Loop top hard X-ray emission in solar flares: Images and statistics

The discovery of hard X-ray sources near the top of a flaring loop by the Hard X-Ray Telescope on board the Yohkoh satellite represents a significant progress toward the understanding of the basic processes driving solar flares. In this paper we extend the previous study of limb flares by Masuda by including all Yohkoh observations up through 1998 August. We report that from 1991 October to 1998 August, Yohkoh observed 20 X-ray bright limb flares (where we use the same selection criteria as Masuda), of which we have sufficient data to analyze 18 events, including eight previously unanalyzed flares. Of these 18 events, 15 show detectable impulsive loop top emission. Considering that the finite dynamic range (about a decade) of the detection introduces a strong bias against observing comparatively weak loop top sources, we conclude that loop top emission is a common feature of all flares. We summarize the observations of the footpoint-to-loop top flux ratio and the spectral indexes. We present light curves and images of all the important newly analyzed limb flares. Whenever possible we present results for individual pulses in multipeak flares and for different loops for multiloop flares. We then discuss the statistics of the fluxes and spectral indexes of the loop top and footpoint sources, taking into account observational selection biases. The importance of these observations ( and those expected from the scheduled HESSI satellite with its superior angular spectral and temporal resolution) in constraining acceleration models and parameters is discussed briefly.