CO-SPATIAL WHITE LIGHT AND HARD X-RAY FLARE FOOTPOINTS SEEN ABOVE THE SOLAR LIMB

Abstract

We report analysis of three solar flares that occur within 1? of limb passage, with the goal to investigate the source height of chromospheric footpoints in white light (WL) and hard X-rays (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 photosphere or 300?450 km above the limb height. Because of the extreme limb location of the events we study, emissions from such low altitudes are influenced by the opacity of the atmosphere and projection effects. STEREO images reveal that for SOL2012-11-20T12:36 the projection effects are smallest, giving upper limits of the absolute source height above the nominal photosphere for both wavelengths of ?1000 km. To be compatible with the standard thick target model, these rather low altitudes require very 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. If the WL emission is from low-temperature ( 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 not responsible for chromospheric evaporation of hot ( K) plasma, but that their energy is lost through radiation in the optical range.