Stellar Energetic Particle Transport in the Turbulent and CME-disrupted Stellar Wind of AU~Microscopii

TL;DR

This study uses simulations to analyze how energetic particles from AU Microscopii propagate through its turbulent stellar wind, especially after CME events, revealing significant flux fluctuations and particle confinement effects.

Contribution

It provides the first detailed modeling of energetic particle transport in AU Mic's stellar wind, including the impact of CMEs on particle flux and magnetic field structure.

Findings

Particle flux peaks 2-3 orders of magnitude higher than Earth's solar flux.

CME passage compresses magnetic fields, reducing particle scattering mean free path.

Post-CME magnetic field remains highly turbulent, affecting particle propagation.

Abstract

Energetic particles emitted by active stars are likely to propagate in astrospheric magnetized plasma turbulent and disrupted by the prior passage of energetic Coronal Mass Ejections (CMEs). We carried out test-particle simulations of $\sim$ GeV protons produced at a variety of distances from the M1Ve star AU~Microscopii by coronal flares or travelling shocks. Particles are propagated within the large-scale quiescent three-dimensional magnetic field and stellar wind reconstructed from measured magnetograms, and { within the same stellar environment following passage of a $10^{36}$~erg kinetic energy CME}. In both cases, magnetic fluctuations with an isotropic power spectrum are overlayed onto the large scale stellar magnetic field and particle propagation out to the two innnermost confirmed planets is examined. In the quiescent case, the magnetic field concentrates the particles onto two regions near the ecliptic plane. After the passage of the CME, the closed field lines remain inflated and the re-shuffled magnetic field remains highly compressed, shrinking the scattering mean free path of the particles. In the direction of propagation of the CME-lobes the subsequent EP flux is suppressed. Even for a CME front propagating out of the ecliptic plane, the EP flux along the planetary orbits highly fluctuates and peaks at $\sim 2 -3$ orders of magnitude higher than the average solar value at Earth, both in the quiescent and the post-CME cases.