Multidirectional Investigations on Quiet Time Suprathermal Ions Measured by ASPEX-STEPS on Board Aditya L1

Abstract

The origin, acceleration, and anisotropy of suprathermal ions in the interplanetary medium during quiet time have remained poorly understood issues in solar wind physics. Using measurements (in the energy range of 0.12−1.33 MeV n−1) by the four detectors that are part of the Supra-Thermal and Energetic Particle Spectrometer (STEPS) of Aditya Solar Wind Particle Experiment (ASPEX) on board the Aditya L1 spacecraft, we address the variations in spectral indices with directions in shorter durations during 2024 January–November, which coincides with the maximum phase of Solar Cycle 25. Three out of four detectors on STEPS—Parker Spiral, Intermediate, Earth Pointing—are in one plane, while the fourth detector—North Pointing—is in a mutually orthogonal plane. The derived spectral indices are found to be −1.99 ± 0.06 regardless of directions, suggesting directionally near isotropic behavior during quiet times. It is also shown that the influence of the Compton–Getting effect is negligible in this assessment of directional isotropy. This result has important ramifications as directional isotropy is assumed while solving the Parker transport equation to explain the acceleration of energetic particles. Further analysis of elemental abundance ratios (3He/4He, Fe/O, and C/O) during the same quiet times obtained from the Ultra Low Energy Isotope Spectrometer on board the Advanced Composition Explorer spacecraft suggests possible contributions from the leftover ions from the previous solar energetic particle events in the quiet time suprathermal ion pool.