Gisela A. M. Dreschhoff

Solar cosmic ray events for the period 1561–1994: 1. Identification in polar ice, 1561–1950

The geophysical significance of the thin nitrate-rich layers that have been found in both Arctic and Antarctic firn and ice cores, dating from the period 1561–1991, is examined in detail. It is shown that variations of meteorological origin dominate the record until the snow has consolidated to high-density firn some 30 years after deposition. The thin nitrate layers have a characteristic short timescale ( 30 MeV solar proton fluence. The proton fluences (omnidirectional fluence cm−2) derived from the 70 largest impulsive nitrate events between 1561 and 1950 are tabulated. The proton fluence probability distribution derived from these large impulsive nitrate events are in good agreement with earlier studies of the cumulative probabilities of solar proton events and with the observation of cosmogenic isotopes in moon rocks. The cumulative probability curve derived from the impulsive nitrate events indicates a rapidly decreasing probability of occurrence of >30 MeV solar proton events having an omnidirectional fluence exceeding 6 × 109 cm−2. It is concluded that the impulsive nitrate events are reliable indicators of the occurrence of large fluence solar proton events and that they provide a quantitative measure of these events. It is further concluded that the impulsive nitrate events will permit the study of solar activity for many thousands of years into the past.

Serpentinization and the Origin of Hydrogen Gas in Kansas

Hydrogen gas occurs in ten Kansas wells near the Mid-Continent rift system. Since 1982, two wells have yielded small amounts of gases containing an average of 29-37 mole % H2, the remainder being chiefly N2 with only traces of hydrocarbons. Isotopic compositions for hydrogen (^dgrD = -740 to -836 ^pmil) imply near-ambient (about 10°C) equilibration temperatures for the gases, which are among the most deuterium-depleted in nature and resemble the H2-rich gases described from ophiolites in Oman. Isotopic values for the Kansas N2 differ slightly from those of the atmosphere, but not enough to rule out an atmospheric origin. Because they are low in CH4 and CO2, expected byproducts of biogenic activity, the gases are probably abiogenic in origin. The existence of such gases near a major rift system, containing mafic rocks, and not far from known kimberlites is consistent with an origin from reactions involving Fe+2 oxidation, for example during serpentinization. Because the gases may be associated with kimberlites and deep-seated rifting, mantle outgassing is possible, but such an origin would be difficult to reconcile with the low isotopic temperatures. The H2 gases from Kansas (and elsewhere) seem to be too low in pressure to have commercial value. However, neither the Kansas gases nor those from other H2 occurrences have been adequately examined to assess their importance as potential resources.

Evidence of individual solar proton events in Antarctic snow

The high-resolution nitrate analyses of a snow sequence in Antarctica reveals clear evidence that the snow contains a chemical record of ionization from charged particles incident upon the upper atmosphere of the Earth. The Antarctic continent acts as a cold trap that effectively freezes out this signal and retains it in the stratigraphy of the ice shelves and the continental ice sheet. The signal that we measure results from the ionization of nitrogen and oxygen, the two primary constituents of the Earths atmosphere, which subsequently react to form oxides of nitrogen. A large portion of the nitrogen oxides produced are ultimately oxidized to nitric acid and incorporated in snow crystals together with nitrates from tropospheric sources that also contribute to the general background. The nitrate concentration in a firn core was measured in Antarctica by ultraviolet spectrophotometry under tightly controlled experimental procedures. Based on uninterrupted, high-resolution sampling, variations in nitrate concentration were found to average about 53% (one standard deviation) of the mean concentration for the entire core. Short pulses of high nitrate concentration were found to show a variance of up to 11 standard deviations above the mean. At the series mean, the precision of analysis is better than 2%.The firn core was drilled by hand to a depth of 21.7 m corresponding to 62 years and including more than 5 solar cycles. The time series that resulted from a total of 1393 individual analyses shows a statistically significant modulation of the background signal that is clearly tracable to solar activity. Several anomalously large concentration peaks were observed that have been dated and found to correlate with the major solar proton events of August 1972, July 1946, and the white-light flare of July 1928.

Anomalous nitrate concentrations in polar ice cores—Do they result from solar particle injections into the polar atmosphere?

Nitrate concentrations and electrical conductivity in an ice core from central Greenland have been measured simultaneously on 1.5 cm sections along the entire length of the 122 m core. This method of micro-resolution (time resolution is 1 week-1 month) provides for statistically significant determinations of rapid variations in nitrate fallout from the polar stratosphere superimposed on a background from all other sources. The seasonal NO3− background variations and the volcanic signal contained in the electrical conductivity data provide accurate dating of the ice core. This type of micro-resolution nitrate measurement appears to be useful as a tool to investigate the possible sources of polar nitrate anomalies such as solar proton events.

