Background on Impulsive Nitrate Enhancements in Polar Ice as Proxies for Historical SPEs

1. Background on Impulsive Nitrate Enhancements in Polar Ice as Proxies for Historical SPEs Don F. Smart and M. A. Shea sssrc@msn.com  The initial work on the solar proton event – impulsive nitrate event relationship was by Dreschhoff and Zeller (Solar Phys., 127, 333, 1990). The extreme values had a correspondence to known solar flare and large solar particle events. The “raw” Windless Bight, (70° S, 167° E) Antarctica NO(y) data are shown below: Note that there are nitrate ‘peaks’ that do not correspond to outstanding solar-terrestrial events. These are associated with terrestrial weather storms (Zeller, private communication, 1995; Laird et al., Ant. J. 19, 68, 1985). The Windless Bight location on the Ross Ice Shelf is subject to ocean storms. Storms from the sea can result in additional nitrate concentrations. As a result, polar ice cores from locations near oceans have considerably more “noise” in the nitrate record than deep interior sites. The analysis of a second core at Windless Bight in 1991 (about 2 km distance from the first site) verified the results of the first core.

2. In 1992 an ice core (named GISP2-H) was obtained at Summit, Greenland (72° N, 38° W), and the upper 125.6 meters was specifically designed for high resolution nitrate studies. This deep interior polar location is removed from the coast, and fluctuations in nitrate concentration of meteorological origin are minimal. The meteorological data indicate that this site in central Greenland has approximately uniform precipitation throughout the year (Bromwich et al., JGR, 104, 22013, 1999). Dating of the 125.6 meter core established that the precipitation was deposited in the years between 1561 and 1992. The green spikes are volcanic eruptions. The integrated NO(y) above the background is used to obtain the solar proton fluence)

3. It is our thesis that energetic solar protons penetrating to lower altitudes provide the energy for an exothermic reaction that converts O3 into NO(y) (Jackman et al., JGR, 85, 7495, 1980; Jackman et al., JGR, 95, 7417, 1990). The energetic solar protons penetrate into the atmosphere down to heights where the majority of the EUV has already been absorbed and thus is unlikely to contribute significantly to the disassociation of NO(y). We assume these NO(y) radicals become attached to aerosols which, by gravitational sedimentation, are transported downwards into the troposphere and are precipitated into the polar ice within a ~6-week time period. Recognizing that the nitrate deposition in polar ice would be over a few week time period, we used the total fluence for a solar activity episode of activity such as the multiple of events in August 1972 and October 1989. Details of the verification of the nature of the nitrate events, and the calibration technique including allowances made for seasonal variations and ice density are given by McCracken et al. (JGR, 106, 21585, 2001). Using the derived calibration between nitrate concentration and solar proton events, the initial analysis of the GISP2-H core resulted in the identification of 70 impulsive nitrate enhancements that were five standard deviations above the background level. Using the derived calibration between modern era nitrate concentrations and solar proton events we find that these impulsive nitrate events correspond to a >30 MeV omni-directional fluence>2 x 109 cm-2. (Note that a 109 cm-2omni-directional fluence event of > 30 MeV protons is a very large and very rare solar proton event).

4. Other correlations and anti Correlations Palmer et al. (GRL, 28, 1953, 2001), preformed a statistical analysis of the frequency of NO(y) increases found in ice cores from Law Dome (66° S, 112° E; at an altitude of about 1300 meters but in a high precipitation area near the ocean). They compared the average annual nitrate cycle as with annual nitrate cycles containing documented significant solar particles events. The found that there may be a statistical significant association suggesting that they may be a solar contribution to the nitrate in polar ice. Wolff et al. (Atmos. Chem. Phys., 8, 5627, 2008) did a systematic analysis of the daily nitrate deposition in the precipitation during 2004 and 2005 at a coastal site (Halley Bay, 75° S, 333° E). They found no association of nitrate deposition with solar events. But the solar proton event they noted in July and August 2004 had a total integrated omni- directional >30 MeV fluence of 6.5 x 106 cm-2 - far below our established NO(y) detection threshold of 1.0 x 109 cm-2. The GLE of 20 January 2005 had a >30 MeV omni-directional fluence of 1.0 x 109 cm-2, but Wolff et al. did not observe a time-associated NO(y) increase. Kepko et al. (JASTP, 71, 1840, 2009) analyzing an independent “short core” from Summit, Greenland, using the continuous flow technique resulting in ~400 samples per year, found an impulsive NO(y) increase for each of the large solar cosmic ray ground- level event in the 1940-1950 decade.

5. Both Greenland cores and the Antarctic cores see the same large events where they overlap in time Top: Nitrate data from the 2004 Greenland core with annotated solar events. (~400 samples/year) Bottom: Nitrate deposition from 1988-1989 Antarctic ice cores. (1.5 cm resolution = ~20 samples/year) Figure adopted from Kepko et al., JASTP, 71, 1840, 2009.

6. The NO(y) events identified in the GIPS2-H core also have a association with low-latitude aurora The association with ancient Korean Aurora (date determined by a lunar calendar) The association with mid-latitude aurora in Europe and North America