Guan Le

During fast mode intervals the Magnetospheric Multi‐Scale (MMS) Fast Plasma Investigation (FPI) operates continuously in ion burst capacity at 150 msec cadence. From the burst data and at the same cadence, FPI computes four ion moments (ion density, ion flux) termed trigger data. Only a few percent of the full burst data are downloaded, but trigger data are downloaded over the entire fast mode interval. The trigger data were not envisioned as a science product, so the sign bits of the 3 ion flux components were dropped, and only absolute values were telemetered to the ground. Using 4.5 s FAST mode data and minimization techniques, we reconstruct the ion flux sign bits and computed ion velocity, validating the methodology through comparisons with burst mode velocity for 103 burst mode intervals. We demonstrate that calibrated FPI “trigger” data are scientifically valid in the magnetosheath and its boundary layer, and particularly useful for wave studies whose frequency is greater than 0.1 and less than 3 Hz. As an example of the use of the trigger data, we present an analysis of ion cyclotron waves (ICW) observed by the MMS constellation on 7 October 2015, in the afternoon magnetosheath.

The ion foreshock is the region upstream of Earth's bow shock where magnetic field lines connect to the quasi‐parallel shock surface. Within this region, there exist a variety of backstreaming ion populations from the shock ramp that can generate ultra‐low frequency (ULF) waves through wave‐particle interactions. In this work, we use data from the Magnetospheric Multiscale (MMS) mission to study such ULF waves and backstreaming ions during a prolonged interval of above‐average solar wind helium abundance, embedded in a multi‐day period of strong solar activity driven by a coronal mass ejection. When interplanetary magnetic field orientations positioned MMS within the ion foreshock, the spacecraft captured the complete evolutionary sequence of the backstreaming ion velocity distributions: the initial formation of a reflected ion beam, followed by phase bunching and generation of coherent ULF waves, and eventual thermalization and randomization in velocity space to form diffuse ions. Intervals with elevated energetic He++ flux exhibited broadened and frequency‐downshifted wave spectra, consistent with heavy‐ion cyclotron resonance effects. The unusually rapid beam‐to‐diffuse transitions observed near the foreshock boundary likely result from the combined effects of multi‐species wave‐particle interactions and higher backstreaming ion densities during this active interval. These findings underscore the need for simulations and modeling that incorporate multi‐species effects.

On 10 May 2024, Earth was hit by a CME that drove the largest geomagnetic storm in 20 years. Multi‐spacecraft observations previously showed that the ∼100 nT north‐south IMF variation was driven by Kelvin‐Helmholtz waves with wavelength ∼250 and reconnection jets in the ‐direction (Nykyri, 2024a, https://doi.org/10.1029/2024gl110477 ). Here we show that Magnetospheric Multiscale (MMS), located upstream of the Earth's bow shock at , detected additional reconnection jets not seen by ARTEMIS at lunar orbit ∼60 away, perpendicular to CME propagation. One jet coincided with high‐energy ions and electrons streaming anti‐parallel to the magnetic field. The ions showed dispersion followed by apparent counter‐dispersion, whereas electrons above 100 keV arrived nearly simultaneously. Strong electromagnetic wave activity between these phases is consistent with magnetosonic‐Whistler waves, suggesting that MMS traversed very near the particle acceleration region.

Identifying the primary physical mechanisms and associated secondary processes (e.g., different plasma wave modes, instabilities and magnetic reconnection), that are responsible for plasma heating at the planetary magnetospheres and astrophysical plasmas is often difficult with a single spacecraft or even with a constellation of spacecraft within a single‐plasma scale. Here, we present a statistical study using data from four Magnetospheric Multiscale (MMS) satellites to characterize the cross‐scale plasma‐heating distributions associated with Kelvin‐Helmholtz instability (KHI) and drift mirror instability (DMI). We find that characteristic heating frequency (CHF) distributions follow distinct tail‐scaling regimes: KHI events exhibit low‐ power‐law‐like tails, whereas DMI events occupy larger‐ regimes closer to quasi‐Gaussian behavior. Comparison of a KHI event with Particle‐in‐Cell (PIC) simulations yields consistent tail‐scaling, supporting the physical interpretation. We also analyze an event with periodic encounters of diamagnetic cavities (DMCs) at the Earth's dayside magnetopause, and find that the CHF scaling together with KH growth‐rate analysis is consistent with KHI modulation of reconnection. These results establish statistical discrimination between KHI‐ and DMI‐associated heating, and can be used in future works to establish an automated identification of primary mesoscale processes driving cross‐scale non‐adiabatic heating in collisionless plasmas.

