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

TL;DR

This study models how planetary magnetic fields influence the flux of Galactic cosmic rays reaching exoplanet atmospheres, revealing that weak or absent magnetic fields significantly increase cosmic-ray exposure.

Contribution

It provides a quantitative analysis of cosmic-ray flux at exoplanet atmospheres based on planetary magnetic field strength, filling a gap in understanding cosmic-ray shielding.

Findings

Weak magnetic fields allow more cosmic rays to reach atmospheres.

Magnetic shielding effectiveness depends on planetary magnetic moment.

Flux increases by over three orders of magnitude without magnetic protection.

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 (${M}=0.05\,{M}_{\oplus}$), 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 ($M=10.0\,{M}_{\oplus}$), 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.