Shock-excited radio burst from reconnection outflow jet?

Abstract

Models of dynamic (two-ribbon-, arcade) flares involve the formation of a system of standing slow and possibly also fast mode shock waves associated with the fast reconnection process below the erupting filament. These shocks are anticipated theoretically, but are not unambiguously confirmed by observations. In this paper we identify for the first time the radio signature of a fast mode outflow termination shock in a dynamic radio burst spectrogram. The standing fast mode shock is revealed by a zero-drift type II burst recorded between 300 and 400 MHz. It started almost 1 hour after the impulsive phase of the 7 April 1997 flare and lasted for more than 30 min. The burst shows a characteristic herringbone fine structure and a band split of (10 ′ 6)% of the emission frequency. No fundamental-harmonic pattern was observed, and we argue that the feature is fundamental mode emission. Simultaneous imaging observations (Ha, Yohkoh SXT, SOHO EIT) show a relaxed postflare loop arcade with a bright soft X-ray cusp commonly interpreted as a typical reconnection pattern. Conditions for termination shock formation and excitation of radio emision are investigated. Favourable circumstances for the radio detection of a termination shock in the reconnection outflow are a comparatively large height of the diffusion region, a low plasma to magnetic pressure ratio β upstream of the slow shocks, and a small angle between the reconnecting field lines. Finally, we discuss why similar radio signatures are not observed more frequently, and why it appeared so late in the event. We stress the implications and point to some inconsistencies which might be a consequence of commonly practiced ad hoc application of idealized model results to realistic conditions.