Galactic cosmic rays on extrasolar Earth-like planets

I. Cosmic ray flux

¹ LPC2E – Université d’Orléans/CNRS, 3A, Avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France
e-mail: jean-mathias.griessmeier@cnrs-orleans.fr
² Station de Radioastronomie de Nançay, Observatoire de Paris – CNRS/INSU, USR 704 – Univ. Orléans, OSUC, route de Souesmes, 18330 Nançay, France
³ Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK
⁴ Technische Universität Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany
⁵ Zentrum für Astronomie und Astrophysik (ZAA), Technische Universität Berlin (TUB), Hardenbergstr. 36, 10623 Berlin, Germany
⁶ Now at: Extrasolare Planeten und Atmosphären (EPA), Institut für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Rutherfordstr. 2, 12489 Berlin, Germany
⁷ Blue Marble Space Institute of Science, 1200 Westlake Ave N Suite 1006, Seattle, WA 98109, USA

Received: 2 December 2014
Accepted: 22 May 2015

Abstract

Context. Theoretical arguments indicate that close-in terrestial exoplanets may have weak magnetic fields, especially in the case of planets more massive than Earth (super-Earths). Planetary magnetic fields, however, constitute one of the shielding layers that protect the planet against cosmic-ray particles. In particular, a weak magnetic field results in a high flux of Galactic cosmic rays that extends to the top of the planetary atmosphere.

Aims. We wish to quantify the flux of Galactic cosmic rays to an exoplanetary atmosphere as a function of the particle energy and of the planetary magnetic moment.

Methods. We numerically analyzed the propagation of Galactic cosmic-ray particles through planetary magnetospheres. We evaluated the efficiency of magnetospheric shielding as a function of the particle energy (in the range 16 MeV ≤ E ≤ 524 GeV) and as a function of the planetary magnetic field strength (in the range 0 M_⊕ ≤ M ≤ 10 M_⊕). Combined with the flux outside the planetary magnetosphere, this gives the cosmic-ray energy spectrum at the top of the planetary atmosphere as a function of the planetary magnetic moment.

Results. We find that the particle flux to the planetary atmosphere can be increased by more than three orders of magnitude in the absence of a protecting magnetic field. For a weakly magnetized planet (ℳ = 0.05 ℳ_⊕), only particles with energies below 512 MeV are at least partially shielded. For a planet with a magnetic moment similar to that of Earth, this limit increases to to 32 GeV, whereas for a strongly magnetized planet (ℳ = 10.0 ℳ_⊕), partial shielding extends up to 200 GeV. Over the parameter range we studied, strong shielding does not occur for weakly magnetized planets. For a planet with a magnetic moment similar to that of Earth, particles with energies below 512 MeV are strongly shielded, and for strongly magnetized planets, this limit increases to 10 GeV.

Conclusions. We find that magnetic shielding strongly controls the number of cosmic-ray particles reaching the planetary atmosphere. The implications of this increased particle flux are discussed in a companion article.