Different Types of Ion Populations Upstream of the 2013 October 8 Interplanetary Shock

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 2013 October 8. The ARTEMIS P 1 and P 2 spacecraft first observed field-aligned ions ( P 1) and gyrating ions ( P 2) arriving from the shock. These were followed by intermediate ions and later by a diffuse population. At the location of the P 2 the shock exhibited an Alfvénic Mach number of M A  = 5.7 and was marginally quasi-perpendicular ( ). At P 1 spacecraft the shock was weaker ( M A  = 4.9) and more perpendicular ( ). Consequently, the observed suprathermal ion and ultra-low-frequency wave properties were somewhat different. At P 2 the ultra-low-frequency 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 ∼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.