Kareem Sorathia

We study the development of the Kelvin‐Helmholtz instability (KHI) along the dusk dayside Earth's magnetopause with and without the presence of enhanced plasma density at the shear interface. Using three‐dimensional MAGE global simulations, we establish general KHI growth rate diagrams, which depend sensitively on the transverse profiles of both flow velocity and density. We find multiple KHI onsets promoting various growths along the magnetopause. Their associated KH frequencies and growth rates are found to follow the flow structure and decrease downstream. The magnetopause displacement, caused here by an abrupt southward‐to‐northward IMF turning, shifts KH onsets upstream and promotes density leakage that seeds an additional mixing layer at the interface. The presence of enhanced density modifies KHI frequency response suppressing small‐scale KHI structures while amplifying larger‐scales, as a low‐pass filter. We complement the analysis with dedicated high‐resolution scaled 2‐D Cartesian simulations, Doppler‐corrected for comparison with the global 3‐D results, and find good agreement with the linear KH growth rates. We show that the flow evolution along the magnetopause lowers the KHI frequencies down to Pc5–Pc4 ranges. Density effects further reorganize the frequency range of the KH unstable modes inducing the amplification of Pc6 to low‐Pc5 frequencies, suggesting that KHI can amplify fast magnetosonic waves propagating along standing field lines. The high‐resolution 2‐D simulations suggest that, in the nonlinear stage, the presence of a density pocket can suppress secondary instabilities.

The Juno mission, in conjunction with past missions and Earth‐based observations, has provided a wealth of information on Jupiter's magnetosphere. This magnetosphere is governed by Jupiter's strong magnetic field, rapid 10‐hr rotation, and a significant internal plasma source, the volcanic moon Io. Jupiter's magnetodisc exhibits plasma temperatures that deviate strongly from adiabatic expectations, increasing by nearly two orders of magnitude between the Io plasma torus and ∼25 . The physical mechanisms responsible for this heating remain poorly understood, particularly in the middle magnetosphere where advective transport dominates. In this study, we use a global magnetohydrodynamic (MHD) simulation to investigate the processes controlling plasma temperature evolution. Fluid tracing and test particle simulations are performed to analyze radial transport and energization within the magnetodisc. Our results reproduce key features of the observed temperature profiles, including the pronounced increase beyond 10 , without the need for additional heating terms. We demonstrate that a combination of tail reconnection, adiabatic compression, and large‐scale radial and corotational transport can explain the simulated temperature profiles, providing insight into the mechanisms governing plasma heating in Jupiter's magnetodisc. These findings highlight the importance of global scale MHD processes in shaping the thermal structure of the middle magnetosphere and offer a framework for interpreting Juno observations in regions not directly observed.

Saturn has been shown to exhibit periodicities ranging from minutes to days. A comprehensive understanding of the various physical drivers, their characteristic periodicities, and their potential interconnectedness is lacking, particularly in the context of overall mass (Saturn's net source/loss ∼50–100 kg/s) and magnetic flux (net = 0) transport. The paper focuses on periodicities ranging from Saturn's ubiquitous quasi‐period of 60 min (QP60) to less than the planetary rotation period (∼10.7 hr). For this study, we utilized a magnetohydrodynamics (MHD) simulation in conjunction with data from the Cassini spacecraft. We implemented the mutual information (MI) analysis on a source‐free Grid Agnostic MHD for Extended Research Applications (GAMERA) simulation to investigate the different quasi‐periodicities present in Saturn's magnetosphere and their implications for transport. We found a feedback relation between duskward‐propagating Kelvin‐Helmholtz (KH) surface waves forming at the subsolar magnetopause boundary and injection flows due to magnetotail reconnection. The KH surface waves have a natural period of roughly 1 hour; however, the feedback relationship occasionally amplifies the KH waves, leading to a 2–4 hr period.

Ultra‐low frequency (ULF) waves are a ubiquitous carrier of energy in geospace. However, their efficiency in transferring solar wind energy into the upper atmosphere remains a fundamental and not well‐understood question. This is due to their global presence, which cannot be fully quantified by spatially limited observations, and the need for self‐consistent global modeling to account for their dependence on dynamic, inhomogeneous magnetic fields and plasma densities. In this study we use a purely global magnetohydrodynamic model to investigate energy inputs to the ionosphere in the form of Poynting flux. Oscillations in solar wind dynamic pressure excite field line resonances in the magnetosphere. The total Alfvénic Poynting flux entering the ionosphere can be comparable to the total quasi‐steady Poynting flux under northward interplanetary magnetic field. The efficiency of this energy transfer via ULF waves depends on the driving frequency of the solar wind and the ionospheric Pedersen conductance.

