Jerry Goldstein

We perform a basic model experiment: we run two nearly identical plasmapause simulations, differing only by their initial conditions. The first run contains initial fine‐scale and mesoscale structure, and the second run is initialized with a featureless, circular plasmapause. We show that residual or pre‐existing density structure can have an important impact on plasmaspheric dynamics.

Subauroral ion drift (SAID) is a narrow and rapid westward ion flow observed in the subauroral ionosphere during geomagnetic storms and substorms. It is more localized and intense than subauroral polarization streams (SAPS), typically appearing equatorward of auroral boundaries and often associated with midlatitude troughs. This study analyzes ion drifts and plasma density variations using DMSP F16, F17, and F18 data from June 1, 2013, in the Southern Hemisphere. Using multi-satellite observations from three DMSP spacecraft, we systematically examine the spatiotemporal evolution of a SAID event and its associated midlatitude troughs, focusing on their relation to geomagnetic storm phases and substorm activity. We develop an ad hoc empirical model that reproduces SAID spatial distribution and temporal evolution by establishing a quantitative relationship between SAID velocity and the AE index. From the results, we present two key findings: first, we identified a previously unreported two-stage development pattern of SAID: equatorward expansion with minimal width change and moderate potential drop in the early main phase, followed by latitudinal stabilization, width variation, and stronger electric fields in the late main phase. Second, we newly identified that the midlatitude trough developed through three distinct stages: mild density gradient associated with the initial AE increase, sharp density drop at the plasmapause boundary after the first AE decrease, and persistent deep trough after first AE peak and throughout the second AE peak lasted for three hours. These findings and our empirical modeling approach provide new quantitative insights into the distinct temporal evolution patterns of SAID and midlatitude troughs, advancing further understanding of the connection between ionospheric disturbances and geomagnetic storms. Graphical

Measurements with the Hot Plasma Composition Analyzer (HPCA) instruments of the Magnetospheric Multiscale (MMS) mission yield the highest time resolution energetic and compositional analysis of a storm-driven magnetosphere. During a single inbound pass through the magnetosheath boundary layer, low-latitude boundary layer, and the magnetopause during the recovery phase of the Gannon storm of 10–13 May 2024, the HPCA instruments measured doubly and singly charged oxygen and nitrogen ions, and molecular ions, identified as molecular nitrogen in the time-of-flight spectra, originating from the ionosphere. These in situ observations are analyzed with precision peak fitting and validated against instrument calibration data. The four-spacecraft MMS constellation observed these ions as it traversed the magnetosphere and several boundary layers. The ion populations had assumed different energies (0.4 – 20 keV) due to accelerating reconnection processes in the low-latitude and magnetosheath boundary layers. In particular, molecular nitrogen ions were observed in the magnetosheath boundary layer escaping from the magnetosphere at an as-yet-unobserved average high velocity of 352 km/s.

Cold plasma distribution in the ionosphere‐plasmasphere system governs wave‐particle interactions, plasma energization and loss, and radio wave propagation. A longstanding observational gap at altitudes 800–8,000 km has largely prevented studying the coupled dynamics of the two regions. Here, we show that observations by JAXA's Arase mission can bridge this gap. Electron densities inferred from the upper hybrid resonance frequencies measured by Arase are highly consistent with radio occultation profiles from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) mission, with a median difference of 5%. Using the combined COSMIC‐Arase data set, we provide a convenient way to reconcile the two regions in empirical models based on the analytical Chapman function inversion for scale height. Our results enable studying fundamental questions about the ionosphere‐plasmasphere coupling, their transition, and life cycle of cold plasma in near‐Earth space.

On 10 June and 27 September 2024, two workshops were held at GFZ Potsdam under the umbrella of the Geo. X Research Network of Geosciences to discuss the unresolved question of the overestimation and lack of scattering of modeled ring current electrons during geomagnetic storms. At the workshops, we discussed the potential contributions to the lack of scattering of electron cyclotron harmonic (ECH) waves, chorus waves, time‐domain‐structures (TDS), the non‐linear effects of wave‐particle interactions, and induced electric fields. A case study shows that the scattering by ECH waves is insufficient to account fully for the missing electron loss. More work must be done to understand the potential effects of inaccuracies in the assumed chorus wave models, TDS, and the non‐linear effects of wave‐particle interactions. Including induced electric fields in ring current simulations is an important step to describe the electron drifts more accurately. Explaining the missing loss process is crucial for space weather applications of surface charging effects, which rely on accurate predictions of ring current electron fluxes.

