  We have reevaluated recent studies of the effects on Earth by cosmic rays
(CRs) from nearby supernovae (SNe) at 100 and 50 pc, in the diffusive transport
CR case, here including an early-time suppression at lower CR energies
neglected in the previous works. Inclusion of this suppression leads to lower
overall CR fluxes at early times, lower atmospheric ionization, smaller
resulting ozone depletion, and lower sea-level muon radiation doses.
Differences in the atmospheric impacts are most pronounced for the 100 pc case
with less significant differences in the 50 pc case. We find a greater
discrepancy in the modeled sea-level muon radiation dose, with significantly
smaller dose values in the 50 pc case; our results indicate it is unlikely that
muon radiation is a significant threat to the biosphere for SNe beyond 20 pc,
for the diffusive transport case. We have also performed new modeling of the
effects of SN CRs at 20 and 10 pc. Overall, our results indicate that,
considering only the effects of CRs, the "lethal" SN distance should be closer
to 20 pc rather than the typically quoted 8-10 pc. Recent work on extended SN
X-ray emission indicates significant effects out to 50 pc and therefore the
case is now strong for increasing the standard SN lethal distance to at least
20 pc. This has implications for studies of the history of life on Earth as
well as considerations of habitability in the Galaxy.
