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

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). This has been controversial, 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 midstratosphere. These simulations suffer from deficiencies: they consider only soft SPEs and narrow energy ranges; spectral fits are poorly chosen; and 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 that 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 SPEs 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.

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

“…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. Accordingly, an analysis of an extreme SEP event is a crucial point to ultimately resolve this question.…”

Reply to comment by E. W. Wolff et al. on “Low time resolution analysis of polar ice cores cannot detect impulsive nitrate events”

“…Using a percentage enhancement dilutes the expected effect. Instead, calculations of the absolute amount of additional, SPE‐produced, inorganic, nitric acid available for prompt deposition are consistent with the observed nitrate increase [ Melott et al , ].…”

“…Consequently, we emphatically dispute the extrapolations by Wolff et al [, ], Duderstadt et al [], Duderstadt et al [], and others that no SPEs can be found in polar ice core nitrates and that these records cannot be used to obtain statistics of past, very large SPEs. On the contrary, recent work, showing direct experimental and model evidence of substantial lower stratosphere/troposphere ionization from SPEs (e.g., Usoskin et al [, ], Nicoll and Harrison [], and Melott et al []) indicates exactly the opposite may be the case. Thus, the SPE/polar ice nitrate hypothesis is still very much alive.…”

“…Wolff et al 's [] mention that they await more detailed modeling studies, which are in progress. Such a study of the 23 February 1956 SPE has recently been conducted where the amount of nitrate available from this SPE has been computed using full spectrum modeling involving high‐energy showers [ Melott et al , ]. These results indicate that nitrate production in the stratosphere is roughly 2–10 times greater than estimated by past limited energy range, analytic models.…”

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.

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