Evolution of interplanetary CME-driven shocks from remote-sensing and in-situ observations

Coronal mass ejections (CME)-driven shocks are the most efficient accelerators of gradual solar energetic particles (SEPs), which pose risks to technological infrastructure and human activity in space. Knowing the physical properties of expanding shocks is critical in order to prevent SEPs hazard and to understand their impact to the near-Earth environment. However, a thorough picture on how the properties of shocks evolve from the corona to the heliosphere remains poorly constrained.  We present a study of a unique event, a shock driven by a circumsolar CME on 2023 March 13, observed from multiple spacecraft, using both remote sensing observations from STEREO-A/COR2 and in-situ data from Parker Solar Probe (PSP), Solar Orbiter (SolO), and Wind. We focused on the determination of some key parameters, such as the density compression ratio and the Alfvénic Mach number. The analysis of remote-sensing data has required advanced modelling of the 3D geometry of the observed shock complemented by raytracing simulation of the Thomson scattered emission, which was compared with the brightness measured from STEREO-A/COR2.
Following the evolution of the parameters, we have found that closer to the Sun, both the density compression ratio and the Alfvénic Mach number remain almost constant, while they increase at larger radial distances. These results highlight a non-trivial evolution of the properties of the shock during its journey throughout the interplanetary medium, with implications for SEP acceleration and space-weather forecasting.

This study was carried out within the "Data-based predictions of solar energetic particle arrival to the Earth: ensuring space data and technology integrity from hazardous solar activity events" (CUP H53D23011020001) funded by Next Generation EU’ PIANO NAZIONALE DI RIPRESA E RESILIENZA (PNRR), and the Space It Up! project, funded by the Italian Space Agency (ASI) and the Ministry of University and Research (MUR), under Contract Grant Nos. 2024-5-E.0-CUP n.I53D24000060005.