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(abridged abstract) Theoretical arguments indicate that close-in terrestialexoplanets may have weak magnetic fields. As described in the companion article(Paper I), a weak magnetic field results in a high flux of galactic cosmic raysto the top of the planetary atmosphere. We investigate effects that may resultfrom a high flux of galactic cosmic rays both throughout the atmosphere and atthe planetary surface. Using an air shower approach, we calculate how theatmospheric chemistry and temperature change under the influence of galacticcosmic rays for Earth-like (N_2-O_2 dominated) atmospheres. We evaluate theproduction and destruction rate of atmospheric biosignature molecules. Wederive planetary emission and transmission spectra to study the influence ofgalactic cosmic rays on biosignature detectability. We then calculate theresulting surface UV flux, the surface particle flux, and the associatedequivalent biological dose rates. We find that up to 20% of stratospheric ozoneis destroyed by cosmic-ray protons. The reduction of the planetary ozone layerleads to an increase in the weighted surface UV flux by two orders of magnitudeunder stellar UV flare conditions. The resulting biological effective dose rateis, however, too low to strongly affect surface life. We also examine thesurface particle flux: For a planet with a terrestrial atmosphere, a reductionof the magnetic shielding efficiency can increase the biological radiation doserate by a factor of two. For a planet with a weaker atmosphere (with a surfacepressure of 97.8 hPa), the planetary magnetic field has a much strongerinfluence on the biological radiation dose, changing it by up to two orders ofmagnitude.

Galactic cosmic rays on extrasolar Earth-like planets: II. Atmospheric implications