Flux ropes in space plasma can play an important role in particle acceleration. However, the mechanisms of electron energization and the resulting electron energy distribution associated with flux ropes are still not fully understood. This study examines a flux rope chain with four ion-scale flux ropes observed by the Magnetospheric Multiscale (MMS) mission in a turbulent plasma region in Earth’s magnetotail. We investigate the active electron acceleration mechanisms, including parallel electric fields, betatron acceleration, and magnetic curvature-driven Fermi-type acceleration, based on MMS’s four spacecraft in situ measurements. Our results demonstrate that Fermi acceleration is the dominant mechanism with betatron acceleration providing an important contribution. The electron energy spectra are further fitted by Kappa and Flattop distributions aiming to characterize the energy densities of thermal and suprathermal populations. It shows that although suprathermal electrons comprise only 10% of the number number density, they could contribute up to 30% of the total energy density. While the energy densities of both thermal and suprathermal electrons increase within the flux ropes, the relative portions of suprathermal electrons decrease. This suggests that flux rope could confine both thermal and suprathermal electrons; however, they are likely more efficient to energize thermal electrons.

Energetic particles injected from dayside reconnection serve as a source for ionospheric particle precipitation in the cusp region. The cusp comprises poleward-moving plasma structures, cusp plasma filaments due to discontinuous reconnection events which are considered footprints of flux transfer events (FTEs). These structures remain relatively unexplored on Earth, however an examination of MESSENGER data from Mercury's magnetosphere suggests that cusp filaments represent the magnetospheric extensions of FTEs originating at the magnetopause due to localized magnetic reconnection. This underscores the need for a comprehensive investigation of similar phenomena within Earth's magnetosphere. Reconnection often takes place at the dayside magnetopause, where the solar wind's magnetic field interacts with the Earth's magnetic field. This interaction leads to the merging and rearrangement of magnetic field lines, creating open magnetic field lines that connect the high-latitude magnetospheric cusp to the solar wind forming FTEs which are flux-rope-like structures filled with magnetosheath plasma. In this study, data obtained from the MMS spacecraft was examined which is strategically positioned to traverse the cusp region. The spacecraft's orbit facilitates multi-spacecraft in situ measurements within the cusp, providing crucial data for the analysis of phenomena such as cusp plasma filaments. Cusp filaments were analyzed using multi-spacecraft analysis techniques and high-resolution measurements were utilized to reconstruct and analyze the internal plasma structure of these cusp filaments. Characteristics and spatial distribution of cusp plasma filaments within Earth's cusp region were also investigated. The primary focus was to comprehend the role of these filaments in particle precipitation and their correlation with dayside magnetic reconnection events. Our preliminary results suggest that cusp plasma filaments are indeed low latitude, high altitude footprints of FTEs. Moreover, there appears to be a correlation between the presence of plasma filaments and dayside reconnection events.

The ion foreshock is the region upstream of Earth’s bow shock where magnetic field lines connect to the quasi-parallel shock surface. Within this region, there exist a variety of backstreaming ion populations from the shock ramp that can generate ultra-low frequency (ULF) waves through wave-particle interactions. In this work, we use data from the Magnetospheric Multiscale (MMS) mission to study such ULF waves and backstreaming ions during a prolonged interval of above-average solar wind helium abundance, embedded in a multi-day period of strong solar activity driven by a CME. When interplanetary magnetic field (IMF) orientations positioned MMS within the ion foreshock, the spacecraft captured the complete evolutionary sequence of the backstreaming ion velocity distributions: the initial formation of a reflected ion beam, followed by phase bunching and generation of coherent ULF waves, and eventual thermalization and randomization in velocity space to form diffuse ions. Intervals with elevated energetic He++ flux exhibited broadened and frequency-downshifted wave spectra, consistent with heavy-ion cyclotron resonance effects. The unusually rapid beam-to-diffuse transitions observed near the foreshock boundary likely result from the combined effects of multi-species wave-particle interactions and higher backstreaming ion densities during this active interval. These findings underscore the need for simulations and modeling that incorporate multi-species effects.

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.

We investigate the empirical relationship between the spacecraft potential ( V _s ) measured by the Electric Field Double Probes, and the electron density ( N _e ) measured by the Fast Plasma Instrument on the MMS spacecraft. We derive their relationship during fast‐mode intervals when the Active Spacecraft Potential Control Devices are off. Then we apply this relationship to slow‐mode intervals during the perigee passes where V _s can be less than +2 V and N _e can exceed 1,000 cm ⁻³ . Because such a parameter range is never observed by MMS during fast‐mode intervals, we define this part of the relationship using simultaneous observations from the Van Allen mission where N _e is measured up to 3,000 cm ⁻³ while V _s is less than +2 V. We compare the empirical relationship to the predictions by an orbital motion limited theory. This suggests how to model the photoelectron current above +5 V and the collection of ambient electrons for moderate Debye lengths. We apply the empirical relationship (i.e., theoretical curves are not used here) to several consecutive plasmapause crossings by MMS during two magnetic storms. The erosion rates of the plasmasphere were ∼60 cm ⁻³ /day at 7 MLT and ∼30 cm ⁻³ /day at 14 MLT. In the duskside, the erosion rate decreases with L due to increasing flux tube volumes, being about 10–20 cm ⁻³ /day at L ∼ 7. The refilling rates are similar to the erosion rates, but in the dawnside, the refilling is delayed due to a slow expansion of the plasmapause to higher L .

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.