24 research outputs found
Isolation and Phase-Space Energization Analysis of the Instabilities in Collisionless Shocks
We analyze the generation of kinetic instabilities and their effect on the
energization of ions in non-relativistic, oblique collisionless shocks using a
3D-3V simulation by , a hybrid particle-in-cell code. At
sufficiently high Mach number, quasi-perpendicular and oblique shocks can
experience rippling of the shock surface caused by kinetic instabilities
arising from free energy in the ion velocity distribution due to the
combination of the incoming ion beam and the population of ions reflected at
the shock front. To understand the role of the ripple on particle energization,
we devise the new instability isolation method to identify the unstable modes
underlying the ripple and interpret the results in terms of the governing
kinetic instability. We generate velocity-space signatures using the
field-particle correlation technique to look at energy transfer in phase space
from the isolated instability driving the shock ripple, providing a viewpoint
on the different dynamics of distinct populations of ions in phase space. We
generate velocity-space signatures of the energy transfer in phase space of the
isolated instability driving the shock ripple using the field-particle
correlation technique. Together, the field-particle correlation technique and
our new instability isolation method provide a unique viewpoint on the
different dynamics of distinct populations of ions in phase space and allow us
to completely characterize the energetics of the collisionless shock under
investigation.Comment: 32 pages, 14 figures, accepted by the Journal of Plasma Physic
The Importance of Heat Flux in Quasi-Parallel Collisionless Shocks
Collisionless plasma shocks are a common feature of many space and
astrophysical systems and are sources of high-energy particles and non-thermal
emission, channeling as much as 20\% of the shock's energy into non-thermal
particles. The generation and acceleration of these non-thermal particles have
been extensively studied, however, how these particles feed back on the shock
hydrodynamics has not been fully treated. This work presents the results of
self-consistent hybrid particle-in-cell simulations that show the effect of
self-generated non-thermal particle populations on the nature of collisionless,
quasi-parallel shocks. They contribute to a significant heat flux density
upstream of the shock. Non-thermal particles downstream of the shock leak into
the upstream region, taking energy away from the shock. This increases the
compression ratio, slows the shock down, and flattens the non-thermal
population's spectral index for lower Mach number shocks. We incorporate this
into a revised theory for the Rankine-Hugoniot jump conditions that include
this effect and it shows excellent agreement with simulations. The results have
the potential to explain discrepancies between predictions and observations in
a wide range of systems, such as inaccuracies of predictions of arrival times
of coronal mass ejections and the conflicting radio and x-ray observations of
intracluster shocks.
These effects will likely need to be included in fluid modeling to accurately
predict shock evolution.Comment: 7 pages, 3 figures, a lot of appendi