Geoffrey D. Reeves

Solar wind/magnetosphere coupling functions are widely used to quantify geoeffective energy transfer. However, their effectiveness as proxies for energetic electron precipitation (EEP) has not been adequately characterized. This study evaluates the performance of the Kan‐Lee (KL) electric field coupling function and the Akasofu ε parameter using >30, >100, and >300 keV precipitating electron fluxes. Correlation, multivariate regression, and superposed epoch analyses were performed using instantaneous, optimally lagged, and temporally integrated forcing representations. Both coupling functions exhibited significant correlations with EEP, with peak correlations occurring when the forcing led to precipitation by approximately 1–3 h. Regression analyses showed that explained variance increased systematically from instantaneous to lagged forcing and improved further when temporally integrated forcing was employed. For >30 keV precipitation, coefficients of determination increased from 0.136 (instantaneous) to 0.172 (lagged) and 0.420 (integrated) for KL, with corresponding values of 0.079, 0.201, and 0.408 for ε, respectively. Although KL generally exhibits stronger statistical associations with EEP, differences between the coupling functions were smaller than the improvements associated with incorporating response delays and cumulative forcing histories. Additional analyses using the auroral electrojet index showed that internal magnetospheric activity accounts for much of the variance associated with instantaneous solar wind coupling, while integrated coupling retained additional explanatory power. These results indicate that EEP is best represented as a temporally integrated response to coupled external solar wind forcing and internal magnetospheric dynamics operating across multiple timescales.

It has been proposed that the cyclic rising tone in frequency observed in rising tone quasi-periodic (QP) fast magnetosonic waves is due to the cyclic diffusion of the inner energy edge of the source proton shell distribution toward lower energies. Linear theory shows that as the inner edge of the proton shell diffuses inwards the unstable frequency rises leading to a rising tone. We present the first observations of this diffusion of the proton shell distribution for this type of wave event, observed by the HOPE instrument on Van Allen Probes B, where this diffusion is clearly observed in two back-to-back QP wave cycles. Linear instability is performed on the observed proton shell distribution, and the linear growth rate is compared with the observed wave frequency, the linear theory applied to the observed proton shell distributions was qualitatively but not quantitatively consistent with observed wave frequencies. Nearby Waveform Receiver burst mode measurements showed that the spacing of the emission bands in frequency was very close the proton cyclotron frequency f _cP , suggesting near source. However, a ±2 nT spacecraft induced oscillation at ½ the spacecraft spin period in the background magnetic field magnitude could not allow us to conclude that the emission bands where at integer multiples of f _cP , a stronger condition for source proximity.

It has been proposed that the cyclic rising tone in frequency observed in rising tone quasi‐periodic (QP) fast magnetosonic waves is due to the cyclic diffusion of the inner energy edge of the source proton shell distribution toward lower energies. Linear theory shows that as the inner edge of the proton shell diffuses inwards the unstable frequency rises leading to a rising tone. We present the first observations of this diffusion of the proton shell distribution for this type of wave event, observed by the HOPE instrument on Van Allen Probes B, where this diffusion is clearly observed in two back‐to‐back QP wave cycles. Linear instability is performed on the observed proton shell distribution, and the linear growth rate is compared with the observed wave frequency; the linear theory applied to the observed proton shell distributions was qualitatively but not quantitatively consistent with the observed wave frequencies. Nearby Waveform Receiver burst mode measurements showed that the spacing of the emission bands in frequency was very close to the proton cyclotron frequency f _cP , suggesting a near source. However, a ±2 nT spacecraft induced oscillation at ½ the spacecraft spin period in the background magnetic field magnitude could not allow us to conclude that the emission bands where at integer multiples of f _cP , a stronger condition for source proximity.

Chorus waves play a significant role in the dynamic evolution of energetic electrons in the inner magnetosphere. Therefore, understanding the spatial and temporal dynamics of these electrons requires global distributions of chorus waves, which in turn necessitates combining data products from multiple satellite missions to achieve sufficient spatial coverage. In this study, we use 11 years of data from both the Van Allen Probes and the Arase satellite to create a global model of the magnetic intensity of chorus waves. The agreement between the two satellite missions was assessed using observations during close conjunctions. The statistical model is based on data with good spatial coverage up to 40° magnetic latitude, across all magnetic local times (MLT), and at high L-shells, resulting in a model with excellent spatial continuity. The model is generated for both Upper-Band Chorus (UBC; 0.5 fce

We investigate the injection and transport of energetic particles from the Earth's plasma sheet into the inner magnetosphere during the three consecutive substorms that occuurred on 7 September 2017. Using coordinated observations from the MMS, LANL, and Van Allen Probes spacecraft, we track the evolution of ion and electron fluxes from the mid‐tail to the inner magnetosphere. Using the dipole field approximation to trace particle drift orbits from LANL satellites at geosynchronous orbit, we identified the equatorial injection region to extend from ∼2000 to ∼0400 MLT. Subsequently, the injected particles were detected further earthward at Van Allen Probes (L ∼ 4), particularly on the dayside. The multi‐point data reveal that stronger substorms injected ions over a wide energy range (1–200 keV) with significant dayside penetration, while weaker substorms resulted in narrower energy injections (5–30 keV). Back‐tracing the proton drift paths further, calculated using the electric and magnetic fields from the MHD simulation, indicates that convection and adiabatic drifts alone are insufficient to account for the ion injections observed at the Van Allen Probe locations. This suggests that non‐adiabatic processes, such as substorm‐driven impulsive electric fields, are required for particles to reach the same regions. In contrast, electrons at Van Allen Probes lacked clear injection signatures, suggesting the influence of localized electric fields. These results highlight the spatiotemporal complexity of substorm injections and emphasize the value of multi‐point observations and simulations to understand particle transport in the inner magnetosphere.

