Enhanced ion acceleration due to high-shear tangential discontinuities
upstream of quasi-perpendicular shocks
Abstract
Collisionless shock waves are efficient ion accelerators. Previous
numerical and observational studies have shown that quasi-parallel (Q∥)
shocks are more effective than quasi-perpendicular (Q⊥) shocks at
generating energetic ions under steady upstream conditions. Here, we use
a local, 2D, hybrid particle-in-cell model to investigate how ion
acceleration at super-critical Q⊥ shocks is modulated when tangential
discontinuities (TDs) with large magnetic shear are present in the
upstream plasma. We show that such TDs can significantly increase the
ion acceleration efficiency of Q⊥ shocks, up to a level comparable to Q∥
shocks. Using data from the hybrid model and test particle simulations,
we show that the enhanced energization is related to the magnetic field
change associated with the discontinuity. When shock-reflected ions
cross the TD during their upstream gyromotion, the sharp field change
causes the ions to propagate further upstream, and gain additional
energy from the convection electric field associated with the upstream
plasma flow. Our findings illustrate that the presence of upstream
discontinuities can lead to bursts of energetic ions, even when they do
not trigger the formation of foreshock transients. These results
emphasize the importance of time-variable upstream conditions when
considering ion energization at shocks.