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 (Saturnʼs 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 Saturnʼs 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 the 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).

An in situ Comparison of Electron Acceleration at Collisionless Shocks under Differing Upstream Magnetic Field Orientations

“…This study follows directly from the results presented by Masters et al (2016), who searched for evidence of electron acceleration at these Cassini shock crossings. These authors identified three crossings with particularly strong energetic electron signatures, which cannot be explained as a result of leakage of energetic electrons from within Saturn's magnetic field environment.…”

“…These crossings took place predominantly on the dayside of the shock surface, and under a range of upstream conditions. Data taken during shock crossings by Cassini have been surveyed by Sulaiman et al (2016) and Masters et al (2016), who discussed the extent of information about each crossing that can be extracted from Cassini data sets. These studies provide an estimated Alfvén Mach number of each crossing, as well as a separation of the crossings by shock geometry (quasiparallel/quasi-perpendicular).…”

“…We refer the reader to Masters et al (2011Masters et al ( , 2016 and Sulaiman et al (2016) for a discussion of the information that can be reliably extracted from Cassini data taken at Saturn's bow shock. The location of the shock crossing shown in Figure 1 and the mean upstream magnetic field strength of ∼0.8 nT indicates an Alfvén Mach number of M A ∼15 (assuming a stationary shock in the planetary rest frame; see Sulaiman et al 2016).…”

Whistler mode waves upstream of Saturn

“…Observations first made at Earth and more recently extended to Saturn have shown a relationship between M w and electron acceleration [ Oka et al ., ; Masters et al ., ]. The spectral index of the electron energy spectrum was found to be controlled by MA / M w — i.e., electron acceleration was prominent at supercritical shocks.…”

“…There are two critical Mach numbers associated with quasi-perpendicular collisionless shocks. The first represents the upper limit in solar wind flow speed at which the shock has the capacity to sufficiently dissipate the incident flow by anomalous resistivity and/or wave dispersion [Marshall, 1955;Kennel et al, 1985]. Above this, the shock adopts an alternative mechanism, namely, ion reflection, to convert the excess ram energy into thermal energy [Paschmann et al, 1981;Treumann, 2009].…”