Natural light displays known as the aurora provide a captivating glimpse into the electromagnetic dynamics in space plasmas. Aurorae are not exclusive to Earth but also observed on celestial bodies including planets and even comets. Previous studies have unveiled two fundamental auroral acceleration mechanisms: electric potential and Alfvénic acceleration. However, the relation of the energy processes associated with the auroral acceleration region has remained unclear mainly due to the lack of quantitative analysis. Employing quantitative assessment of energy budget using multi-platforms from the magnetosphere to auroral ionosphere, our findings underscore the interplay between these two mechanisms. Here we show that energy carried by Alfvén waves travels from the magnetosphere to the auroral acceleration region, forming an electric potential drop that accelerates particles to produce aurorae. Similarities in auroral particle behaviors between Earth and Jupiter suggest the applicability of the terrestrial scenario to Jupiter and potentially other celestial bodies in the Universe.

Our study presents the first simultaneous multi‐point observations of the local‐time asymmetry of 10's–100's of keV energy particle pitch angle distributions (PADs), and associated wave activity during the main phase of the geomagnetic storm that occurred on 7 September 2017. This study is a companion to Pandya et al. (2025), who identified a rare conjunction in which RBSP‐A and RBSP‐B were located in the evening and morning sectors, respectively, enabling the comparison of ion populations across local time. Building on that work, we identify an additional rare conjunction during the same interval in which RBSP‐A and MMS‐1 were co‐located in the evening sector for ∼15 min (2,325–2340 UT) within Δ L < 0.2, ΔMLT < 2 min and ΔMLAT ∼ 20°. Our analysis shows strong local time asymmetries. Protons in the evening sector exhibited freshly injected, enhanced fluxes with nearly isotropic PADs at >60 keV energies and pancake PADs at <60 keV energies, whereas dawn‐noon proton fluxes retained pancake PADs characteristic of older populations. In contrast, electrons showed a significant flux decrease in the evening sector and isotropic enhancement in the morning sector, consistent with their eastward gradient‐B and curvature drifts. Concurrent wave observations revealed strong ULF, magnetosonic and hiss waves in the evening sector, while hiss waves dominated in the morning sector. Our results demonstrate that storm‐time magnetic field reconfiguration, rather than wave‐particle interactions, primarily governs keV particle PAD asymmetries. The findings highlight the importance of coordinated multi‐point measurements for advancing our understanding of ring current asymmetries and storm‐time particle transport.

On 15 February 2018 a co‐rotating interaction region (CIR) from an equatorial coronal hole reached the Earth. The CIR initiated a moderate and slowly intensifying geomagnetic storm, which began with a large and strong substorm injection. The substorm injection was exceptionally well‐observed by an array of spacecraft including LANL‐GEO satellites, Van Allen Probes (RBSP), Arase (ERG), and MetOp/POES, as well as ground‐based instruments. These observations enable the unambiguous identification of several important features that have been impossible to measure directly in other events. The substorm injection extended well inside the geosynchronous orbit. A fortuitous conjunction of RBSP‐A (moving inbound) and Arase (simultaneously moving outbound at the same magnetic local time) allows us to establish, very precisely, the location of the inner edge of the injection region at L = 3.8−3.9. In supporting observations, North American riometers saw precipitation extending down to L ≈ 4 but not lower. Arase and RBSP‐A also observed whistler‐mode hiss waves inside the plasmasphere. Analysis of the resonance conditions shows, conclusively, and for the first time, that they were produced by the drifting injected electrons. RBSP‐A observations also show the injection (or transport) of electrons into or through the slot region within hours of the substorm injection onset. Previous studies were not able to clearly connect or separate substorm injections and slot‐filling processes. These new observations clearly identify slot‐filling as a spatially and temporally separate process that is not a direct result of substorm injection.

Our study presents the first simultaneous multi‐point observation of the local‐time asymmetry of 10's–100's of keV energy ion fluxes during the main phase of the geomagnetic storm that occurred on 7 September 2017. During this event, Van Allen Probe‐A and Van Allen Probe‐B observed two different tendencies. The ion fluxes increased by an order of magnitude in the noon‐dusk sector, while decreasing by one order or more in the dawn‐noon sector, offering a unique opportunity to investigate this asymmetry. Numerical simulations employing the Comprehensive Inner Magnetosphere‐Ionosphere model with time‐dependent electric fields from Global Magnetohydrodynamic (MHD) simulations revealed that the local time asymmetry in ion fluxes is associated with a sharp southward turning of the interplanetary magnetic field (IMF) and long‐duration persistent westward electric field. These factors cause ions to drift toward the dusk sector, while preexisting ions on the dayside drift sunward and escape the inner magnetosphere. Our findings provide the first direct observational evidence of ring current asymmetry, complementing and supporting prior statistical studies and simulation results.

We recently reported the finding of elementary rising‐tone emissions embedded within each harmonic of magnetosonic waves, by investigating wave electric field waveforms measured by Van Allen Probes. The present study further uncovers a new set of fine structures of magnetosonic waves, namely, each elementary rising‐tone may consist of a series of mini harmonics spaced around the O ⁺ gyrofrequency. The measured ion distributions suggest that the proton ring distribution provides free energy to excite the waves, whilst the O ⁺ ions suppress the wave growth around multiples of O ⁺ gyrofrequency, resulting in the formation of mini harmonics. Further investigation suggests that the warm plasma dispersion relation, that is, the ion Bernstein mode instabilities, may contribute to the formation of mini harmonics. The mini harmonic structure implies a new mechanism of energy redistribution among ion species in space plasmas, potentially providing a new acceleration mechanism for O ⁺ ions in the magnetosphere.