Investigation on upstream ion events from L1 point observation: New Insights

TL;DR

This study analyzes upstream ion events at L1, revealing spectral softening related to ion mass-to-charge ratios and suggesting cross-field diffusion influenced by magnetic field orientation as key factors in ion acceleration and modulation.

Contribution

It provides new insights into the spectral characteristics and origin of upstream ions, highlighting the role of cross-field diffusion and magnetic field orientation near Earth's bow shock.

Findings

Upstream ions show a spectral softening compared to solar wind ions.

Energy cut-off for upstream events is observed below 300 keV.

Spectral softening correlates with decreasing mass-to-charge ratio.

Abstract

Origin of energetic upstream ions propagating towards the Sun from the Earth's bow shock is not understood clearly. In this letter, relationship between solar wind suprathermal and upstream ions has been investigated by analyzing fluxes of H, 4He, and CNO obtained from multidirectional in-situ measurements at the first Lagrange point of the Sun-Earth system during 2012-2014. 49 upstream events have been selected based on flux enhancements of the upstream ions in comparison with the solar wind suprathermal ions. An energy cut-off at less than 300 keV is observed for the upstream events. This is attributed to the efficacy of the particle acceleration process near the bow shock. Interestingly, spectra of upstream ions soften systematically as compared to the spectra of their solar wind counterpart with decreasing mass of elements. The degree of spectral softening increases with decreasing mass-to-charge ratio of the species. Since during most of the events the interplanetary magnetic field was radial, we argue that cross-field diffusion of upstream ions gives rise to the modulation (spectral softening) of upstream ions, which is dependent on the mass-to-charge ratio of species. Our work indicates towards a systematic change in solar wind suprathermal ions after interaction with the bow shock.