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The Earth’s magnetosphere responds to the external changes of interplanetary magnetic field and solar wind conditions showing a multiscale dynamics, manifesting in the occurrence of fluctuations over a very wide range of timescales. Here, using an approach based on a Langevin/Fokker-Planck description we investigate the nature of the fast (short-) and slow (long-timescale) fluctuations of SYM-H index during geomagnetic storms. The results point towards a different origin of the fast ( τ  < 200 min) and slow ( τ  > 200 min) fluctuations, which are characterized by state functions of different nature. In detail, the state function associated with the slow dynamics shows the evidence of the occurrence of first-order-like topological phase transition during the different phases of a geomagnetic storm, while the fast dynamics seems to be characterized by a quasi-invariant quadratic state function. A modeling in terms of stochastic Langevin equation is discussed and the relevance of our results in the framework of Space Weather studies is outlined.

Understanding the complex behavior of the near-Earth electromagnetic environment is one of the main challenges of Space Weather studies. This includes both the correct characterization of the different physical mechanisms responsible for its configuration and dynamics as well as the efforts which are needed for a correct forecasting of several phenomena. By using a nonlinear multi-scale dynamical systems approach, we provide here new insights into the scale-to-scale dynamical behavior of both quiet and disturbed periods of geomagnetic activity. The results show that a scale-dependent dynamical transition occurs when moving from short to long timescales, i.e., from fast to slow dynamical processes, the latter being characterized by a more regular behavior, while more dynamical anomalies are found in the behavior of the fast component. This suggests that different physical processes are typical for both dynamical regimes: the fast component, being characterized by a more chaotic and less predictable behavior, can be related to the internal dynamical state of the near-Earth electromagnetic environment, while the slow component seems to be less chaotic and associated with the directly driven processes related to the interplanetary medium variability. Moreover, a clear difference has been found between quiet and disturbed periods, the former being more complex than the latter. These findings support the view that, for a correct forecasting in the framework of Space Weather studies, more attention needs to be devoted to the identification of proxies describing the internal dynamical state of the near-Earth electromagnetic environment.

We report for the first time transient isolated auroral spots at Saturn's southern polar region, based on Hubble Space Telescope (HST) FUV images. The spots last several minutes and appear distinct from the rest of the auroral emissions. We study two sets of HST and Cassini observations during which Cassini instrumentation detected signatures of energetic particle injections close to the region where, on the same day, HST observed transient auroral spots. On the basis of the simultaneous remote and in situ observations, we discuss the possibility that the transient features are associated with the dynamical processes taking place in the Kronian magnetosphere. Given the limitations in the available observations, we suggest the following possible explanations for the transient aurora. The injection region could directly be coupled to Saturn's ionosphere by pitch angle diffusion and electron scattering by whistler waves, or by the electric current flowing along the boundary of the injected cloud. The energy contained in the injection region indicates that electron scattering could account for the transient aurora process.

The Jovian magnetosphere is highly dynamic, influenced by both solar wind and internal processes associated with the rapid planetary rotation and Io's volcanic activities. Accompanying the mass and energy circulations driven by the magnetospheric dynamics, the magnetic configuration also changes dramatically. One of the crucial parameters to characterize the magnetic configuration is magnetic field line curvature (FLC), which generally describes how stretched the field line is. The curvature is pivotal to influence particle behaviors, for example, pitch angle scattering which may lead to auroral particle precipitation. In this work, a method is proposed to investigate the real‐time magnetic FLC in Jovian current sheet using the magnetic field data from the Juno spacecraft. The results indicate that the FLC scattering of ions and relativistic electrons are common in Jovian magnetosphere, providing a crucial insight to understand the particle behaviors. Plain Language Summary Both the Earth and the Jupiter have intrinsic magnetic field. When the planetary magnetic field interacts with the solar wind, a region called magnetosphere is formed. Particle behaviors in different planetary systems are different, due to the different magnetospheric dynamics. The curvature of magnetic field, describing the stretch level of a magnetic field line, is a basic parameter to describe a planetary space system, and it can significantly influence particle behaviors, for example, to scatter the magnetospheric particles to planetary atmosphere, causing auroral emissions. In this work, we proposed a method to calculate the magnetic field line curvature (FLC) near the equatorial plane inside the Jupiter's magnetosphere using Juno data set, for the first time to provide a global picture on the magnetic FLC. By comparing with the radius of particles' gyration motions, we suggest that ions and electrons can be strongly scattered by the magnetic FLC. We believe that the results in this study provide useful information on the different particle behaviors between the terrestrial system and the Jovian system. Key Points We proposed a method to investigate the magnetic field line curvature (FLC) in Jupiter's current sheet using data from Juno data set 50 events are selected by specific criteria. The magnetic FLC and different particles' Larmor radius are investigated The FLC will scatter ions and relativistic electrons as a potential cause of auroral precipitation

