Galactic cosmic rays on extrasolar Earth-like planets I. Cosmic ray flux

Abstract

(abridged abstract) 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. 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. 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 $\le$ E $\le$ 524 GeV) and as a function of the planetary magnetic field strength (in the range 0 $M_{\oplus }$ $\le$ {M} $\le$ 10 $M_{\oplus }$). 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. 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 ($M=0.05M_{\oplus }$), only particles with energies below 512 MeV are at least partially shielded. For a planet with a magnetic moment similar to Earth, this limit increases to 32 GeV, whereas for a strongly magnetized planet ($M=10.0M_{\oplus }$), partial shielding extends up to 200 GeV. 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.

10 pages, 9 figures; accepted in A&A

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