Shock Surface Undulation and Particle Acceleration at Oblique Shocks

Details Shock Surface Undulation and Particle Acceleration at Oblique Shocks Shock Surface Undulation and Particle Acceleration at Oblique Shocks

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsh54a..03k&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #SH54A-03

Other

2114 Energetic Particles (7514), 2139 Interplanetary Shocks, 7851 Shock Waves (4455), 7867 Wave/Particle Interactions (2483, 6984)

Scientific paper

Considering the average Parker spiral magnetic field configuration, CME-driven interplanetary (IP) shocks within 1 AU should have oblique portions over much of their domain. Indeed, CME-driven shocks observed close to Earth are often oblique. However, it is well known that the standard diffusive shock acceleration mechanism, which relies on self-consistent wave generation via upstream propagating ions and their scattering, becomes increasingly inefficient with greater shock normal angle. Not only is a higher threshold energy required for the ions to leave the shock upstream, but also, approximately-parallel propagating waves are more quickly convected back into the shock, and the growth rate for waves propagating normal to the shock (the ones with the largest convective growth) decreases. As a result, typical, small-scale hybrid simulations of oblique shocks only show a dilute upstream beam, similar to what is often observed at the oblique Earth's bow shock - and no scattered, highly-energized ions. On the other hand, there are many "energetic storm particle" (ESP) events associated with oblique shocks that have significant fluxes of energetic ions. Recently, we have found that when run for a long time, our hybrid simulations (kinetic ions, electron fluid) show that the initial, weak beam is sufficient to generate compressive, steepening upstream waves. These waves are capable of disturbing the shock surface, resulting in an undulation that is propagating along the surface and growing in amplitude over time. The process is akin to that of the well-known reformation occurring at sufficiently strong quasi-parallel shocks. However, here the perturbations require at least two dimensions, show a strong spatial correlation, and travel along the shock surface. This process not only leads to enhanced ion acceleration, but also means that the shock characteristics are difficult to pinpoint, observationally: both the local jumps and the shock normal angle are highly variable. Shock undulation is also of interest to electron acceleration, since the undulated surface gives locally much larger shock normal angles and provides multiple mirroring and escape opportunities to accelerated electrons. We compare our simulations with a set of oblique shocks that we compiled from ACE observations, and discuss the results in the context of developing quantitative models of the flux and spectrum of energetic ions at IP shocks.

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