  Understanding the impact of active M-dwarf stars on the atmospheric
equilibrium and surface conditions of a habitable zone Earth-like planet is key
to assessing M dwarf planet habitability. Previous modeling of the impact of
electromagnetic (EM) radiation and protons from a single large flare on an
Earth-like atmosphere indicated that significant and long-term reductions in
ozone were possible, but the atmosphere recovered. These stars more
realistically exhibit frequent flaring with a power-law distribution of
energies. Here we use a coupled 1D photochemical and radiative-convective model
to investigate the effects of repeated flaring on the photochemistry and
surface UV of an Earth-like planet unprotected by an intrinsic magnetic field.
We use time-resolved flare spectra obtained for the dM3 star AD Leo, combined
with flare occurrence frequencies and total energies (typically 10$^{30.5}$ to
10$^{34}$ erg) from the 4-year Kepler light curve for the dM4 flare star
GJ1243. Our model results show repeated EM-only flares have little effect on
the ozone column depth, but that multiple proton events can rapidly destroy the
ozone column. Combining the realistic flare and proton event frequencies with
nominal CME & SEP geometries, we find the ozone column for an Earth-like planet
can be depleted by 94% in 10 years, with a downward trend that makes recovery
unlikely and suggests further destruction. For more extreme stellar inputs O3
depletion allows a constant 0.1-1 W m$^{-2}$ of UV-C at the planet's surface,
which is likely detrimental to organic complexity. Our results suggest that
active M dwarf hosts may comprehensively destroy ozone shields and subject the
surface of magnetically-unprotected Earth-like planets to long-term radiation
that can damage complex organic structures. However, this does not preclude
habitability, as a safe haven for life could still exist below an ocean
surface.
