Seminar of October 29th, 2013
Particle Acceleration at Interplanetary Shock Waves
An outstanding problem in astrophysics is to explain the origin of the almost featureless cosmic ray spectrum extending up to energies of some 1020 eV. A very small feature is apparent at between about 1013 – 1015 eV, the “knee.” In the late 1970’s, a suite of papers was published establishing the idea of diffusive shock acceleration for cosmic rays, essentially a first-order Fermi mechanism, which appeared to provide an explanation for the observed cosmic ray spectrum up to the knee. Diffusive shock acceleration is probably the most widely used particle acceleration mechanism in astrophysics and space physics, yet the theory is based on some important simplifications. The detailed [plasma] physics of the acceleration mechanism requires elucidation. We are fortunate in that very detailed observations of particle acceleration at shock waves, particularly in the guise of Space Weather, are providing considerable experimental insight into the basic physics of particle acceleration at a shock wave. Indeed, understanding the problem of particle acceleration at interplanetary shocks is assuming increasing importance, especially in the context of understanding the space environment. Detailed interplanetary observations are not easily interpreted in terms of the simple original models of particle acceleration at shock waves. Three fundamental aspects make the interplanetary problem much more complicated than the typical astrophysical problem: the time dependence of the acceleration and the solar wind background; the geometry of the shock; and the long mean free path for particle transport away from the shock wave. An interplanetary shock is not steady, as it decelerates and expands into an expanding, temporal solar wind. Furthermore, the shock geometry varies from quasi-parallel to quasi-perpendicular along a shock front, and multiple shocks can be present simultaneously in the solar wind. Consequently, the shock itself introduces a multiplicity of time scales, ranging from shock propagation time scales to particle acceleration time scales at parallel and perpendicular shocks, and many of these time scales feed into other time scales (such as determining maximum particle energy scalings, escape time scales, etc.). We will discuss the basic physics of particle acceleration via scalings, their relationship to particle acceleration models, observations and geometry in both an astrophysical and space physics context. This will include discussing the physics of perpendicular and parallel shocks, upstream turbulence, particle spectra, and particle injection and the seed population. After acceleration of particles at an interplanetary shock, the transport of energetic particles is non-diffusive because of their large mean free path in the quiet solar wind. The complications of coupling diffusive (at the shock) to non-diffusive transport will be addressed. In particular, we will address the coupled acceleration and transport of heavy ions, Fe/O ratios, the variability among individual events, and seed particle populations. We will discuss theoretical models and address recent modeling efforts.
Prof. G.P. Zank - Center for Space Plasma and Aeronomic Research (CSPAR) and Physics Department, University of Alabama in Huntsville
Prof. Catia Grimani
Since seminar time and Data lessons time overlap, 2nd year students cannot request that credits be recognized.
|Aula Magna, Piazza della Repubblica 13, Urbino||October 29th, 2013||16:00-18:00||0.125|