  An interplanetary (IP) shock wave was recorded crossing the Magnetospheric
Multiscale (MMS) constellation on 2018 January 8. Plasma measurements upstream
of the shock indicate efficient proton acceleration in the IP shock ramp: 2-7
keV protons are observed upstream for about three minutes (~8000 km) ahead of
the IP shock ramp, outrunning the upstream waves. The differential energy flux
(DEF) of 2-7 keV protons decays slowly with distance from the ramp towards the
upstream region (dropping by about half within 8 Earth radii from the ramp) and
is lessened by a factor of about four in the downstream compared to the ramp
(within a distance comparable to the gyroradius of ~keV protons). Comparison
with test-particle simulations has confirmed that the mechanism accelerating
the solar wind protons and injecting them upstream is classical shock drift
acceleration. This example of observed proton acceleration by a low-Mach,
quasi-perpendicular shock may be applicable to astrophysical contexts, such as
supernova remnants or the acceleration of cosmic rays.
