Comment on "Atmospheric ionization by high-fluence, hard spectrum solar proton events and their probable appearance in the ice core archive" by A.L. Melott et al

TL;DR

This paper critiques previous claims that nitrate spikes in ice cores can reliably indicate specific solar proton events, emphasizing the importance of background nitrate levels and arguing that nitrate is not a valid proxy for SPEs.

Contribution

It provides a critical analysis showing that nitrate spikes in ice cores cannot be used to identify individual solar proton events due to background nitrate levels and modeling limitations.

Findings

Background nitrate levels are much higher than SPE-induced increases.

SPEs produce less than 5% increase in atmospheric nitrate burden.

Nitrate peaks in ice cores cannot be reliably linked to SPEs.

Abstract

Melott et al. [2016] suggest that individual solar proton events (SPEs) are detectable as nitrate ion spikes in ice cores. They use the high fluence, high energy (hard spectrum) SPE of 23 February 1956 to calculate an enhancement of HNO3 from the surface to 46 km that is equivalent to a ~120 ng cm-2 nitrate ion spike observed in the GISP2H ice core. The Melott et al. [2016] approach is fundamentally flawed, since it considers only the absolute column burden of SPE-produced nitrate and not the pre-existing nitrate in the stratosphere. Modeling studies supported by extensive observations [Duderstadt et al., 2014, 2016, and this comment] show background HNO3 in the lower and middle stratosphere equivalent to 2000 to 3000 ng cm-2 nitrate. These high levels of background nitrate must also be included when estimating SPE enhancements to the deposition of nitrate ions that might eventually be preserved in an ice core. The 1956 SPE results in less than a 5% increase in the column burden of atmospheric HNO3, not large enough to explain the nitrate spike seen in the GISP2H ice core. Even extreme SPE enhancements cannot explain nitrate peaks (typically hundreds of percent increases) observed in the ice record [Duderstadt et al., 2016]. Realistic mechanisms linking nitrate ions in ice cores to SPEs have not been established. It is time to move the search for indicators of SPEs away from nitrate ions: Nitrate ions cannot be used as proxies for individual SPEs in the ice core record.