The Solar Wind at ‘short scales’: Observations at second and sub-second cadences with PAS/SWA.

The Proton Alpha Sensor (PAS), part of the Solar Wind Analyzer (SWA) onboard Solar Orbiter, has been designed to measure the full 3D ion velocity distribution function (VDF) at time cadence larger than one sample per second, thus, faster than the typical proton cyclotron period (Burst mode). Unfortunately, due to software difficulties, this capability to explore ion kinetic processes has been only activated during the first months of operation (2020-mid 2021) so that the ‘normal mode’ cadence (1 VDF each 4 s) was systematically used from 2021 to 2024. After in-depth analysis, a new software version has finally been implemented at the end of 2024, restoring in part the PAS burst mode capability. As a result, an impressive set of continuous full 3D VDF measurements at 1 s cadence has been obtained in 2025. More recently, we have fully restored the PAS burst mode. Since December 2025, PAS is then operated at a continuous 1 s cadence, with 8 bursts of 5 minutes per day during which full 3D measurements at 2 or 4 Hz, thus below the proton cyclotron period, are performed. As illustrated by examples (waves, sharp gradients, turbulence, shocks), this obviously re-opens a window to study various dynamical plasma phenomena at ion kinetic scales.