Statistical Properties of Super-Hot Solar Flares

Observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) have shown that "super-hot" (T > 30 MK) plasma temperatures appear to be commonly achieved by intense, GOES M- and X-class flares. Recent studies of individual X-class flares (Caspi & Lin 2010, ApJ 725, L161; Longcope et al. 2010, SolPhys 267, 107) showed that the super-hot thermal component is spectrally and spatially distinct from the ubiquitous ~10-20 MK plasma observed by GOES, suggesting that the two populations are heated by different physical mechanisms. However, how and why some flares achieve super-hot temperatures while others do not, and on what physical parameters this depends, remains unknown; consequently, the origins of super-hot plasmas remain poorly understood. We present results from a survey of 37 M- and X-class flares observed by RHESSI and GOES, which show that the maximum plasma temperature measured by RHESSI is strongly correlated with GOES class, such that super-hot temperatures are achieved almost exclusively by X-class flares. This correlation is significantly steeper than that observed for the peak GOES temperature. The maximum (instantaneous) thermal energy of the RHESSI-observed plasma, which occurs after the peak RHESSI temperature, is also correlated with GOES class, and super-hot flares are observed to be strongly associated with high number densities and (instantaneous) thermal energy densities compared to cooler flares, suggesting that strong magnetic fields are a requirement for the formation of super-hot plasma. We discuss these results and their implications for flare plasma heating, temperature distribution evolution, and energy transport.