ring current has its contributions to space weather effects, both in terms of particles, ions and electrons, which constitute it, and magnetic and electric fields produced and modified by it at the ground and in space. We address the main aspects of the space weather effects from the ring current starting with brief review of ring current discovery and physical processes and the Dst-index and predictions of the ring current and storm occurrence based on it. Special attention is paid to the effects on satellites produced by the ring current electrons. The ring current is responsible for several processes in the other inner magnetosphere populations, such as the plasmasphere and radiation belts which is also described. Finally, we discuss the ring current influence on the ionosphere and the generation of geomagnetically induced currents (GIC).

One of the most intense geomagnetic storms of recent times occurred on 10–11 May 2024. With a peak negative excursion of Sym‐H below −500 nT, this storm is the second largest of the space era. Solar wind energy transferred through radiation and mass coupling affected the entire Geospace. Our study revealed that the dayside magnetopause was compressed below the geostationary orbit (6.6 RE) for continuously ∼6 hr due to strong Solar Wind Dynamic Pressure (SWDP). Tremendous compression pushed the bow‐shock also to below the geostationary orbit for a few minutes. Magnetohydrodynamic models suggest that the magnetopause location could be as low as 3.3RE. We show that a unique combination of high SWDP (≥15 nPa) with an intense eastward interplanetary electric field (IEFY ≥ 2.5 mV/m) within a super‐dense Interplanetary Coronal Mass Ejection lasted for 409 min–is the key factor that led to the strong ring current at much closer to the Earth causing such an intense storm. Severe electrodynamic disturbances led to a strong positive ionospheric storm with more than 100% increase in dayside ionospheric Total Electron Content (TEC), affecting GPS positioning/navigation. Further, an HF radio blackout was found to occur in the 2–12 MHz frequency band due to strong D‐ and E‐region ionization resulting from a solar flare prior to this storm.

Based on the predictions of global 3D hybrid simulations, we present a new transport/acceleration path for escaped O+ ions in the upstream solar wind region resulting from the impact of a particular IMF tangential discontinuity (TD) with negative (positive) IMF Bz on the discontinuity's anti‐sunward (sunward) side. For O+ ions escaping to the duskside magnetosheath and with gyro‐radii larger than the TD thickness, when they encounter the TD, they can first go sunward into the upstream solar wind. They then gyrate clockwise to the pre‐noon side and get accelerated within the solar wind region and circulate back to the dawnside magnetosphere. These ions may be accelerated to well within the ring current energy range depending on the solar wind electric field strength. This new transport/acceleration path can bring some of the escaped ions into the inner magnetosphere, thus providing a new mechanism for generating an O+ ring current population. Plain Language Summary O+ ions in the magnetosphere only come from the Earth's ionosphere. For O+ ions escaping the magnetosphere, scientists have been treating them as being lost. Using simulations that can describe the O+ ion's kinetic dynamics, we find that, due to the impact of a particular solar wind structure, some escaped O+ ions can circulate back to the magnetosphere via a transport path in the upstream solar wind region and some of them can even enter the inner magnetosphere. Additionally, they are also energized by solar wind electric field and thus can contribute to the ring current population. Therefore, this study shows a new journey of escaped O+ ions. Key Points First global 3D hybrid simulations to investigate the fate of O+ ions after escaping the dayside magnetosphere New transport/acceleration path for escaped O+ ions in upstream solar wind region after impact of an IMF tangential discontinuity New transport/acceleration path brings some of escaped O+ ions back to the inner magnetosphere, contributing to O+ ring current pressure

The properties of organic molecules can be influenced by magnetic fields, and these magnetic field effects are diverse. They range from inducing nuclear Zeeman splitting for structural determination in NMR spectroscopy to polaron Zeeman splitting organic spintronics and organic magnetoresistance. A pervasive magnetic field effect on an aromatic molecule is the aromatic ring current, which can be thought of as an induction of a circular current of π-electrons upon the application of a magnetic field perpendicular to the π-system of the molecule. While in NMR spectroscopy the effects of ring currents on the chemical shifts of nearby protons are relatively well understood, and even predictable, the consequences of these modified electronic states on the spectroscopy of molecules has remained unknown. In this work, we find that photophysical properties of model phthalocyanine compounds and their aggregates display clear magnetic field dependences up to 25 T, with the aggregates showing more drastic magnetic field sensitivities depending on the intermolecular interactions with the amplification of ring currents in stacked aggregates. These observations are consistent with ring currents measured in NMR spectroscopy and simulated in time-dependent density functional theory calculations of magnetic field-dependent phthalocyanine monomer and dimer absorption spectra. We propose that ring currents in organic semiconductors, which commonly comprise aromatic moieties, may present new opportunities for the understanding and exploitation of combined optical, electronic, and magnetic properties.