Evidence of individual solar proton events in Antarctic snow

Abstract

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 Earth's 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 back~ound. 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 1 l 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.

A history of solar activity over millennia

“…However, after traversing some amount of matter (the nuclear interaction mean-free path is on the order of 100 g/cm 2 for a proton in the air) the CR particle may collide with a nucleus in the atmosphere, producing a number of secondaries. These secondaries have their own fate in the atmosphere, in particular they may suffer further collisions and interactions forming an atmospheric cascade (e.g., Dorman 2004). Because of the thickness of the Earth's atmosphere (1033 g/cm 2 at sea level) the number of subsequent interactions can be large, leading to a fully-developed cascade (also called an air shower) consisting of secondary rather than primary particles.…”

A History of Solar Activity over Millennia

“…), the abundance of compounds, related to the stratospheric chemistry such as nitrate ions (NO − 3 ), provides a unique and long-term (potentially up to 10 5 years) record of the radiation environment and climate in polar atmosphere. The concentration of nitrates has been measured in polar ice from both the Southern (South Pole, e.g., Dreschhoff and Zeller, 1990) and Northern (Greenland, e.g., Zeller and Dreschhoff, 1995;Dreschhoff and Zeller, 1998) polar caps. The Greenland series of ultra-high resolution (better than 30 equispaced samples per year) provides a unique record in which (groups of) individual strong SEP events can be identified for as far back as 1562.…”

Solar Influences on Global Change

“…Two-dimensional modeling studies, as well as spacecraft observations of middle atmospheric ozone abundances, suggest that this may indeed be a viable coupling mechanism Solomon and Garcia, 1984). Changes in nitrate content of antarctic snow associated with solar activity may reflect these processes (Dreschhoff and Zeller, 1990 (Joselyn, 1990). auroral Joule heating, and energetic neutral atoms produced from extraterrestrial ring current flows.…”