Different types of ion populations upstream of the 8 October 2013 interplanetary shock

TL;DR

This study reveals the coexistence of different suprathermal ion populations upstream of a single interplanetary shock, showing how shock strength and orientation influence ion distributions and wave activity.

Contribution

It is the first to identify multiple ion populations upstream of one interplanetary shock and links their properties to shock parameters and geometry.

Findings

Different ion populations observed upstream of the shock.

Ion energies are around 20 keV, higher than Earth's bow-shock.

ULF wave intensity varies with ion population and shock parameters.

Abstract

We show for the first time that different types of suprathermal ion distributions may exist upstream of a single interplanetary shock. ACE and the two ARTEMIS satellites observed a shock on 8 October 2013. The ARTEMIS P1 and P2 spacecraft first observed field-aligned ions (P1) and gyrating ions (P2) arriving from the shock. These were followed by intermediate ions and later by a diffuse population. At the location of the P2 the shock exhibited an Alfv\'enic Mach number of M$_A$=5.7 and was marginally quasi-perpendicular, ($\theta_{Bn}$=47$^\circ$). At P1 spacecraft the shock was weaker (M$_A$=4.9) and more perpendicular ($\theta_{Bn}$=61$^\circ$). Consequently the observed suprathermal ion and ultra low frequency wave properties were somewhat different. At P2 the ULF waves are more intense and extend farther upstream from the shock. The energies of field aligned and gyrating ions in the shock rest frame were $\sim$20~keV, which is much more than in the case of the stronger (M$_A$=6-7) Earth's bow-shock, where they are less than 10~keV