Computed contributions to odd nitrogen concentrations in the Earth’s polar middle atmosphere by energetic charged particles

Abstract A two-dimensional photochemical transport model which has inputs that characterize the odd nitrogen production associated with galactic cosmic rays, solar particle events (SPEs), and lower thermospheric contributions (auroral electrons and solar EUV and soft X-rays) is used to compute odd nitrogen concentrations in the polar middle atmosphere from 1 January 1970 to 31 December 1994. We are able to separate out of the total odd nitrogen budget the contributions of the energetic charged particles according to type. The SPE contributions to annual average odd nitrogen concentrations in the polar stratosphere (latitudes > 50°) are computed to be significant (>10%) only for the larger events of August 1972 and October 1989. The SPE contributions to odd nitrogen concentrations in the polar middle atmosphere are found to be asymmetric with respect to hemispheres. The computed SPE contributions to odd nitrogen concentrations at 30 km are significant more often over the South Pole than the North Pole. The thermospheric contributions to odd nitrogen concentrations in the polar middle atmosphere are asymmetric with respect to hemispheres. A stronger thermospheric influence in the stratosphere is computed over the South Pole than the North Pole. An attempt has been made to compare the modeled odd nitrogen of the polar middle atmosphere to an ultra-high resolution polar ice cap nitrate sequence to examine the hypothesis that the nitrate sequences exhibit a signal associated with energetic particles. Variations of odd nitrogen production and modeled concentrations associated with energetic particles themselves cannot explain all of the huge variations observed in the fine structure present in nitrate data from the polar ice cap nitrates, but may be able to explain parts of some of them.

Cometary airbursts and atmospheric chemistry: Tunguska and a candidate Younger Dryas event

We find agreement between models of atmospheric chemistry changes from ionization for the A.D. 1908 Tunguska (Siberia region, Russia) airburst event and nitrate enhancement in Greenland Ice Sheet Project 2 (GISP2H and GISP2) ice cores, plus an unexplained ammonium spike. We then consider a candidate cometary impact at the Younger Dryas onset (YD). The large estimated NO x production and O 3 depletion are beyond accurate extrapolation, but the ice core peak is much lower, possibly because of insufficient sampling resolution. Ammonium and nitrate spikes in both Greenland Ice Core Project (GRIP) and GISP2 ice cores have been attributed to biomass burning at the onset of the YD. A similar result is well resolved in Tunguska ice core data, but that forest fire was far too small to account for this. Direct input of ammonia from a comet into the atmosphere is adequate for YD ice core data, but not for the Tunguska data. An analog of the Haber process with hydrogen contributed by cometary or surface water, atmospheric nitrogen, high pressures, and possibly catalytic iron from a comet could in principle produce ammonia, accounting for the peaks in both data sets.

Identification of major proton fluence events from nitrates in polar ice cores

Large transient concentrations of nitrates in polar ice cores have been identified as the signature of some major solar proton fluence events between 1940 and 1991. We review this solar proton proxy identification technique using nitrate concentration measurements in ice cores from the Arctic and Antarctic. Using this identification technique we go back in time in an attempt to identify major solar proton events during the past several centuries. There is a very large nitrate increase corresponding to the Carrington flare of 1859 evident in the Arctic ice core. Other significant nitrate increases may indicate that major solar proton events occurred toward the end of the last century. The problems associated with this new technique of using nitrates as proxies to identify solar proton events are discussed.

The distribution of nitrate content in the surface snow of the Antarctic Ice Sheet along the route of the 1990 International Trans‐Antarctica Expedition

Previous work showed that nitrate measured at very high resolution (1.5 cm) in snow depositional sequences in Antarctica could be correlated with short-term phenomena such as solar proton events (Dreschhoff and Zeller, 1990). It was clear that deposition of the ionization products in the snow is strongly dependent upon precipitation and atmospheric conditions during and immediately after the event. Information about the geographic distribution of the nitrate fallout over Antarctica was limited to only a few sites, however. A unique opportunity to examine this aspect of the nitrate distribution and to test more fully the hypothesis that atmospheric ionization from solar-charged particles is responsible for a significant portion of nitrate was presented to us by a set of surface snow samples collected by the International Trans-Antarctica Expedition foot traverse. The set of 95 samples of the upper 25 cm was collected by one of us (Qin) at roughly equal distances along the 5736-km route from July 27, 1989, to March 3, 1990. Samples are distributed along a track from 65°05′S, 59°35′W, through 90°S, to 66°33′S, 95°39′E, which represents geomagnetic latitudes 50°S, west longitude, to 77°S, east longitude. The profiles of nitrate concentration and flux along the route were plotted and indicate that (especially at the higher elevation of the polar plateau) the distribution may be affected by electron precipitation.

Major solar flares and long-term variability in Antarctic ice cores

In-Situ data acquisition of high-resolution nitrate concentration in Antarctic snow resulting from ionization in the polar atmosphere reveals (a) very large solar proton events can be resolved, (b) a signal from thermospheric and mesospheric sources is found across Antarctica within the average boundaries of the auroral oval, (c) long-term periods of high or low solar activity, such as the Maunder Minimum are present in the nitrate record.

Effect of space charge on F centers near the stopping region of monoenergetic protons

Single crystals of sodium chloride were bombarded with 1–2‐MeV protons from a Van de Graaff accelerator at low temperature (77 K) and at room temperature. In the case of low temperature, a layered structure was found to be developed within the irradiated part of the crystal. This was indicated by the color‐center formation. The color‐center distribution in the irradiated area along the particles’ trajectories was studied using thin sections under a petrographic microscope and etched crystal surfaces in a scanning electron microscope. Three zones are visible: (1) strong color‐center development in the upper region, followed by (2) a colorless layer, and (3) the end of the particles’ path is marked by a deeply colored zone caused mainly by severe lattice damage. The relative degree of lattice damage throughout the irradiated region is clearly visible in scanning electron microscope photographs. During the irradiation, the terminal layer constitutes a region of positive charge. This space charge gives rise t...