  Recent 60Fe results have suggested that the estimated distances of supernovae
in the last few million years should be reduced from 100 pc to 50 pc. Two
events or series of events are suggested, one about 2.7 million years to 1.7
million years ago, and another may at 6.5 to 8.7 million years ago. We ask what
effects such supernovae are expected to have on the terrestrial atmosphere and
biota. Assuming that the Local Bubble was formed before the event being
considered, and that the supernova and the Earth were both inside a weak,
disordered magnetic field at that time, TeV-PeV cosmic rays at Earth will
increase by a factor of a few hundred. Tropospheric ionization will increase
proportionately, and the overall muon radiation load on terrestrial organisms
will increase by a factor of 150. All return to pre-burst levels within 10kyr.
In the case of an ordered magnetic field, effects depend strongly on the field
orientation. The upper bound in this case is with a largely coherent field
aligned along the line of sight to the supernova, in which case TeV-PeV cosmic
ray flux increases are 10^4; in the case of a transverse field they are below
current levels. We suggest a substantial increase in the extended effects of
supernovae on Earth and in the lethal distance estimate; more work is
needed.This paper is an explicit followup to Thomas et al. (2016). We also here
provide more detail on the computational procedures used in both works.
