Phase Space Energization in Quasi-Perpendicular and Oblique Collisionless Shocks*

There is growing interest in characterizing the energetics of collisionless plasmas by directly diagnosing the energy transfer between the plasma and electromagnetic fields in phase space. Unlike bulk, fluid quantities such as J ● E, diagnostics like the field-particle correlation that leverage the full 6D phase space allow us to identify how separate populations of the plasma are energized. To demonstrate the power of this phase-space analysis, we have performed a suite of 3D hybrid-kinetic quasi-perpendicular and oblique collisionless shock simulations and characterized the energization of the ions via these shocks. In particular, we can cleanly separate the energization of the different populations of ions that reflect multiple times off the shock and thus gain additional energy via the classical shock-drift acceleration mechanism. We thus identify a new phase space energization signature, sometimes referred to as shock-surfing acceleration, which can be compared with observations on the quasi-perpendicular side of Earth’s bow shock. Such comparisons will prove vital for determining the importance of shock-drift versus shock-surfing acceleration in real collisionless shocks.