  We study the physics of electron acceleration at collisionless shocks that
move through a plasma containing large-scale magnetic fluctuations. We
numerically integrate the trajectories of a large number of electrons, which
are treated as test particles moving in the time dependent electric and
magnetic fields determined from 2-D hybrid simulations (kinetic ions, fluid
electron). The large-scale magnetic fluctuations effect the electrons in a
number of ways and lead to efficient and rapid energization at the shock front.
Since the electrons mainly follow along magnetic lines of force, the
large-scale braiding of field lines in space allows the fast-moving electrons
to cross the shock front several times, leading to efficient acceleration.
Ripples in the shock front occuring at various scales will also contribute to
the acceleration by mirroring the electrons. Our calculation shows that this
process favors electron acceleration at perpendicular shocks. The current study
is also helpful in understanding the injection problem for electron
acceleration by collisionless shocks. It is also shown that the spatial
distribution of energetic electrons is similar to in-situ observations (e.g.,
Bale et al. 1999; Simnett et al. 2005). The process may be important to our
understanding of energetic electrons in planetary bow shocks and interplanetary
shocks, and explaining herringbone structures seen in some type II solar radio
bursts.
