Partial limb-occultation will frequently provide view angles from which purely coronal emission (e.g. Hudson 1978) can be readily seen because the footpoint sources are occulted. This will allow us to study faint coronal sources that otherwise would have been lost in the limited dynamic range (≤20) of indirect imaging instruments. Coronal hard X-ray emission in the absence of hard X-ray footpoint emissions, but with the thermal flare loop still partially visible, is best studied for flares occurring up to ~20° behind the limb. This suggests that for a single spacecraft about 20% of the observed flares have occulted footpoints with the main flare loop still partially visible above the limb. The following figure shows RHESSI observations of partially occulted-limb flares. In at least 90% of all such events, non-thermal coronal emission is observed, most prominently during the rise of the thermal emission (Krucker & Lin 2008). Most often the non-thermal emission is seen close to the thermal loop, but occasionally from above the thermal flare loops similar to what was reported for the Masuda flare (Masuda et al. 1994). As electron acceleration is thought to occur in the corona, hard X-ray imaging spectroscopy provides crucial information on the location and spectrum of energetic electrons before they precipitate to the chromosphere. STIX will provide frequent partially limb-occulted observations at very high sensitivity and will be able to see up to 15 weaker coronal emissions than RHESSI. This will allow us to image hard X-ray emission from the corona at the highest ever sensitivity, free from the intense footpoint sources, thus providing unique information about the supra-thermal electrons closest to the site in the corona where their acceleration is believed to occur.
Coronal phenomena in hard X-rays associated with CMEs:
The partial occultation of a solar flare by the solar limb is an excellent opportunity for studying faint coronal HXR emissions without competition from the very bright emission of the footpoint sources. For flares occurring >25° behind the solar limb (i.e. occultation height larger than 0.1 of a solar radius), not are only the hard X-ray footpoints occulted, but also the main thermal and non-thermal emissions from the corona. Nevertheless, these highly occulted events also show hard X-ray emission (e.g. Kane et al. 1992) that is associated with fast backside CMEs (Krucker et al. 2007). The emission is faint but has a rather flat (hard) spectrum indicating that the emission is produced by non-thermal bremsstrahlung of energetic electrons. Multi-spacecraft observations reveal that the onset of the emission is simultaneous with the onset of the main hard X-ray emission seen in footpoints (e.g. Kane et al. 1992), but has a much larger source size that expands with time. These sources move rapidly (~1,000 km s-1) upwards (Hudson et al. 2001, Krucker et al. 2007) in the same direction as the associated CME. The high coronal emissions may be produced by flare-accelerated energetic (>10 keV) electrons trapped in magnetic structures related to the CME, or they may be accelerated in CME current sheets or other coronal magnetic restructuring related to the CME. However, the details are not understood. The relative number of non-thermal electrons is observed to be about 10% of the number of thermal electrons in the high coronal source and the pressure exerted by the non-thermal electrons may, therefore, be comparable to that of the thermal plasma itself. High-altitude coronal hard X-ray sources are believed to be a common phenomenon since RHESSI detected them in association with all fast (>1500 km s ) backside CMEs with flare locations between ~25° and ~50° behind the limb (Krucker et al. 2007). However, present-day observations are only sensitive to high coronal emissions related to large X-class flares. With the 15-times enhanced sensitivity of STIX when near the Sun, high coronal emission from backside CMEs related to M-class flares will regularly be detected.