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Main sequence M stars pose an interesting problem for astrobiology: theirabundance in our galaxy makes them likely targets in the hunt for habitableplanets, but their strong chromospheric activity produces high energy radiationand charged particles that may be detrimental to life. We studied the impact ofthe 1985 April 12 flare from the M dwarf, AD Leonis (AD Leo), simulating theeffects from both UV radiation and protons on the atmospheric chemistry of ahypothetical, Earth-like planet located within its habitable zone. Based onobservations of solar proton events and the Neupert effect we estimated aproton flux associated with the flare of $5.9\times 10^{8}$ protons cm$^{-2}$sr$^{-1}$ s$^{-1}$ for particles with energies >10 MeV. Then we calculated theabundance of nitrogen oxides produced by the flare by scaling the production ofthese compounds during a large solar proton event called the "Carringtonevent". The simulations were performed using a 1-D photochemical model coupledto a 1-D radiative/convective model. Our results indicate that the ultravioletradiation emitted during the flare does not produce a significant change in theozone column depth of the planet. When the action of protons is included, theozone depletion reached a maximum of 94% two years after the flare for a planetwith no magnetic field. At the peak of the flare, the calculated UV fluxes thatreach the surface, in the wavelength ranges that are damaging for life, exceedthose received on Earth during less than 100 s. Flares may therefore notpresent a direct hazard for life on the surface of an orbiting habitableplanet. Given that AD Leo is one of the most magnetically-active M dwarfsknown, this conclusion should apply to planets around other M dwarfs with lowerlevels of chromospheric activity.

The Effect of a Strong Stellar Flare on the Atmospheric Chemistry of an Earth-like Planet Orbiting an M dwarf