A Tale of Two Shocks

TL;DR

This study uses high-time-resolution data from Solar Orbiter to reveal microphysical processes near interplanetary shocks, such as ion reflection and non-equilibrium particle features, advancing understanding of particle acceleration in space plasmas.

Contribution

It provides the first detailed observations of microphysics at shocks using unprecedented high-time-resolution data, revealing phenomena like ion reflection and rapid particle variations.

Findings

Proton beams follow magnetic fields instantaneously on short timescales.

Energetic particles vary on the order of their gyro-radius convection time.

Ion reflection occurs at a current sheet near shocks.

Abstract

Energetic particles in interplanetary space are normally measured at time scales that are long compared to the ion gyroperiod. Such observations by necessity average out the microphysics associated with the acceleration and transport of 10s - 100s keV particles. We investigate previously unseen non-equilibrium features that only become observable at very high time resolution, and discuss possible explanations of these features. We use unprecedentedly high-time-resolution data that were acquired by the in situ instruments on Solar Orbiter in the vicinity of two interplanetary shocks observed on 2023-11-29 07:51:17 UTC and 2023-11-30 10:47:26 UTC at $\sim 0.83$ astronomical units from the Sun. The solar-wind proton beam population follows the magnetic field instantaneously, on time scales which are significantly shorter than a gyro-period. Energetic particles, despite sampling large volumes of space, vary on remarkably short time scales, typically on the order of the convection time of their gyro-radius. Non-equilibrium features such as bump-on-tail distributions of energetic particles are formed by small-scale magnetic structures in the IMF. High-time-resolution observations show previously unobserved microphysics in the vicinity of two traveling interplanetary shocks, including ion reflection at a current sheet, which may explain where ions are reflected in shock acceleration.