Energy Release and Transport in Solar Flares

Solar flares are sudden brightenings of the solar atmosphere that usually occur near sunspots. Among other emissions, flares copiously emit X-rays, and here there are at least two problems. First, the energy spectrum of the electrons that emit the hard X-rays is not known. Second, the source of the hot coronal plasma that emits soft X -rays has been the subject of some controversy; chromospheric upflow are a popular candidate. There are indications that the coronal density and temperature affect the production and propagation of energetic electrons. This may be the key to understanding solar flare energy release. In this work soft X-ray resonance line spectra and observations of hard X-ray burst emission obtained with instruments on board the Yohkoh and Compton Gamma Ray Observatory satellites are analyzed for over thirty flares that exhibited upflows. We find that the hot upflows are visible in both hard and soft X-rays, and that the rate of increase of soft X-ray emission is apparently linked to the upflow. This is the Neupert Effect. It also seems that the decay of the hard X-ray burst is due to the density increase during the flare. The coronal density and the hard X-ray flare are connected by the processes responsible for the production and transport of energetic electrons. The energy rapidly injected into the energetic electrons during a solar flare is popularly conjectured to have been slowly built up in the corona as the coronal current system responds to changes in the photospheric boundary conditions. Magnetic reconnection, however, acts as a safety valve and tends to bleed excess free energy from the system. We study the stability of current sheets to the resistive tearing instability in a line-tied magnetic flux tube by numerical simulation, and calculate growth rates using an energy principle derived from a radical new model called averaged MHD (AMHD; Pfirsch and Sudan, 1994). We found reasonably good agreement between the resistive MHD and the AMHD calculations. We discuss the implications of these results for our understanding of solar flare energy release.