Atmospheric ionization by high-fluence, hard spectrum solar proton events and their probable appearance in the ice core archive

TL;DR

This study improves simulations of solar proton events' atmospheric ionization, demonstrating that high-fluence, hard spectrum SPEs can be recorded in ice cores, providing a new archive of solar activity over millennia.

Contribution

The paper introduces enhanced simulation methods that accurately model proton-induced air showers and their atmospheric effects, linking high-fluence SPEs to ice core nitrate records.

Findings

High-fluence, hard spectrum SPEs can produce detectable nitrate layers in ice cores.

The 1956 SPE event's nitrate peak aligns with model predictions of atmospheric ionization and transport.

Soft-spectrum SPEs like the 1972 event may not produce clear nitrate signatures in ice cores.

Abstract

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.