Global numerical simulations are valuable tools for understanding the Sun‐Earth interaction as they provide a more thorough picture of the system when compared to typically sparse observations. Yet, comparisons of these global models against observations is complicated by the inherent uncertainty that the observed phenomena occurred at the same time and location in the simulation domain. Therefore, aggregate measures of the Sun‐Earth interaction, such as activity indices (e.g., AL, AU, Dst, cross polar‐cap potential), are often used, with the downside of missing important underlying physical processes, especially at the mesoscales. Metrics or parameters that provide deeper insight into the physics but do not rely precisely on the location of the observations are therefore needed. Data assimilative models can be used to reconstruct high‐latitude ionospheric electrodynamic quantities using sparse observations, providing a new avenue for data‐model comparisons. We demonstrate this new approach to data‐model comparisons by assimilating global simulation output from Multiscale Atmosphere‐Geospace Environment (MAGE) into Local mapping of the polar ionospheric electrodynamics (Lompe). We directly compare and find good agreement between the simulation output and real‐data‐assimilation patterns of field‐aligned currents and their constituent terms deriving from the divergence of the ionospheric Ohm's Law. We further calculate reconnection electric fields and voltages and find good agreement during the initial expansion phase, but different behavior later in the event as MAGE enters into steady‐magnetospheric convection, in contrast to the observations, which indicated a classic substorm.

We present a case study of a moderate geomagnetic storm, which exhibits an abrupt relativistic enhancement in the heart of the radiation belt accompanied by low‐latitude purple auroral emission. The relativistic enhancement occurs on a timescale of tens of minutes and reaches MeV energies deep within the inner magnetosphere. The enhancement is concurrent with mesoscale purple patches which protrude below the pre‐existing auroral oval. We analyze the event using a novel combination of observational data sets and physics‐based modeling. This includes multispectral auroral imaging, dosimeter data from the Global Positioning System constellation, auroral transport modeling, and global geospace modeling. Our analysis reveals the cause of both the radiation belt enhancement and purple emission to be direct, mesoscale injections of electrons reaching relativistic energies. Finally, we show that these injections are most likely caused by near‐Earth reconnection which produces deeply depleted entropy bubbles capable of trapping and transporting energetic particles into the inner magnetosphere.

A systems science approach based on canonical correlation analysis (CCA) is applied as a new, behavioral way to validate global geospace models. The biggest novelty of the technique is that it validates models at a system level, whereby a side‐by‐side comparison is performed of CCA applied to a 30‐day observational and the corresponding simulation data sets comprising quiet, moderate and active times. The simulation used the Multiscale Atmosphere‐Geospace Environment (MAGE) model. It is shown that (a) CCA must be combined with sensitivity analysis to be effective, (b) the MAGE model generally reproduces the observed behavior (more so for quieter time intervals), quantified by the intercorrelations between different variables and (c) the technique identifies the SuperMAG SML index as a quantity for which refinements of the model are needed.

Jupiter's rapidly rotating magnetosphere, with internal plasma sources such as the volcanic moon Io, provides a unique natural laboratory for studying internally driven planetary magnetospheres. Using the Grid Agnostic Magnetohydrodynamics for Extended Research Applications (GAMERA) model, we simulated Jupiter's magnetosphere with variable ionospheric Pedersen conductances, which is mainly responsible for energy dissipation between the ionosphere and magnetosphere though convection. We chose values ranging from 0.5 to mho to investigate the Pedersen conductance's role in controlling mid‐magnetosphere region's dynamics and the closing of magnetosphere‐ionosphere currents. Simulated density, temperature, and radial and azimuthal flows in the equator are compared with observations from the Jovian Auroral Distributions Experiment (JADE) on the Juno spacecraft. All simulation cases exhibit dawn‐dusk asymmetries, in both the ionosphere and magnetosphere. The 0.5 case showed the best agreement with JADE observations, while the 1 case exhibited a magnetic topology more consistent with the auroral observations from the Hubble Space Telescope and Juno. These results enhance our understanding of Jupiter's magnetosphere‐ionosphere coupling, provide context for observations, and inform the background parameters of future test particle simulations and data‐model comparisons using the GAMERA model.

During substorms, Earth's magnetotail undergoes rapid dipolarization, driving Earthward plasma flows that decelerate and dissipate energy upon encountering the dipole magnetic field in the nightside transition region. This region mediates the interaction between the magnetotail, inner magnetosphere, and the ionospheric auroral zone, though significant mapping uncertainties obscure the precise link and particle acceleration processes. Using data from THEMIS, TREx, and ELFIN, we analyze a storm‐time substorm on 4 September 2022, establishing a relationship, that is, not a causation, between magnetospheric and ionospheric dynamics. Following a dipolarization, the auroral bulge overlapped with the footprints of the electron isotropy boundary (IB) and the outer radiation belt. Notably, the precipitating electron energies reached at least 2 MeV in the bulge, exceeding previous reports. By comparing the latitudes of the electron IB with respect to the auroral bulge, we deduce that the sources of both auroral and relativistic precipitation were confined in the dipolarized region.