2025 THEMIS SCIENCE NUGGETS
Ion Moment Variability Across Substorm Phases: Statistical Insights
Sanjay Kumar
Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI,
USA
Introduction
Earth’s magnetotail is crucial for storing and releasing energy during substorms—sudden disturbances caused by solar wind—where magnetic energy converts into plasma motion, particle acceleration, and auroras. While numerous studies have explored fast and slow ion flows and dawn-dusk asymmetries in the plasma parameters within the plasma sheet, questions remain about how these flows and plasma properties evolve throughout the substorm cycle, particularly the statistical distributions of earthward and tailward flows and asymmetries in ion density and pressure during different phases.
Results
This study uses five years of THEMIS and MMS data to provide a comprehensive statistical analysis of ion flows and plasma parameters across substorm phases. By examining spatial distributions and asymmetries in key plasma characteristics, we aim to improve our understanding of substorm-driven dynamics in Earth’s magnetotail.
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| Figure 1. (a–f) Color-coded maps of slow earthward velocity perpendicular to the magnetic field (0 < V⊥x < 100 km/s) in the plasma sheet (plasma β > 0.1), averaged over 1 RE × 1 RE bins in the geocentric solar magnetospheric (GSM) XY plane during the growth, expansion, and recovery phases of substorms. Panels (a–c) use combined data from THEMIS-A, D, and E; panels (d–f) use MMS-1 data. Each map represents a 15-minute average prior to the substorm onset, peak, and end. The black circle marks the geosynchronous orbit (~6.6 RE), and the gray shaded region indicates the inner magnetosphere (within 3 RE), which is excluded from analysis. Panels (g–i) show histograms comparing the V⊥x in dawn and dusk sectors, highlighting dawn–dusk variations in V⊥x for 0 < V⊥x < 100 km/s using MMS data. |
Figure 1 shows the distribution of slow earthward ion flows (0 < V⊥x < 100 km/s) during substorm phases, using data from THEMIS-A, D, E (a–c) and MMS-1 (d–f). In the growth phase, both missions observe that flows are strongest near Earth (X > −15 RE), especially on the dusk side, indicating a dawn–dusk asymmetry. During the expansion phase, flows extend toward the midnight and post-midnight regions. In the recovery phase, flow strength decreases, but dusk-side dominance persists.
Panels (g–i) present V⊥x histograms for dawn (blue) and dusk (yellow) sides, using a Two-Sample T-Test to assess statistical differences. The dusk side consistently shows stronger, more variable earthward flows, while the dawn side displays slower, more stable flows. This confirms a persistent dawn–dusk asymmetry in ion convection during substorm phases, as illustrated in Figure 1 (a-f).
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| Figure 2. Top panels (a-f): These panels present the average ion number density within Earth’s plasma sheet during various substorm phases (Onset-15, Peak-15, End-15), as observed by the MMS-1 (a-c) and THEMIS-A, D, E (d-f) (2016-2020). The squares in the grid represent a region of 1 RE by 1 RE . Bottom panels (a-f): Similar to the top panels, these panels show the average ion pressure in the different substorm phases observed by the MMS-1 (a-c) and THEMIS-A, D, E (d-f). |
Upper panels (a–c) of Figure 2 show average ion density from MMS-1 in 1 RE × 1 RE bins during substorm phases. In the growth phase (a), highest densities occur mainly post-midnight beyond X > −25 RE, with some near Earth on the pre-midnight side (X > −15 RE). During expansion (b), high densities appear near and just outside the geosynchronous orbit, especially post-midnight. By late recovery (c), densities form a narrow band near the geosynchronous orbit, showing clear dawn-dusk asymmetry. Panels (d–f) show ion densities from THEMIS-A, D, E spacecraft, confirming the dawn-dusk asymmetry seen in MMS data. However, THEMIS densities are more spatially confined (mostly within X > −8 RE and −10 < Y < 5 RE) and remain relatively stable across all substorm phases.
Lower panels (a–c) show ion pressure from MMS-1, with highest pressures near Earth in the post-midnight region during growth. Pressure increases near and outside the geosynchronous orbit in expansion and late recovery phases, following density trends and showing a clear dawn-dusk asymmetry with increasing pressure and narrowing distribution over time. Lower panels (d–f) show ion pressure from THEMIS-A, D, E confirming a dawn-dusk asymmetry. Compared to MMS, THEMIS data are more confined (mostly within X > −10 RE, −10 < Y < 5 RE) and stable, with less spatial variation across substorm phases.
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| Figure 3. A series of histograms show the occurrence of V⊥x values on the dawn and dusk sides for density (a–c) and pressure (d–f) during Onset−15, Peak−15, and End−15, using MMD data. These histograms illustrate the variations in these parameters between the two regions, highlighting potential differences in their distributions. |
Figure 3 shows histograms of dawn-dusk asymmetry in ion density (a–c) and pressure (d–f) across substorm phases, with statistical differences assessed using a Two-Sample T-Test. For density (a–c), dusk-side values dominate during growth (a), but in expansion and recovery (b–c), higher densities occur more often on the dawn side, with dusk values peaking lower. Similarly, pressure histograms (d–f) show no significant asymmetry during the growth phase, but reveal consistently higher-pressure values on the dawn side during expansion and recovery, mirroring the density patterns. These results strongly support the dawn-dusk asymmetry patterns observed in Figure 2.
Conclusion
In summary, we analyzed a five-year dataset from THEMIS-A, D, E, and MMS-1 to study earthward ion flows, along with average ion density and pressure during substorm phases in Earth’s magnetotail plasma sheet. Our results show that ion flows averaged up to around 35 km/s near Earth (X > −15 RE) as observed by THEMIS, and up to approximately 60 km/s further tailward (X < −15 RE) according to MMS-1 data, exhibiting a consistent dawn-dusk asymmetry in V⊥x. Additionally, a pronounced dawn-dusk asymmetry was evident in both ion density and pressure from THEMIS and MMS data, with higher values observed on the dawn side throughout the substorm phases.
Biographical Note
Sanjay Kumar is a postdoctoral researcher in the Department of Climate and Space Sciences and Engineering at the University of Michigan, Ann Arbor. His research primarily focuses on magnetospheric processes, particularly substorms and related phenomena in the magnetotail.
References
Kumar, S., Pulkkinen, T. I., and Pitkänen, T. (2025). Ion moment variability across substorm phases: Statistical insights. Journal of Geophysical Research: Space Physics, 130, e2024JA032953. https://doi. org/10.1029/2024JA032953
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