Suprathermal population associated with stream interaction regions observed by STEREO-A: New insights

TL;DR

This study investigates the variability of suprathermal ion populations associated with stream interaction regions at 1 au, revealing diverse spectral indices and proposing magnetic island dynamics as a key mechanism.

Contribution

It provides new insights into the mechanisms influencing suprathermal ion spectral indices, highlighting the role of magnetic islands near 1 au in spectral uniformity.

Findings

Spectral indices vary widely among elements during SIRs.

A unique case shows nearly identical spectral indices for different elements.

Magnetic islands near 1 au may cause spectral index uniformity.

Abstract

Stream interaction regions (SIRs) are often thought to be responsible for the generation of suprathermal population in the interplanetary medium. Despite the source being same, wide variations in spectral indices of suprathermal populations are observed at 1 au during SIRs. This poses significant uncertainty in understanding the generation of suprathermal ion populations by SIRs and indicates interplay of multiple source mechanisms. In the present work, by analyzing variations in suprathermal 4He, O, and Fe for 20 SIR events recorded by STEREO-A during 2007 - 2014, we find that the spectral indices of these elements vary in the range of 2.06-4.08, 1.85-4.56, and 2.11-4.04 respectively for 19 events. However, in one special case, all the three suprathermal elements show nearly identical (1.5) spectral indices. We offer possible mechanisms, which can cause significant variations in the spectral indices of suprathermal particles. More importantly, we show the possible role of merging and/or contraction of small-scale magnetic islands near 1 au in producing nearly identical spectral indices for three different elements with different first ionization potential and mass-to-charge ratio. The occurrence of these magnetic islands near 1 au also supports the minimal modulation in spectral indices of these particles. The role of a possible solar flare in generating these magnetic islands near the heliospheric current sheet is also suggested.