Electron acceleration in a wavy shock front

Astronomical Institute of the Academy of Sciences of the Czech Republic, 25165 Ondřejov, Czech Republic
e-mail: vandas@ig.cas.cz

Received: 12 November 2010
Accepted: 22 April 2011

Abstract

Context. It is known that electrons are accelerated at nearly perpendicular shocks by the drift mechanism. And it is also known that energy gain of electrons caused by this mechanism is not very high. Therefore it was suggested in the past that the energy gain might be increased if shocks had wavy fronts. For instance, there were attempts to explain coronal type II burst and their fine structure by electron acceleration in a wavy shock front.

Aims. We studied electron acceleration numerically at nearly perpendicular wavy shocks for coronal conditions and compared it with analytical results on electron acceleration at nearly perpendicular plane shocks.

Methods. An analytical model of a wavy shock front was used and trajectories of electrons in it and around it were calculated numerically in a guiding centre approximation.

Results. We found that energy gains of electrons at a wavy shock front and a corresponding smoothed-into-plane shock on the average were comparable. That is why they do not depend significantly on the shock thickness, magnetic field profile inside the shock, and shock wavy form. They do depend on the angle between the smoothed shock front and ambient magnetic field.

Conclusions. On average, a wavy shock front does not significantly increase an acceleration efficiency. Energy gain remarkably exceeds an average level for some combinations of initial parameters. Distribution functions of accelerated electrons have a patchy structure, which is prone to inducing plasma instabilities that will generate plasma waves. This may have relevance to the problem of type II burst origin.