  Solar energetic particles ionize the atmosphere, leading to production of
nitrogen oxides. It has been suggested that some such events are visible as
layers of nitrate in ice cores, yielding archives of energetic, high fluence
solar proton events (SPEs). There has been controversy, due to slowness of
transport for these species down from the upper stratosphere; past numerical
simulations based on an analytic calculation have shown very little ionization
below the mid stratosphere. These simulations suffer from deficiencies: they
consider only soft SPEs and narrow energy ranges; spectral fits are poorly
chosen; with few exceptions secondary particles in air showers are ignored.
Using improved simulations that follow development of the proton-induced air
shower, we find consistency with recent experiments showing substantial excess
ionization down to 5 km. We compute nitrate available from the 23 February 1956
SPE, which had a high fluence, hard spectrum, and well-resolved associated
nitrate peak in a Greenland ice core. For the first time, we find this event
can account for ice core data with timely (~ 2 months) transport downward
between 46 km and the surface, thus indicating an archive of high fluence, hard
spectrum SPE covering the last several millennia. We discuss interpretations of
this result, as well as the lack of a clearly-defined nitrate spike associated
with the soft-spectrum 3-4 August 1972 SPE. We suggest that hard-spectrum SPEs,
especially in the 6 months of polar winter, are detectable in ice cores, and
that more work needs to be done to investigate this.