Plasmaspheric fine‐scale structure (FSS) comprises density irregularities below 0.1–0.2 Earth radii in size. In this paper, we investigate FSS within dayside plasmaspheric plumes as they convect sunward from geosynchronous orbit to the magnetopause. We perform a statistical study of Magnetospheric Multiscale ion data, analyzing 39,018 ion moments from 122 plume events. We find FSS grows inside sunward‐moving dayside plumes, increasing exponentially with 8–11 hr timescale. Spatially, FSS becomes concentrated in the outer duskside region where ion drift paths converge toward the magnetopause. We also investigate basic properties of plume ions. We confirm that plume ion temperature increases with distance, and find that for most plumes light ion densities are correlated to each other. In older plumes (18% of our database) with preferential heating of , mesoscale and fine‐scale structures grow more strongly correlated with protons, and light‐ion correlation decreases. The average plume ion bulk flow is sunward and consistent with penetration of the solar wind electric field. From Fourier analysis, FSS scale sizes extend down to the lower limit of the instrumental sampling range, with evidence of structures below that limit but too small to measure. Plume events exhibit discrete peaks in the Fourier spectrum, but the specific peak structure changes with event. As dayside plumes age, global FSS spectral power migrates radially outward, and shifts to smaller spatial scales. Power‐law fitting of density spectra suggests that turbulence is involved in generating FSS, possibly aided by convective elongation of existing structure and the gradient‐drift instability.

The warm plasma cloak (WPC) is a plasma population located in the outer magnetosphere. It is composed primarily of electrons and ions with energies of tens to hundreds electronvolts and can contain significant amounts of . We conduct a statistical survey of the WPC using 9 years of observations in the dayside magnetosphere from the Magnetospheric Multiscale mission. WPC is found in of the observations and it is 1.6 times more abundant in the dawn than in the dusk sector. ‐rich WPC is detected in of the observations, being 3 times more probable in the dawn sector. We find that after a peak of strong geomagnetic activity, it takes approximately 9 hr to detect the WPC in the dayside magnetosphere.

We present the result of optical testing of a prototype of the High-Resolution Plasmasphere Observatory (HRPO), an imaging camera designed to measure the macroscale dynamics of fine-scale plasmaspheric density structure, i.e., features with a scale size of ≤0.1 RE. We report laboratory measurements of the HRPO prototype to validate the model-predicted average spot diameter of 0.05 deg ± 0.01 deg [root-mean-squared (RMS)] or 0.08 deg ± 0.03 deg (geometric). From testing at nine different field angles, we find the average measured spot diameter is 0.05 deg ± 0.01 deg (RMS) or 0.09 deg ± 0.02 deg (full-width 1/10 maximum), demonstrating that HRPO’s lab-measured angular resolution is consistent with the model prediction and between four and seven times smaller than the 0.36 deg needed for Nyquist (two-point) sampling of 0.1 RE fine-scale plasmasphere structure from a notional 8 RE geocentric radius orbit. We compute simulated HRPO images to demonstrate the imager’s expected performance for realistic plasmaspheric brightness distributions. HRPO’s improved angular resolution is capable of capturing smaller density structures, but only with sufficient count rates either from the brighter regions of images or with adaptive space or time binning in the dimmer regions.

The plasmasphere is a key component of Earth's magnetosphere, regulating numerous energy transfer processes. The plasmasphere is also a cold multi‐species plasma. Tracing differences in dynamics between low and high mass cold ions is important for identifying the processes that drive plasmaspheric evolution. At the same time, measurements of cold ion fractional composition within the plasmasphere are sparse and challenging to obtain. Seeking to overcome this challenge, this work presents a novel concept for combining extreme ultraviolet (EUV) photon imaging and Global Navigation Satellite System (GNSS) pseudorange observations to measure cold ion fractional composition throughout the plasmasphere. The feasibility of this concept is demonstrated using a model plasmaspheric density structure combined with known properties of GNSS transmitters and signals. Implementation of this measurement concept on a future space mission has the potential to enable significant new progress understanding processes that drive plasmaspheric and, extension magnetospheric dynamics.