Reply to comment by K.A. Duderstadt et al. on "Atmospheric ionization by high-fluence, hard spectrum solar proton events and their probable appearance in the ice core archive"

TL;DR

This paper defends the validity of nitrate spikes in ice cores as indicators of solar proton events, emphasizing the importance of accurate modeling and calling for detailed ice core studies to resolve ongoing debates.

Contribution

It clarifies the limitations of previous models, demonstrates the consistency of nitrate enhancements with observed data, and advocates for targeted ice core research to confirm SPE proxies.

Findings

Nitrate enhancements from extreme SPEs can reach hundreds of percent.

Current models are limited by uncertainties in atmospheric conditions.

Ice core nitrate spikes are consistent with solar proton event predictions.

Abstract

Duderstadt et al. [2016b] comment that the Melott et al. [2016] study of nitrate formation by solar proton events (SPEs) and comparison with the ice core archive is "fundamentally flawed," because it does not include pre-existing HNO3 in the stratosphere. We show that they exaggerate both the enhancement predicted by our findings and pre-industrial HNO3 levels in their model, and fail to prove this assertion. Our feasibility study matched expected SPE nitrate production with ground truth measurements. It is not clear that their approach is more realistic and absence of a detailed mechanism does not disprove our results. Models can be no better than the information they are provided and in this case there continue to be significant unknowns and uncertainties, especially in the role of polar stratospheric clouds (PSCs) and possible interactions with cosmic rays that constitute lower boundary conditions. Duderstadt et al. [2014; 2016a] used incomplete, poorly-constrained and incorrect initial and boundary conditions, and they continue to advocate on the basis of uncertain results. Meanwhile, Smart et al. [2014] identified a series of ice core nitrate spikes that have since been confirmed in 10Be by McCracken and Beer [2015]. Melott et al. [2016] computationally reproduced the ionization profile of the only major balloon measurements to date. We show that our calculated nitrate enhancement is consistent with measured results, given current levels of uncertainty, and that extreme SPEs can potentially produce occasional nitrate spikes with hundreds of percent increases. Instead of repeating old arguments to dismiss nitrates as proxies of SPEs, it is past time for a dedicated, fine-resolution, multi-parameter, replicate ice core field campaign to resolve this debate.