Occulted Hard X-ray Flare Observations with Sentinels

Multi-spacecraft hard X-ray observations provided by the Sentinel mission will give different view angles of solar flares, including the possibility of partially occulted observations. Occultation of the main flare emission by the solar limb allows us to study fainter coronal emission that otherwise would be hidden by the limited dynamic range of the observations. Furthermore, it will allow to measure directivity and reconstruct the 3 dimensional geometry of X-ray sources. We use statistical results from RHESSI observations to derive the probability of occulted flare observations with Sentinels. A statistical study of 19 giant flares with fast CMES (v>1500 km/s) occurring behind the solar limb is presented. We find that all events occurring 45 degrees or less behing the limb (corresponding to an occultation height h < 0.4~Rsun) show X-ray enhancements, while the three events occurring more than 50 degrees (h>1~Rsun) behind the limb do not. This result indicates that on the average, two Sentinels will see the total flare, while one spacecraft will see the flare partially occulted. The most prominent event in our RHESSI survey will be discussed in detail. It has an occultation high of 0.3 solar radius, but still shows emission up to 80 keV. The observed hard X-ray source is extended, with a size of about 200", and moves upwards with a velocity of ~1000 km/s.