Study of the Emission Heights of White-Light Solar Flares and their Hard X-Ray Sources

A solar flare is the most energetic explosion observed in the solar system. Different ways of viewing solar flares include X-rays, radio and chromospheric spectral lines. A large percentage of flares are recognized as a result of associated brightening in the visible light spectrum, and thus are called white-light flares. White-light emission is a signature of large solar flares and can be observed even on other stars. Though the white light emission was the first manifestation of a solar flare ever detected, in the famous so-called Carrington event of September 1, 1959, its origin is still not understood. This study will compare observation and modeling of the heights in the solar atmosphere where the white-light and hard-X-ray flares originate and through this be able to elucidate and distinguish between competing flare generation mechanisms. In this way, the investigation tackles fundamental issues related to flare energy transfer in the lower solar atmosphere and the results will provide important information about the generation, energetics and dynamics of solar flares and coronal mass ejections, providing crucial understanding of the initiation of space weather events. The research effort will also have major consequences for the study of flaring stars, since the same acceleration and radiation generation processes are at work. These stellar flares are detected in white light, but hard X-ray emission from accelerated electrons is too faint to be directly measured. The results from this effort will better define the relationship between white light and hard X-ray emissions from solar flares. This information, in turn, will aid the investigation of stellar flares, including particle acceleration and atmospheric conditions of flaring stars.

It is puzzling that the actual mechanism by which solar flares generate white-light (WL) continuum radiation in the lower atmosphere is not better understood. The association of WL continuum with the impulsive phase of a solar flare, that is, the phase in which particles are accelerated, has been interpreted as an indication that non-thermal particles accelerated in the corona can penetrate deep into the lower solar atmosphere, heating those layers and causing intense WL emission. However, flare-accelerated electrons hitting the chromosphere have relatively short collisional ranges, and so their ability to penetrate deeply, as indicated by the close correlation of WL and hard X-ray (HXR) time profiles, is surprising. Other methods of generating the observed WL continuum include energy deposition by protons (instead of electrons) or 'radiative back-warming' by the UV or X-ray continua. Alfvenic wave transport of energy could also be significant, as could recombination radiation in the chromosphere. Comparison between the heights of WL and HXR emissions in the solar atmosphere will support or rule out many of these proposed generation mechanisms, thereby providing a greater understanding of WL flare generation as well as the overall energetics of solar flares. The main objective of this study is the determination of the heights in the solar atmosphere of white-light and hard X-ray (HXR) sources for a large sample of flares. Both absolute heights in the solar atmosphere and heights of the two types of emission relative to each other will be studied. The observed heights will be compared with the results of theoretical modeling performed via the radiation hydrodynamic model RADYN and the 3D radiative MHD model RADMHD. The derived heights and model comparisons will reveal in which layers of the outer solar atmosphere WL flare sources originate and will provide new insight into the generation of WL flare emission, an important and poorly-understood problem in solar physics. Source heights will be determined using data from the HMI instrument aboard the SDO spacecraft (for white-light sources), the RHESSI (for hard X-ray sources), and the EUVI instruments on the twin STEREO spacecraft, as context data, in order to provide an alternate viewing angleand thus precise height measurements. The ground-based GONG++ network will be used to validate the modeling results. Multiwavelength observations from the ROSA and IBIS instruments will be used to characterize white-light flare energetics.