Low time resolution analysis of polar ice cores cannot detect impulsive nitrate events

Ice cores are archives of climate change and possibly large solar proton events (SPEs). Wolff et al. (2012) used a single event, a nitrate peak in the GISP2-H core, which McCracken et al. (2001a) time associated with the poorly quantified 1859 Carrington event, to discredit SPE-produced, impulsive nitrate deposition in polar ice. This is not the ideal test case. We critique the Wolff et al. analysis and demonstrate that the data they used cannot detect impulsive nitrate events because of resolution limitations. We suggest re-examination of the top of the Greenland ice sheet at key intervals over the last two millennia with attention to fine resolution and replicate sampling of multiple species. This will allow further insight into polar depositional processes on a sub-seasonal scale, including atmospheric sources, transport mechanisms to the ice sheet, post-depositional interactions, and a potential SPE association.Comment: 22 pages, 7 figures in Journal of Geophysical Research: Space Physics 119, 201

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

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). There has been controversy, 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 mid stratosphere. These simulations suffer from deficiencies: they consider only soft SPEs and narrow energy ranges; spectral fits are poorly chosen; 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 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 SPE 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.Comment: JGR Atmospheres, in pres

Estimating the frequency of extremely energetic solar events, based on solar, stellar, lunar, and terrestrial records

The most powerful explosions on the Sun [...] drive the most severe space-weather storms. Proxy records of flare energies based on SEPs in principle may offer the longest time base to study infrequent large events. We conclude that one suggested proxy, nitrate concentrations in polar ice cores, does not map reliably to SEP events. Concentrations of select radionuclides measured in natural archives may prove useful in extending the time interval of direct observations up to ten millennia, but as their calibration to solar flare fluences depends on multiple poorly known properties and processes, these proxies cannot presently be used to help determine the flare energy frequency distribution. Being thus limited to the use of direct flare observations, we evaluate the probabilities of large-energy solar explosions by combining solar flare observations with an ensemble of stellar flare observations. We conclude that solar flare energies form a relatively smooth distribution from small events to large flares, while flares on magnetically-active, young Sun-like stars have energies and frequencies markedly in excess of strong solar flares, even after an empirical scaling with the mean activity level of these stars. In order to empirically quantify the frequency of uncommonly large solar flares extensive surveys of stars of near-solar age need to be obtained, such as is feasible with the Kepler satellite. Because the likelihood of flares larger than approximately X30 remains empirically unconstrained, we present indirect arguments, based on records of sunspots and on statistical arguments, that solar flares in the past four centuries have likely not substantially exceeded the level of the largest flares observed in the space era, and that there is at most about a 10% chance of a flare larger than about X30 in the next 30 years.Comment: 14 pages, 3 figures (in press as of 2012/06/18); Journal of Geophysical Research (Space Physics), 201

Solar proton events for 450 years: The Carrington event in perspective

Using high resolution measurements of the impulsive nitrate events in polar ice as identifiers of solar proton events in the past, we have identified 19 events over the period 1561–1950 that equal or exceed the \u3e30 MeV fluence measured during the August 1972 episode of solar proton events. The largest nitrate impulsive deposition event (and largest solar proton fluence above 30 MeV) occurred in late 1859 in time association with the Carrington flare of September 1859. The Carrington flare occurred near the central meridian of the sun; the interplanetary disturbance associated with the solar activity rapidly traveled toward the earth resulting in an extremely large geomagnetic storm commencing within 17.1 h of the visual observation of the solar flare. While this event was remarkable by itself, historical records indicate that the Carrington event was part of a sequence of solar activity as an active region traversed the solar disk. We compare the derived omni-directional solar proton fluence for the Carrington event of 1.9 × 1010 cm−2 above 30 MeV with the solar proton fluence from the past and from more recent episodes of solar activity. The Carrington event is the largest solar proton event identified in our ∼450 year period, having almost twice the \u3e30 MeV solar proton fluence than the second largest event in 1895, and approximately four times the solar proton fluence of the August 1972 events

United States Department of Energy Grand Junction Office Report 1642-1

From introduction: This report covers the results of an ERDA-sponsored study to provide a preliminary evaluation of the uranium potential of the central portion of the Great Plains. It was carried out by the University of Kansas under contract number AT (05-1)-1642. This work is part of the ERDA National Uranium Resource Evaluation (NURE) Program. Texas Instruments Incorporated contributed to the study as a subcontractor