Astrospheric evolution and cosmic-ray particle transport

Abstract

The modulation of galactic cosmic rays (GCRs) in astrospheres requires knowledge of large and small scale structures and plasma parameters. These are known for the heliosphere from different spacecraft observations, but observations of all the required parameters are not generally possible or available for astrospheres. Therefore, knowledge of heliospheric conditions needs to be applied. This study presents the use of magneto-hydrodynamic models to calculate some of the plasma parameters that are needed as input for a cosmic ray modulation model. For Luminous Blue Variable (LBV) type stars, periodic variation in their stellar outflow parameters are found to have no influence on their shock structure. However, an eruptive event is able to increase the size of their astrospheres by pushing the astropause or bow shock further out. A one-dimensional stochastic differential equation (SDE) transport model is used to calculate intensities in an astrosphere of a LBV-type star and Proxima Centauri. The calculated intensities in a LBV-type star are higher than observed within the heliosphere, while intensities in Proxima Centauri are higher than reported by other studies; however, the influence of the eruptive event on GCR intensities highlighted the limitations implicit in 1D transport modelling by overestimating diffusive shock acceleration which may be reduced by 3D effects such as drifts. A three-dimensional SDEtransport model is therefore used to calculate intensities at exoplanet locations within the selected astrospheres of Proxima Centauri, Trappist-1, LHS 1140 and GJ 436. The investigation of GCR intensities pointed out the importance of the stellar rotation period and winding angle of the astrospheric magnetic field for the modulation of GCRs. For Proxima Centauri this led to counter-intuitive results based on what is known for the heliosphere, and further emphasises the need for three-dimensional models to be used to model the transport of these particles.