Reconnection and energy release rates in a two-ribbon flare

Abstract

Aims. The aim of this study was to derive the local reconnection rate (coronal electric field) and the global reconnection rate (magnetic flux change rate), as well as the energy release rate (Poynting flux), in a two-ribbon flare from chromospheric/photospheric observations. Furthermore, we tested whether equal shares of positive and negative magnetic flux are involved in the flare process. Methods. A well-observed GOES M3.9 two-ribbon flare was analyzed. The required observables (ribbon expansion velocity, newly brightened area, and magnetic field strength at the ribbon front) were extracted from the TRACE 1600 Å and Kanzelhöhe Hα image time series, and a SOHO MDI magnetogram. Furthermore, the ratio of the converted positive vs. negative magnetic flux was determined. Both RHESSI hard X-ray 20-60 keV full-disk time profiles and subregion time profiles derived from a time series of RHESSI images in the same energy range were used as independent, observable proxies for the energy release rate. The RHESSI images were also used to localize the sites where the bulk of the energy was deposited by fast electrons. Results. We found good temporal correlations between the derived time profiles (local and global reconnection rate, Poynting flux) and observed HXR flux. The local reconnection-rate peak values ranged from 2.7 V cm-1 to 11.8V cm -1, whereas the positive and the negative magnetic flux covered by the flare emission were equal within 5-10%. Conclusions. The results indicate that the local reconnection rate, the global reconnection rate, and the energy release rate in a simple two-ribbon flare can be derived from chromospheric/photospheric observations. Furthermore, it was confirmed that equal shares of positive and negative magnetic flux participated in the reconnection process. © ESO 2006.