  Foreshock transients upstream of Earth's bow shock have been recently
observed to accelerate electrons to many times their thermal energy. How such
acceleration occurs is unknown, however. Using THEMIS case studies, we examine
a subset of acceleration events (31 of 247 events) in foreshock transients with
cores that exhibit gradual electron energy increases accompanied by low
background magnetic field strength and large-amplitude magnetic fluctuations.
Using the evolution of electron distributions and the energy increase rates at
multiple spacecraft, we suggest that Fermi acceleration between a converging
foreshock transient's compressional boundary and the bow shock is responsible
for the observed electron acceleration. We then show that a one-dimensional
test particle simulation of an ideal Fermi acceleration model in fluctuating
fields prescribed by the observations can reproduce the observed evolution of
electron distributions, energy increase rate, and pitch-angle isotropy,
providing further support for our hypothesis. Thus, Fermi acceleration is
likely the principal electron acceleration mechanism in at least this subset of
foreshock transient cores.
