Nonlinear Kinetic Instabilities in Plasma Wakes

Abstract:

Relative motion of a plasma and an embedded perturbing solid object
produces a plasma wake, which is kinetically unstable. For moons,
asteroids, spacecraft, probes, and planets without a magnetosphere the
response is dominantly electrostatic, although generally with a
background magnetic field. Using high-fidelity particle-in-cell
simulations, we have observed the development of kinetic instabilities
and their non-linear consequences in representative wakes. We have
also explained the observations with semi-analytical non-linear
theory. The ion and electron distribution function shapes are strongly
perturbed in the wake region. The ions form two opposite beams
directed inward along the guiding magnetic field, in part because of
the attraction of the wake's electric potential well. The electron
distribution forms a notch or dimple (of reduced phase space density)
localized in velocity to orbits that dwell near the wake axis (because
of repulsion). Those orbits are de-energized by cross-field drift down
the potential-energy ridge. The resulting Langmuir instability spawns
electron holes. The holes that move faster than the ion beams are
accelerated out of the wake by its electrostatic field without growing
substantially. Some holes, however, remain in the wake at essentially
zero parallel velocity. They grow, as a result of the same mechanism
that formed the notch: cross-field drift from a lower to a higher
density. When the density rises by a factor of order two or three,
they grow large enough to perturb the ions, tap their free energy, and
disrupt the ion streams well before they would become ion-ion
unstable. Crucially, these processes depend strongly on the
ion/electron mass ratio and require close to physical ratio (1836) in
simulations, to reveal their characteristics. Electron holes arising
from these processes may be widely present and observable in space
plasma wakes.