Relating magnetic field strengths to hard X-ray emission in solar flares

Abstract

The observation of hard X-ray (HXR) emission in solar flares provides important diagnostic information about the acceleration and subsequent transport of energetic electrons in the flare process. However, while hard X-rays are thought to be emitted from the flare footpoints through thick-target bremsstrahlung interactions, the details of the transport of accelerated electrons through the solar atmosphere still remains unclear. Trapping of the electrons is one particular effect that is expected to occur as a result of the convergence of the magnetic field between the corona and the chromosphere. In this case the brightness of the HXR footpoints should be related to the strength of the magnetic field present and we would expect greater precipitation and higher HXR intensities at the footpoints with lower magnetic field strength. This relationship has been observed to hold in many flares (see Sakao 1994) but interestingly the opposite relationship, where the stronger HXR source is found at the stronger magnetic field region, has also been observed in an event studied by Asai et al. (2002). Using Data from Yohkoh's Hard X-Ray Telescope (HXT) and SOHO's Michelson Doppler Imager (MDI) we have studied the magnetic field strengths at the footpoints of a sample of 32 flares and have compared them to the hard X-ray brightness to determine whether the expected ratios are seen. We find that contrary to the expected relationship the brighter HXR footpoint is found in the region of stronger magnetic field in approximately one third of our sample of events. We discuss the implications of these results in terms of the transport mechanisms.