Title: Particle acceleration in the Heliosphere, from its most extreme shocks to its quietest times Speaker: Domenico Trotta Abstract: Energetic particles pervade the heliosphere, from violent solar eruptions to the quietest solar wind, making it a unique in situ laboratory for processes that operate in distant astrophysical accelerators. Yet a fundamental question remains open: how does particle acceleration start in the first place? With this injection problem as a common thread, I will show examples that address it by leveraging new missions such as Solar Orbiter, from the most extreme acceleration events to the quietest background. I will first present Solar Orbiter observations of more than 170 interplanetary shocks, stressing key aspects of how shocks energise protons, from their overall acceleration efficiency to extreme events that approach regimes typical of astrophysical accelerators. I will also present a strong, quasi-perpendicular shock that accelerated electrons up to 15 MeV, the highest energies ever measured for locally accelerated electrons at a heliospheric shock. Starting from this event and combining observations from several missions, I will then show why a complete picture requires bringing together local acceleration processes and global, mesoscale effects. I will then turn to quiet times, when the solar wind still carries a persistent population of suprathermal “superhalo” electrons whose origin remains unresolved, and discuss how it connects to the processes seen during active periods. Finally, I will outline my research plan at Imperial as a Royal Society University Research Fellow from January 2027: tackling the injection problem by combining a unique set of heliospheric missions with numerical simulations, with the goal of turning injection from a free parameter of acceleration models into a predictable quantity.

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