Hmi and Rhessi Measurements of the Radial Location of Solar Flare Footpoints to Subarcsecond Accuracy
Wednesday, 17 December 2014: 9:00 AM
We report analysis of three solar flares that occur within one degree of limb passage, with the goal to investigate the source height of chromospheric footpoints in white light (WL) and hard X-rays (HXR). The optical observations are from the Helioseismic and Magnetic Imager (HMI) around 617.3 nm, providing high precision observations with an absolute positional accuracy in the radial direction below 0.1 arcsec (~70 km), as referred to the adjacent limb. The Reuven Ramaty Higher Energy Solar Spectroscopic Imager (RHESSI) gives HXR source centroids to a similar accuracy depending on counting statistics. The observed height of the emissions at either wavelength is influenced by the opacity of the atmosphere at that wavelength and the height must correspond to a radial distance from Sun center that is greater than the solar limb at that wavelength (~350 km for WL and ~450 km for HXR). We find the WL and HXR (~30 keV) centroids to be largely co-spatial and from similar heights for all events, with altitudes around 800 km above the height of the photosphere. The observed altitudes are limited by the uncertainty of the precise heliographic locations near the limb and the resulting projection effects. STEREO images reveal that for SOL2012-11-20T12:36 the projection effects are smallest, giving upper limits of the absolute source height of 979+-70 km for the WL emission and 926+-51 km for HXR source. Hence, the peak of the WL and HXR must be below 1000 km. To be compatible with the standard thick target model, these rather low altitudes require low ambient densities within the flare footpoints, in particular if the HXR-producing electrons are only weakly beamed. That the WL and HXR emissions are co-spatial suggests that the observed WL emission mechanism is directly linked to the energy deposition by flare accelerated electrons with energies above ~30 keV. If the WL emission is from low-temperature (~10 000 K) plasma as currently thought, the energy deposition by HXR-producing electrons above ~30 keV seems only to heat chromospheric plasma to such low temperatures. This implies that the energy in flare-accelerated electrons above ~30 keV is lost through radiation in the optical range rather than heating chromospheric plasma to coronal (> MK) temperatures.