Galactic cosmic rays on extrasolar Earth-like planets

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.

Atmospheric effects of stellar cosmic rays on Earth-like exoplanets orbiting M-dwarfs

“…Segura et al (2010) have modelled the photochemical effects of a large UV flare observed on AD Leonis (AD Leo), and find that UV surface fluxes stay within non-lethal dosages. Grießmeier et al (2015b) find that GCR-induced O 3 destruction results in up to 40% increased biologically weighted UV surface flux. However, as the AD Leo spectrum used features lower UV-A and UV-B fluxes and the planetary atmosphere retains enough O 3 to shield from UV-C, total UV fluxes are much lower than on Earth.…”

“…Grenfell et al (2007a) investigated the effect of GCRs on biomarker molecules for biogenic Earth-like planets surrounding an active M-dwarf with varying heliospheric and planetary magnetic shielding. This study was recently built on by Grießmeier et al (2015b) and Grießmeier et al (2015a), who use the updated version of the CR model presented in the current work to examine the effect of the planetary magnetospheric shielding on GCR-induced photochemistry. With a maximum of 20% difference in column-integrated ozone values (and 6% for CH 4 and H 2 O) between no magnetic shielding and ten times the Earth's magnetic field strength, they find GCRs have nearly no effect on the resulting planetary spectra.…”

External Factors that Direct the Biological Clock and Role in Treatment of Disease

“…For example, NASA found that there is a purple light, which is created by nitric oxide, and on Mars it produces its own light [47][48][49][50].…”

A Generalized Two-Component Model of Solar Wind Turbulence and Ab Initio Diffusion Mean-Free Paths and Drift Lengthscales of Cosmic Rays

“…On the other hand, the correct description of the transport of cosmic rays in other astrophysical systems is also of great interest. For example, in astrospheres, i.e., circumstellar regions occupied by stellar winds, it is of high relevance in the context of exoplanet research (e.g., Scalo et al 2007;Grenell et al 2012;Grießmeier et al 2015) and potentially for an understanding of cosmic-ray anisotropy at high energy (Scherer et al 2015). Another example is the, at least partly diffusive, cosmic-ray transport in galactic halos (e.g., Heesen et al 2009;Mao et al 2015).…”