Local-Time Asymmetries in Particle Pitch Angle Distributions and Waves During the September 2017 Geomagnetic Storm: MMS-RBSP Conjunction

Abstract

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 ΔL < 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.