Nitrate ion spikes in ice cores not suitable as proxies for solar proton events

Abstract

Nitrate ion spikes in polar ice cores are contentiously used to estimate the intensity, frequency, and probability of historical solar proton events, quantities that are needed to prepare for potentially society‐crippling space weather events. We use the Whole Atmosphere Community Climate Model to calculate how large an event would have to be to produce enough odd nitrogen throughout the atmosphere to be discernible as nitrate peaks at the Earth's surface. These hypothetically large events are compared with probability of occurrence estimates derived from measured events, sunspot records, and cosmogenic radionuclides archives. We conclude that the fluence and spectrum of solar proton events necessary to produce odd nitrogen enhancements equivalent to the spikes of nitrate ions in Greenland ice cores are unlikely to have occurred throughout the Holocene, confirming that nitrate ions in ice cores are not suitable proxies for historical individual solar proton events.

Atmospheric impacts of the strongest known solar particle storm of 775 AD

“…In contrast to previous hypothetical estimates3, our analysis is based on the latest event strength assessment69, which, as we show, is also supported by good agreement between modelled and observed 10 Be deposition variability. Similarly to prior conclusions1617, our results suggest that even an extreme SEP event cannot be resolved in ice core nitrate time-series devaluating it as a potential proxy for SEP events. Since the analyzed SEP event is the strongest known in the Holocene412, our results can serve as a realistic upper bound (worst-case scenario) of the possible effect of SEP events on the atmosphere.…”

“…Since NO x is a precursor for atmospheric nitrate, it was suggested that strong SEP events can be detected as sharp spikes in records of nitrate in polar ice15. However, this was debated both experimentally for the Carrington event16 and theoretically17. If a nitrate record was indeed a proxy for strong SEP events of the past1118, the extreme event of 774 AD would be clearly identified in polar nitrate series.…”

A history of solar activity over millennia

“…This work was criticized by Smart et al (2014), who stated that the resolution of the analyzed series was not high enough and could have led to missing the spike, which was counter-argued by Wolff et al (2016) that even a fine time resolution does not reveal the peaks. It was also discussed theoretically (Duderstadt et al 2016) that a realistic SEP event can hardly produce sufficient amount of nitrate to leave a strong pulse-like signature in an ice core. Usoskin and Kovaltsov (2012) calculated, from the F 30 fluence proposed by McCracken et al (2001) for the Carrington event, the 10 Be production and showed that, if the Carrington SEP event was so strong, it would have necessarily produce a spike in the annually resolved 10 Be record, which however contradicts to the real data from NGRIP and Dye3 ice cores.…”

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.

“…Duderstadt et al . [] conduct comprehensive chemical transport modeling that includes background HNO 3 . This comment addresses these differences and also corrects misrepresentations regarding Duderstadt et al .…”

“…[] and Duderstadt et al . [] are listed as companion papers, although the contents of Melott et al . [] remained confidential prior to publication.…”