The precipitation of tens to hundreds of keV electrons from Earth's magnetosphere plays a crucial role in magnetosphere‐ionosphere coupling, primarily driven by chorus wave scattering. Most existing simulations of electron precipitation rely on test particle models that neglect particle feedback on waves. However, both theoretical and observational studies indicate that the feedback from energetic electrons significantly influences chorus wave excitation and evolution. In this study, we quantify electron precipitation driven by chorus waves using self‐consistent simulations at L = 6 with typical magnetospheric plasma parameters. Electrons in the ∼10–200 keV range are precipitated, exhibiting energy‐dispersive characteristics. The precipitation intensity reaches ∼108–109 ${10}^{8}\\!\\mathit{\\mbox{--}}\\!{10}^{9}$ keV/s/sr/cm2/MeV $\\mathrm{k}\\mathrm{e}\\mathrm{V}/\\mathrm{s}/\\mathrm{s}\\mathrm{r}/{\\mathrm{c}\\mathrm{m}}^{2}/\\mathrm{M}\\mathrm{e}\\mathrm{V}$, consistent with the typical values in observations. As a comparison, test particle simulations underestimate the precipitation intensity by nearly an order of magnitude. These results highlight the importance of self‐consistent simulations in quantifying electron precipitation and investigating wave‐particle interactions that modulate magnetospheric dynamics.

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.

The cold ions, which are generally “invisible” to most instruments, have strong impacts on plasma wave and magnetic reconnection. Under particular situations, these cold ions could be accelerated and thus become detectable. In this study, we statistically investigated the properties of background cold ions based on Van Allen Probe observations. The cold ions could often be detected near the dusk sector, and a clear dawn‐dusk asymmetry is observed for all ion species with higher density at the dusk side, showing plasmaspheric plume‐like structures. Similar to the cold electrons, cold proton ions show a clear boundary of plasmapause with its location moving toward the Earth as geomagnetic activity increases. Furthermore, the percentage of oxygen increases, and the percentage of protons decreases as geomagnetic activity increases whereas the helium composition is generally small. Our results provide important information on ion compositions for the understanding of cold‐plasma dynamics in the inner magnetosphere. Plain Language Summary The cold ions play an important role in magnetospheric dynamics since they are the source of thermal plasma and they could affect the magnetic reconnection and wave generation. However, the main population of cold ions is difficult to measure due to their low energy and spacecraft charging. Magnetospheric convection and/or induced electric field could increase the energy of cold ions sufficiently above the spacecraft potential so that these ions can be detected by particle instrument. In this study, we investigate the properties of background cold ions when the total ion density is comparable to the background electron density. We found the cold ion could often be measured near the dusk sector and a clear dawn‐dusk asymmetry is observed for all ion species. Similar to the cold electrons, cold protons also show a clear boundary of plasmapause with its location moving toward the Earth as geomagnetic activity increases. Furthermore, the percentage of oxygen ions increases, and the percentage of protons decreases as geomagnetic activity increases whereas the percentage of helium ions is generally small. Our results provide important information on cold ion density for the study of wave‐particle interactions and magnetic reconnection in the Earth's magnetosphere. Key Points We statistically analyzed the cold ion densities and compositions based on Van Allen Probe observations The density above L = 3 decreases as geomagnetic activity increases for all three ion species, suggesting the shrinking of plasmasphere The percentage of cold oxygen ions increases as geomagnetic activity increases

The first major scientific discovery of the Space Age was that the Earth is enshrouded in toroids, or belts, of very high-energy magnetically trapped charged particles. Early observations of the radiation environment clearly indicated that the Van Allen belts could be delineated into an inner zone dominated by high-energy protons and an outer zone dominated by high-energy electrons. The energy distribution, spatial extent and particle species makeup of the Van Allen belts has been subsequently explored by several space missions. Recent observations by the NASA dual-spacecraft Van Allen Probes mission have revealed many novel properties of the radiation belts, especially for electrons at highly relativistic and ultra-relativistic kinetic energies. In this review we summarize the space weather impacts of the radiation belts. We demonstrate that many remarkable features of energetic particle changes are driven by strong solar and solar wind forcings. Recent comprehensive data show broadly and in many ways how high energy particles are accelerated, transported, and lost in the magnetosphere due to interplanetary shock wave interactions, coronal mass ejection impacts, and high-speed solar wind streams. We also discuss how radiation belt particles are intimately tied to other parts of the geospace system through atmosphere, ionosphere, and plasmasphere coupling. The new data have in many ways rewritten the textbooks about the radiation belts as a key space weather threat to human technological systems.