Suprathermal electrons at Saturn's bow shock

Abstract

The leading explanation for the origin of galactic cosmic rays is particle acceleration at the shocks surrounding young supernova remnants (SNRs), although crucial aspects of the acceleration process are unclear. The similar collisionless plasma shocks frequently encountered by spacecraft in the solar wind are generally far weaker (lower Mach number) than these SNR shocks. However, the Cassini spacecraft has shown that the shock standing in the solar wind sunward of Saturn (Saturn's bow shock) can occasionally reach this high-Mach number astrophysical regime. In this regime Cassini has provided the first in situ evidence for electron acceleration under quasi-parallel upstream magnetic conditions. Here we present the full picture of suprathermal electrons at Saturn's bow shock revealed by Cassini. The downstream thermal electron distribution is resolved in all data taken by the low-energy electron detector (CAPS-ELS, <28 keV) during shock crossings, but the higher energy channels were at (or close to) background. The high-energy electron detector (MIMI-LEMMS, >18 keV) measured a suprathermal electron signature at 31 of 508 crossings, where typically only the lowest energy channels (<100 keV) were above background. We show that these results are consistent with theory in which the "injection" of thermal electrons into an acceleration process involves interaction with whistler waves at the shock front, and becomes possible for all upstream magnetic field orientations at high Mach numbers like those of the strong shocks around young SNRs. A future dedicated study will analyze the rare crossings with evidence for relativistic electrons (up to ~1 MeV).

arXiv comment22 pages, 5 figures. Accepted for publication in ApJ

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Cited by in corpus (9)

• Observational Evidence for Stochastic Shock Drift Acceleration of Electrons at the Earth's Bow Shock
• PIC Simulation Methods for Cosmic Radiation and Plasma Instabilities
• Evolution of ion acoustic solitary waves in pulsar wind
• Kinetic simulations of nonrelativistic perpendicular shocks of young supernova remnants. II. Influence of shock-surfing acceleration on downstream electron spectra
• Statistical study of magnetosheath jet-driven bow waves
• Electromagnetic Ion-Ion Instabilities in Space Plasmas: Effects of Suprathermal Populations
• An in situ comparison of electron acceleration at collisionless shocks under differing upstream magnetic field orientations
• Whistler mode waves upstream of Saturn
• Multi-scale simulations of particle acceleration in astrophysical systems