  A leading explanation for the origin of Galactic cosmic rays is acceleration
at high-Mach number shock waves in the collisionless plasma surrounding young
supernova remnants. Evidence for this is provided by multi-wavelength
non-thermal emission thought to be associated with ultrarelativistic electrons
at these shocks. However, the dependence of the electron acceleration process
on the orientation of the upstream magnetic field with respect to the local
normal to the shock front (quasi-parallel/quasi-perpendicular) is debated.
Cassini spacecraft observations at Saturn's bow shock has revealed examples of
electron acceleration under quasi-perpendicular conditions, and the first in
situ evidence of electron acceleration at a quasi-parallel shock. Here we use
Cassini data to make the first comparison between energy spectra of locally
accelerated electrons under these differing upstream magnetic field regimes. We
present data taken during a quasi-perpendicular shock crossing on 2008 March 8
and during a quasi-parallel shock crossing on 2007 February 3, highlighting
that both were associated with electron acceleration to at least MeV energies.
The magnetic signature of the quasi-perpendicular crossing has a relatively
sharp upstream-downstream transition, and energetic electrons were detected
close to the transition and immediately downstream. The magnetic transition at
the quasi-parallel crossing is less clear, energetic electrons were encountered
upstream and downstream, and the electron energy spectrum is harder above ~100
keV. We discuss whether the acceleration is consistent with diffusive shock
acceleration theory in each case, and suggest that the quasi-parallel spectral
break is due to an energy-dependent interaction between the electrons and
short, large-amplitude magnetic structures.
