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- Title
Energetic Ion Reflections at Interplanetary Shocks: First Observations From ARTEMIS.
- Authors
Zhou, Xiaoyan; Gedalin, Michael; Russell, Christopher T.; Angelopoulos, Vassilis; Drozdov, Alexander Y.
- 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. Key Points: Energetic ion reflections (EIRs) are observed at supercritical interplanetary shocks identified using Mc (Kennel, 1987, https://doi.org/10.1029/JA092iA12p13427)EIR ions that have energies in 4–25 keV or higher are speculated to backstream and escape shock upstream, even when θBn > 60°As an observational feature of ion viscosity, EIRs revealed the kinetic physics root of the MHD derived criticality (Kennel, 1987, https://doi.org/10.1029/JA092iA12p13427)
- Subjects
ELECTRON resists; MAGNETIC reconnection; ELECTRODYNAMICS; HYPOTHESIS; QUANTITATIVE research
- Publication
Journal of Geophysical Research. Space Physics, 2020, Vol 125, Issue 11, p1
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2020JA028174