Energetic ($<$ 2 MeV) ion fluxes measured by ASPEX-STEPS on board Aditya-L1 during its earth-bound phase

TL;DR

This study analyzes energetic ion fluxes measured by ASPEX-STEPS on the Aditya-L1 spacecraft during its earth-bound phase, revealing how interplanetary magnetic field polarity influences ion spectra and source contributions.

Contribution

It provides the first detailed spectral analysis of energetic ions in the Earth's magnetosphere during ICME events using ASPEX-STEPS data, highlighting the role of IMF polarity.

Findings

IMF $B_z$ polarity modulates energetic ion spectra.

Spectral indices indicate source contributions vary with IMF conditions.

Directional anisotropy suggests inhomogeneous ion mixing.

Abstract

During its earth-bound phase of the Aditya-L1 spacecraft of India, the Supra-Thermal and Energetic Particle Spectrometer (STEPS) of the Aditya Solar wind Particle EXperiment (ASPEX) was operated whenever the orbit was above 52000 km during 11 - 19 September 2023. This phase of operation provided measurements of energetic ions (with energies 0.1--2 MeV) in the magnetosphere, magnetosheath, and interplanetary medium. Three interplanetary coronal mass ejections (ICME) hit the magnetosphere during this period. This provided opportunity to examine the relative roles of ICME-generated solar energetic particles (SEPs) and substorm generated energetic ions on the magnetosphere. We approach this objective by detailed spectral analyses of energetic ion fluxes measured by two units of ASPEX-STEPS. We identify three distinctly different conditions of the north-south component of the interplanetary magnetic field (IMF $B_z$ = 0, $>$ 0, and $<$ 0) and use the derived spectral indices to understand this relative role. By combining these with the simultaneous energetic ion flux variations from the Advanced Composition Explorer (ACE) around the Sun-Earth first Lagrangian (L1) point and the Geostationary Operational Environmental Satellite (GOES) in the Earth's magnetosphere, we show that the polarity of IMF $B_z$ influences the energetic ion spectra in the magnetosphere by modulating the interplay of the ICME-generated SEP with the energetic particles generated inside the magnetosphere by substorms. Interestingly, ASPEX-STEPS observations also indicate towards directional anisotropy based on spectral indices. This suggests spatially inhomogeneous mixing of energetic ions coming from different source processes.