1930s, when the invention of the ionization chamber allowed more sensitive experiments. Now we know that there are two kinds of cosmic rays, galactic and solar. Galactic cosmic rays (GCRs) are always present, although their intensity varies with solar activity. Solar cosmic rays are present only during very intense SPEs.

Scientists have recorded SPEs indirectly from ground observations since 1942 and directly from spacecraft since 1965. Between 1942 and 1953, the only way to detect SPEs was with ground-based instruments (ionization chambers and muon counters) designed to monitor the intensity of galactic cosmic rays. At that time only particles with extremely high energies (>4 GeV), high enough to penetrate Earth's magnetic shield to the top of the atmosphere, were detectable. As these cosmic rays passed through the atmosphere, they generated a nuclear cascade intense enough that some of the secondaries (primarily muons) reached the cosmic-ray recording instruments on the ground. When an SPE also brought such high-energy particles to Earth, the instruments recorded a transient rise in the count rate above the background set by galactic cosmic rays. From such signals, cosmic ray physicists estimated the dose rate (flux) and total dose (fluence) of the highest energy particles in the SPE. The early 1950s saw the development and deployment of cosmic-ray neutron monitors, which could observe particles with energies nearly an order of magnitude lower than those detected by muon counters or ionization chambers. The SPEs of solar cycle 19 (1954-1965), one of which (February 11, 1956) is famous for its high intensities at high energies, were recorded with neutron monitors.

Before 1957, cosmic ray physicists could infer the occurrence of large fluxes of lower energy particles (<100 MeV) only from the extra ionization they produced in the upper atmosphere. Such ionization absorbed radio signals, thus producing polar cap blackouts, which had (limited) use as a quantitative measure of particle flux or fluence. Since 1957, however, radio techniques have been developed that can measure the flux of lower energy particles through their ionospheric effects.2 More important, the advent of the space age enabled direct observations of solar proton events at lower energies than can be monitored from the ground. Spacecraft studies of SPEs started in the early 1960s, with Explorer 12 in 1961, Explorer 14 in 1962, and Interplanetary Monitoring Program (IMP) 1 in 1963. Routine satellite measurements of SPEs, with good data coverage, started in 1965 and have continued almost uninterrupted over the intervening decades. At present, the NOAA geostationary operational environmental satellites (GOES) are responsible for supplying real-time measurements of energetic particles to SEC for SPE monitoring, while NOAA's National Geophysics Data Center archives SPE data for research. The resulting database, combined with data from heliospheric probes and from ground-based and balloon-borne instruments, has enabled studies that have greatly increased our familiarity with and understanding of SPEs.

Most research on SPEs has focused on characterizing individual events and, as data increased, on statistical studies of those characteristics. The current state of models for SPEs is assessed in Appendix A, in the context of their potential for contributing to the risk assessment effort. Although these studies have not yet revealed how particles are accelerated to relativistic energies or how to predict the flux or fluence from an individual solar event, they are useful for making statistically realistic estimates of the likelihood that SPEs might impact ISS construction.

2.2 PROBABILITY OF SPE IMPACT ON ISS CONSTRUCTION

Turner and Baker3 first estimated the likelihood that SPEs will impact ISS construction using the significant-dose criterion (criterion 2, defined above). They statistically analyzed SPE data from the last four solar cycles to determine the average frequency of occurrence of class 2 SPEs (those capable of satisfying criterion 2) as a function of time from the solar minimum, then tabulated the intervals between the last solar minimum (near the beginning of 1997) and the times of the ISS construction flights, as specified in the then-current manifest, from June 1998 through June 2002. Assuming that the current solar cycle, cycle 23, will resemble the average of the previous four, they found that 11±6 class 2 SPEs would occur from June 1998 through June 2002. This means that between two and four class 2 SPEs probably would occur during an ISS construction flight. This result is easy to comprehend given that, according to Turner and Baker's statistics, the average probability that a class 2 SPE will occur during an arbitrary 2-week ISS construction mission is about 10 percent, and 34 shuttle missions are scheduled.

Turner and Baker's study raises an important question: Is it appropriate to apply statistics derived from averaging the previous four solar cycles (19 through 22) to the current solar cycle, considering the well-documented



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