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 analyse the rare crossings with evidence for relativistic
electrons (up to ~1 MeV).