Coronal Hard X-Ray Emission Associated with Radio Type III Bursts

Abstract

We report on a purely coronal hard X-ray source detected in a partially disk-occulted solar flare by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) that is associated with radio type III bursts and a suprathermal electron event detected near 1 AU by the WIND 3-D Plasma and Energetic Particle (3DP) instrument. Several observational characteristics suggest that the coronal hard X-ray source is thin target bremsstrahlung emission from the escaping electrons that produce the radio type III bursts. The hard X-ray emission correlates in time with the radio type III bursts and originates from a radially elongated source in the corona with a length (~65 Mm) similar to typical coronal density scale heights. Furthermore, the difference between the hard X-ray photon spectral index (γ = 4.1 +/- 0.4) and the electron spectral index of the in situ observed event (δ_{in situ} = 2.9 +/- 0.3) is around 1, consistent with the thin target interpretation. A further test for the thin target scenario is to compare the number of electrons needed to produce the observed hard X-ray emission with the number of in situ observed electrons. However, the number of escaping electrons derived from the single-spacecraft WIND measurement is in the best case an order of magnitude estimate and could easily underestimate the actual number of escaping electrons. Using the WIND observations, the estimated number of escaping electrons is about an order of magnitude too low. Thus, the thin target interpretation only holds if the WIND measurements are significantly underestimating the actual number of escaping electrons. Future multispacecraft observations with STEREO, Solar Orbiter, and Sentinels will resolve this uncertainty.