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

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 ∼ 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 largescale 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 ∼ 2 − 3 orders of magnitude higher than the average solar value at Earth, both in the quiescent and the post-CME cases.

The energetic particle environment of a GJ 436 b-like planet

“…On the other hand, previous stellar wind simulations including CMEs (Alvarado-Gómez et al 2018) have suggested that strong solar-like CMEs may be suppressed by a largescale dipole stellar magnetic field of 75 G which would affect the corresponding stellar energetic particle fluxes. Fraschetti et al (2022) showed for the AU Mic system how the stellar energetic particle flux distribution reaching the planetary orbits is strongly altered by the passage of a CME. To complicate matters further, solar CME-CME interactions account for more than 25% of the major geomagnetic storms observed (Zhang et al 2007).…”

“…We relate the maximum energy of the stellar energetic particles accelerated by stellar flares to stellar magnetic field strength, following Rodgers-Lee et al (2021a), and show how this can be related to stellar X-ray luminosity. Previous studies have investigated the distribution of stellar energetic particles in the Trappist-1 and AU Mic systems (Fraschetti et al 2019(Fraschetti et al , 2022. In the latter case, following a perturbation of the interplanetary medium by a coronal mass ejection (CME).…”