The elusive solar flares: characterizing the trigger of the Sun-heliosphere connection

The Sun manifests its activity across different temporal scales and via transient phenomena that start from solar flares and, through energetic particles, coronal mass ejections, and solar wind, impact the whole heliosphere. The understanding of the causality of this chain of events is hampered by the fact that several open issues still bother a full comprehension of the trigger of such chain, i.e., solar flares. The present talk aims to shed some light on two specific aspects of these elusive phenomena characterizing the active Sun: the determination of the volume of a thermal flaring emission, and the estimate of its effectiveness as particle accelerator. For the first problem, we will show that computer vision applied to hard X-ray observations provided by STIX on-board Solar Orbiter and HXI on-board ASO-S is able to provide the three-dimensional reconstruction of the solar flare thermal emission. For the second problem, the application of an inversion method to STIX visibilities will contribute to settle the long-standing issue concerning the determination of the acceleration rate associated with magnetic reconnection.