5 Solar Energetic Particles in the Past

In addition to galactic cosmic rays, which are always present in the Earth’s vicinity, sometimes sporadic solar energetic particle (SEP) events with a greatly enhanced flux of less energetic particles in the interplanetary medium also occur (e.g., Klecker et al., 2006). Strong SEP events mostly originate from CME-related shocks propagating in the solar corona and interplanetary medium, that lead to effective bulk acceleration of charged particles (e.g., Cane and Lario, 2006). Although these particles are significantly less energetic than GCRs, they can occasionally be accelerated to an energy reaching up to several GeV, which is enough to initiate the atmospheric cascade. Peak intensity of SEP flux can be very high, up to 104 particles (with energy > 30 MeV) per cm2 per second. In fact, the long-term average flux (or fluence) of SEP is mostly defined by rare major events, which occur one to two times per solar cycle, with only minor contributions from a large number of weak events (Shea and Smart, 19902002). As an example, energy spectra of GCR and SEP are shown in Figure 21View Image for the day of January 20, 2005, when an extreme SEP event took place. Such SEPs dominate the low-energy section of cosmic rays (below hundreds of MeV of a particle’s kinetic energy), which is crucial for the radiation environment, and play an important role in solar-terrestrial relations. For many reasons it is important to know the variations of SEPs on long-term scales.

View Image

Figure 21: Daily fluence of solar energetic particles (dashed curve) and galactic cosmic rays for the day of January 20, 2005. Open circles represent space-borne measurements (Mewaldt, 2006).

It is not straightforward to evaluate the average SEP flux even for the modern instrumental epoch of direct space-borne measurements (e.g., Mewaldt et al., 2007). For example, estimates for the average flux of SEPs with an energy above 30 MeV (called F30 henceforth) for individual cycles may vary by an order of magnitude, from 8 cm–2 s–1 for cycle 21 up to 60 cm–2 s–1 for cycle 19 (Smart and Shea, 2002Jump To The Next Citation Point). Moreover, estimates of the SEP flux were quite uncertain during the earlier years of space-borne measurements because of two effects, which are hard to account for (e.g., Reeves et al., 1992Tylka et al., 1997). One is related to the very high flux intensities of SEPs during the peak phase of events, when a detector can be saturated because of the dead-time effect (the maximum trigger rate of the detector is exceeded). The other is related to events with high energy solar particles, which can penetrate into the detector through the walls of the collimator or the detector, leading to an enhanced effective acceptance cone with respect to the “expected” one. Since the SEP fluence is defined by major events, these effects may lead to an underestimate of the average flux of SEPs. The modern generation of detectors are better suited for measuring high fluxes. The average F30 flux for the last five solar cycles (1954 – 2006) is estimated at about 35 cm–2 s–1 (Smart and Shea, 2002Jump To The Next Citation PointShea et al., 2006Jump To The Next Citation Point).

The method of cosmogenic isotopes in terrestrial archives does not allow quantitative estimates of SEP events because of atmospheric and geomagnetic rigidity/energy cutoffs, which are too high for less energetic particles of solar/interplanetary origin. While the SEP effect is negligible in 14C records, the 10Be record may contain information on extreme SEP events in the past (Usoskin et al., 2006bJump To The Next Citation Point). In particular, an additional SEP-related enhancement of the 10Be signal can appear around some solar activity maxima leading to an intermittent 5.5-year quasi-periodicity (McCracken et al., 2002). However, such peaks can be studied only a posteriori, i.e., for known SEP events (e.g., the Carrington flare), which can be associated with strong peaks in 10Be, but the relation is not one-to-one. Therefore, SEP events cannot be unambiguously identified from the 10Be data.

Since energy spectra of the two populations of cosmic rays are significantly different (very soft for SEP and much harder for GCR), they can be separated by their energy. Accordingly, for earlier times, when direct measurements were not possible, one needs a natural spectrometer in order to distinguish between cosmic particles of solar and galactic origin. Such a spectrometer should be able to naturally separate lower and higher energy components of the cosmic-ray spectrum and “record” them in different archives. Fortunately, there are such natural spectrometers, which allow us to study not only galactic cosmic rays but also solar energetic particle flux in the past.

 5.1 Lunar and meteoritic rocks
 5.2 Nitrates in polar ice
 5.3 Summary

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