The height of flare emissions in white light and in hard X-rays

Hard X-ray emission has long been known to correlate well with white-light continuum emission by a solar flare, providing support for the thick-target model of interactions by nonthermal electrons accelerated in the impulsive phase of a solar flare. This model makes specific predictions for the height, in the solar atmosphere, where such emissions can form. In this presentation we extend our earlier work (Martinez Oliveros et al., ApJ 753, 26, 2012) on the determination of the absolute heights of white-light flares and their associated hard X-ray sources. The new work makes use of surveys of the HMI flare observations, which provide a new database with excellent properties for this purpose in conjunction with STEREO and RHESSI. In the earlier work, based on the flare SOL2011-02-24, we found the white-light and hard X-ray sources to coincide, and to occur close to their minimum possible heights in terms of model optical depth. This conclusion is now confirmed with three additional flares: SOL2011-01-28T01:03 (M1.3), SOL2013-05-13T02:17 (X1.7), and SOL2013-05-13T16:01 (X2.8). The relatively low absolute altitude of the hard X-ray sources (of order 500 km above the level of the photosphere, at 500 nm optical depth unity) presents a puzzle for the standard thick-target model, and we discuss some possible explanations.