In contrast, SEP events occur roughly in phase with the solar cycle, with proportionally more events late in the cycle and very significant variations from cycle to cycle. Within this overall statistical pattern, SEP events are unpredictable and can occur at any time during the solar cycle.
Other patterns of short-term variability have been discovered as well: an approximately 22 year cycle in the shape of the GCR peak, for example, and a 27 day modulation of galactic cosmic radiation intensity related to the Sun’s rotational period. But while reliable, continuous direct measurements have made it possible to characterize the temporal behavior of GCR and SEPs within the modern era (as defined by the availability of such measurements), they cover too short a time period to capture longer-term, secular variations that are also important for an understanding of the space radiation environment and the solar processes that influence it. Fortunately, however, when galactic cosmic radiation and solar energetic particles interact with Earth’s atmosphere, they trigger nuclear and chemical reactions, the products of which, deposited and preserved in the polar ice, provide a record of cosmic radiation modulation and SEP activity that extends centuries, even millennia, into the past.
One of the key products of this process is the 10Be isotope, which attaches itself to aerosols and, after a residence of several months in the atmosphere, precipitates onto Earth’s surface. In the polar regions, the precipitated 10Be is preserved in the successive layers of ice that build up over the centuries and that record the history of the local meteorology and of the global influences on it. By boring into the polar ice and extracting core samples from it, researchers can analyze the composition of the different ice layers and measure the amount of 10Be deposited in each. Because 10Be production is proportional to the cosmic radiation flux and because the isotope has a long half-life (1.5 × 106 years), the variations in the 10Be concentration measured in the ice layers can be used to reconstruct the changes in the GCR flux over periods of several millennia.
Analysis of 10Be data has shown that GCR intensities were high though variable during extended periods of low solar activity in the past, such as the Spoerer (1420 to 1540 CE) minimum and the last part of the Maunder (1645 to 1715 CE) minimum. (The level of solar activity in the past is inferred from reports of auroral activity, which date back to the 11th century, and from the sunspot record, which has been kept since around 1600.) The variations in the 10Be concentrations for these periods indicate that—at times substantial—modulation of the GCR fluxes can continue even during periods of low solar activity and that, as in the case of the Maunder minimum, the modulation is not necessarily well correlated with the sunspot number. Further, comparison of GCR intensities in the modern era with those deduced from the 10Be data for earlier epochs indicates that during the past half century, the GCR intensity near Earth has been one of the lowest in the past 1150 years (Figure 1.1.2, McCracken et al., 2004).
Nitrate (NO3) is a product of the chemistry initiated by the interaction of solar energetic particles with Earth’s upper atmosphere. Like 10Be, it is removed from the atmosphere by precipitation and accumulates in the polar ice, although on a much shorter timescale (<1.5 months versus ~1 year). It has recently been demonstrated (McCracken et al., 2001a) that spikes in the concentration of NO3 in polar ice cores provide a record of past large SEP events, just as 10Be data provide a means of deducing the GCR environment in earlier epochs for which direct measurements are not available. Reconstruction of past SEP events from ice core data has revealed that large events (those with fluences >2 × 109 cm−2 for particles with energies >30 MeV) occur