Abstract

Solar flare particle acceleration is an extremely efficient process and understanding the acceleration mechanism remains a major challenge in solar physics. Hard X-ray (HXR) observations provide a direct link to flare-accelerated electrons and HXR directivity is a measure of the electron angular distribution, a prime diagnostic of the unknown electron acceleration mechanism, and various transport properties in the solar corona. However, to- date, HXR directivity has been difficult to measure, with different methods providing conflicting and often unreliable results. However, with the launch of Solar Orbiter/STIX and an upcoming fleet of X-ray instrumentation at Earth (e.g., ASO -S/HXI, Aditya/ HEL1OS , PADRE), reliable stereoscopic observations measuring HXR directivity are now possible. Here, I will discuss the importance of measuring HXR /electron directivity during flares, show the first results of joint stereoscopic measurements with Solar Orbiter/ STIX and Fermi, and explain how the electron directivity can be extracted from observations via comparison with current electron and X-ray modelling.