Energetic Ion Reflections at Interplanetary Shocks: First Observations From ARTEMIS

Xiaoyan Zhou, Michael Gedalin, Christopher T. Russell, Vassilis Angelopoulos, Alexander Y. Drozdov

Research output: Contribution to journalArticlepeer-review

4 Scopus citations

Abstract

Interplanetary shocks accelerate charged particles and have significant space weather effects. The widely accepted categorization of shocks uses the upstream plasma beta (βu), the shock angle (θBn), and the shock Mach number (M, i.e., the fast magnetosonic Mach number). In addition, two critical Mach numbers (Mc) classify shocks based on whether shock dissipation is provided by electron resistivity and conductance alone or also requires ion viscosity. The two critical Mach numbers were defined in studies by Edmiston and Kennel and by Kennel in the 1980s. The corresponding two transition points from subcritical to supercritical are referred to as critical Mach numbers Mc (EK84) and Mc(K87), respectively. In this study, we use the critical Mach numbers and magnetic overshoot as identifiers of supercritical shocks. When the ratio of M/Mc ≥ 1, and/or an overshoot presents, a shock is categorized as a supercritical shock. We found that in more than 80% of supercritical shocks (with M/Mc(K87) ≥ 1), in addition to the regular ion reflection around the magnetic foot, there is another group of ion reflections: energetic ion reflections (EIRs). Typically, regularly reflected ions range from ~1–4 keV, staying at or around the foot, while EIR ions range from ~4–25 keV (or higher), running minutes upstream of the shock, perhaps as a result of ion backstreaming and escaping. EIR is a newly discovered feature observed by Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) at supercritical interplanetary shocks, occurring even when θBn > 60°, which is excluded by simulations due to a strong suppression of the backstreaming reflection in such circumstances. The close correlation between EIR occurrence and the critical Mach number Mc(K87) revealed that the conceptual ion viscosity in the magnetohydrodynamics (MHD)-derived criticality has a root in kinetic microphysics.

Original languageEnglish
Article numbere2020JA028174
JournalJournal of Geophysical Research: Space Physics
Volume125
Issue number11
DOIs
StatePublished - 1 Nov 2020

Keywords

  • critical Mach number
  • energetic ion reflections
  • interplanetary shocks
  • ion viscosity
  • magnetic overshoot
  • shock criticality

ASJC Scopus subject areas

  • Space and Planetary Science
  • Geophysics

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