An overview of HMI off-disk flare observations

Abstract

Context. White-light continuum observations of solar flares often have coronal counterparts, including the classical ‘white-light prominence’ (WLP) phenomenon. Aims. Coronal emissions by flares, seen in the white-light continuum, have only rarely been reported. We seek to use modern data to understand the morphology of WLP events. Methods. We have identified a set of 14 examples of WLPs detected by the Heliospheric and Magnetic Imager (HMI) experiment on board the Solar Dynamics Observatory (SDO) satellite using a new online catalogue that covers 2011–2017. These WLPs invariably accompanied white-light flare (WLF) emission from the lower atmosphere from flares near the limb, as identified by hard X-ray images from the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI). HMI provides full Stokes information, and we have used the linear polarisations (Q and U) to distinguish Thomson scattering from cool material. Results. The event morphologies fit roughly into three categories – ejection, loop, and spike – but many events show multiple phenomena. Conclusions. The coronal white-light continuum, observed by HMI analogously to the observations made by a coronagraph, reveals many examples of coronal emission and dynamics. Using the Stokes linear polarisation, we estimate the masses of hot coronal plasma in 11 of the 14 events and find them to be similar to typical coronal-mass-ejection masses, but without exceeding 1015 g. We note that the HMI observations do not occult the bright solar disk and were not designed for coronal observations, resulting in relatively low signal-to-noise ratios. We therefore believe that future such observations with better optimisation will be even more fruitful.