A Study of Shock
Acceleration
Using Strong Turbulence Methods
Abstract:
Space
plasma shocks and their
associated phenomena are of primary importance in cosmic
electrodynamics, due to their ubiquity in heliospheric and
astrophysical contexts. They condition plasmas downstream and
dramatically energize particles, which, for example, are believed to be
the primary source of cosmic rays. The collisionless shocks observed
near 1A.U. have provided a natural laboratory well suited for the study
of these structures, since Earth's bow shock and traveling
interplanetary shocks span a wide range of applicable parameter space.
The extensive dataset of space plasma shock encounters collected by
satellites within the past four decades has stimulated a tremendous
theoretical effort, including an increasing number of sophisticated
numerical experiments, in an attempt to understand associated
processes. Much particle energization appears to have been explained by
diffusive shock acceleration (DSA) models, but these weak turbulence
methods are often not valid for the strong turbulence that is observed,
and to date do not fully account for energetic particles seen in
association with strongly perpendicular shocks (70o < theta_Bn <
90o). In addition, recent detailed examinations of 3- dimensional ion
distributions in the foreshock exhibit non-thermal energetic tails that
cannot be readily explained by diffusive processes. A similar
conclusion is inferred from recent observations of the X-ray luminosity
of gamma ray bursts. Coherent non-linear structures such as Short
Large-Amplitude Magnetic Structures (SLAMS) are frequently seen in
foreshocks upstream of quasi-parallel shocks (theta_Bn < 45o), but
should not produce diffusive acceleration. Similarly, the nonlinear
cyclic shock-reformation processes expected to occur should likewise
alter the diffusive picture. We propose to bring together a diverse
team of theorists, simulators and observers from space physics and
astrophysics backgrounds to examine in detail the roles of strong
turbulence and singular structures on the acceleration of particles
near shocks. Observationally, our team will focus on energetic particle
distributions produced by shocks and on further elucidating the
descriptions of non-thermal particle distributions and the conditions
leading to them. Our theorists will focus on recent applications of
field theoretical methods to turbulence that hold promise for the study
of heliospheric and astrophysical plasmas. Kinetic simulations will
focus on modeling the roles of singular/non-linear structures in
particle acceleration. We seek support from ISSI to provide
opportunities for our multidisciplinary team to meet, discuss recent
results and share ideas to guide ongoing work, since this endeavor will
require occasional, but intensive, intercomminication. While individual
members of our group will write independent papers on their findings,
we propose to also jointly author a paper summarizing the progress from
the team effort, which we anticipate will have significant impact for
future work in space plasma particle acceleration applications.
List of participants
Program
List of articles
from varies team members