  Astrophysical ionizing radiation events have been recognized as a potential
threat to life on Earth, primarily through depletion of stratospheric ozone and
subsequent increase in surface-level solar ultraviolet radiation. Simulations
of the atmospheric effects of a variety of events (such as supernovae,
gamma-ray bursts, and solar proton events) have been previously published,
along with estimates of biological damage at Earth's surface. In this work, we
employed the TUV radiative transfer model to expand and improve calculations of
surface-level irradiance and biological impacts following an ionizing radiation
event. We considered changes in surface-level UVB, UVA, and photosynthetically
active radiation (visible light) for clear-sky conditions and fixed aerosol
parameter values. We also considered a wide range of biological effects on
organisms ranging from humans to phytoplankton. We found that past work
overestimated UVB irradiance, but that relative estimates for increase in
exposure to DNA damaging radiation are still similar to our improved
calculations. We also found that the intensity of biologically damaging
radiation varies widely with organism and specific impact considered; these
results have implications for biosphere-level damage following astrophysical
ionizing radiation events. When considering changes in surface-level visible
light irradiance, we found that, contrary to previous assumptions, a decrease
in irradiance is only present for a short time in very limited geographical
areas; instead we found a net increase for most of the modeled time-space
region. This result has implications for proposed climate changes associated
with ionizing radiation events.
