Modeling the astrosphere of LHS~1140

Abstract: The cosmic ray (CR) flux, as well as the hydrogen flux into the atmosphere of an exoplanet, can change the composition of the atmosphere. Here, we present the CR and hydrogen flux on top of the atmosphere. To do so, we have to study the 3D multifluid MHD structure of astrospheres. We discuss the shock structure of the stellar wind of LHS 1140 using four different models: HD and MHD single-fluid models, as well as multifluid models for both cases, including a neutral hydrogen flow from the interstellar medium. The CR flux in a multifluid model as well as the ionization rate in an exoplanetary atmosphere are also presented. The astrosphere is modeled using the 3D Cronos code, while the CR flux at LHS 1140 b is calculated using both a 1D and a 3D stochastic galactic CR modulation code. Finally, the atmospheric ionization and radiation dose is estimated using the AtRIS code. Results. It is shown that the 3D multifluid positions of the termination shock differ remarkably from those found in the 3D ideal-single fluid hydrodynamic case. CR fluxes computed using a 1D approach are completely different from those calculated using the 3D modulation code and show an essentially unmodulated spectrum at the exoplanet in question. Utilizing these spectra, ionization rates and radiation exposure within the atmosphere of LHS 1140 b are derived. The termination shock, astropause, and bow shock distances must be taken from the 3D multifluid MHD model to determine the CR fluxes correctly. Moreover, because of the tiny astrosphere, the exoplanet is submerged in the neutral hydrogen flow of the interstellar medium, which will influence the exoplanetary atmosphere. A 3D approach to Galactic\0 cosmic ray (GCR) modulation in astrospheres is also necessary to avoid unrealistic estimates of GCR intensities.