A Study of Shock Acceleration Using Strong Turbulence Methods

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


List of articles from varies team members