Imaging and Spectroscopic Analysis of High Energy Coronal Looptop Sources in Three Solar Flares Observed by RHESSI

We present hard X-ray (HXR) imaging and spectroscopic analysis of three solar flares with rarely seen looptop (LT) emission above 50 keV in the presence of intense footpoint (FP) emission during the impulsive phases as observed by RHESSI. All the LT sources are resolved from the images reconstructed by the MEM_NJIT and Pixon algorithms. The LT sources are nonthermal in nature and show spatial displacement with respect to the thermal loops. HXR observations of the three flares in general agree with the stochastic acceleration model. We focus on the 2003 November 03 solar flare, in which we find that a compact LT source appears even above 100 keV that is spatially distinct from the thermal loop. The LT spectra can be fitted with a thermal function plus a power-law, and the index difference between the LT and FPs is much smaller than commonly seen during the impulsive phase. Theoretical arguments show that the escape time increasing with energy for electrons in the LT acceleration region can account for the small spectral index difference. We also find a second LT up to 80 keV with lower intensity lying above the first LT as seen from the Clean images with slightly softer spectra. We argue that the very energetic LT source in the 2003 November 03 solar flare should be physically real and not much affected by the pulse pileup effect. In the 2002 October 31 solar flare, we find two LT sources above the solar limb with the second LT resolved from the Clean (first grid included) images. In the 2005 September 08 solar flare, we resolve two loops in the HXR images. We further generate the electron flux spectral images based on regularization techniques for the three flares and then obtain the flux spectra for the accelerated electrons in the LT acceleration region and effective radiating electrons in the thick-target FP regions. Based on the spatially resolved electron flux spectra, we derive the energy dependence of the escape time for electrons in the acceleration region and discuss the physical implication for particle acceleration in solar flares.