Solar type II radio emission and the shock drift acceleration of electrons

Many of the properties of energetic electrons and ions accelerated by interplanetary shock waves can be understood in terms of the shock drift acceleration mechanism. In this paper we show that the shock drift acceleration of electrons can be responsible for solar type II radio bursts as well. We review the shock drift acceleration mechanism and show that the streaming distribution of reflected electrons produced upstream of the shock front by this mechanism can be unstable to the generation of electrostatic plasma waves, which in turn interact to produce the observed radio emission. We derive constraints upon the density and energy of suprathermal electrons required to produce a typical type II burst.

The production of type II emission by shock drift accelerated electrons requires that the shock normal be at a high angle (ψ) to the upstream magnetic field. For a 1000 km s^-1 shock in a 2 × 10⁶ K corona, a value of ψ ≈ 80° is required. Reflected electrons are not obtained, however, when ψ is within a few degrees of 90°. We argue that these requirements are consistent with observations and show that a curved shock front propagating across magnetic field lines can naturally result in herringbone structure, herringbone structure without a backbone, or band splitting, the result depending upon the orientation and radius of curvature of the shock front and magnetic field lines and upon the energy of the accelerated electrons. We interpret the occurrence of band splitting and backboneless herringbone structure to the lack of reflected particles when ψ is near 90°. The electrons responsible for each band originate from different regions of the shock front, and, therefore, the emission from each band is predicted to arise from a differential spatial location. Particles transmitted downstream of the shock may contribute to moving type IV radio emission.