disturbances.12,13 The exact relationship between flares and CMEs is complex and the subject of much debate at present; observations indicate that some flares may be initiated by CMEs, but the reverse is rarely if ever true.

As the eruption traverses the heliosphere, the magnetic topology of the CME (and the associated shock that is typically present for a "fast" CME) continue to evolve owing to variations in the surrounding plasma and magnetic field characteristics, compression and draping of the interplanetary field around the propagating disturbance, and interactions with preceding (slower) or following (faster) solar storms and solar-wind structures. Consequently, the configuration of the magnetic disruption that reaches Earth can be much different from its initial configuration near the Sun. Thereafter, the essential factor in determining the severity of a geomagnetic storm is the degree of magnetic misalignment at the point of impact between the solar eruption and Earth's self-generated magnetic field, called the magnetosphere (see below).

Of prime relevance to the core issue of this report is the discovery of the role played by shocks that are caused by fast CMEs. The most energetic SPE particles can reach Earth within 10 to 100 minutes of the solar manifestations of the storm seen in visible or X-ray wavelengths. The time delay depends on whether the source of the particles is a flare or a shock and on the heliographic location of the source as seen from Earth, so particle arrival times vary widely. Moreover, their flux and energy spectra evolve in transit owing to continued acceleration and transport effects.

Earth's magnetosphere is confined and shaped by the magnetized solar wind. As the solar wind passes Earth, it severely compresses the magnetospheric field on the dayside and draws it out into a long, comet like tail (the magnetotail) on the nightside. Many of the field lines that thread the magnetotail are "open": that is, they connect Earth's polar-cap ionosphere to the interplanetary medium and thus ultimately to the Sun or to interstellar space. In contrast, closed magnetospheric field lines have both "feet" on Earth, one in the southern hemisphere and one in the northern, and do not extend into the interplanetary medium.

SPE particles can reach low Earth altitudes directly by spiraling along open field lines. Solar-particle access is not limited entirely to the open field lines of the polar cap, however, for very energetic solar protons can leak onto closed field lines near the polar-cap boundary between open and closed magnetic flux. The more energetic the proton, the farther it can penetrate onto closed field lines. During geomagnetic storms, the solar wind or CME compresses the magnetosphere more severely, more field lines open as a result of magnetic reconnection, and the polar cap grows. Solar particles gain access to larger regions above Earth, particularly at the highest magnetic latitudes, which will be traversed by the ISS. Thus, the dynamic response of the magnetosphere to solar disturbances will increase the solar particle fluxes encountered by ISS.

Earth's radiation belts are composed of trapped energetic particles, which pose an additional radiation hazard for ISS. One component of the belts, the outer-belt MeV electrons, has long been known to be highly variable. Fluxes frequently vary by several orders of magnitude, with an interval of high flux observed typically once a month. Though MeV electrons rarely penetrate into the interior of a spacecraft, they can be hazardous to astronauts performing EVAs. Most of the other components of the belts, including highly penetrating energetic protons, are generally much more stable and predictable. However, we now know that that stability is not absolute. Radiation-belt experts were taken by surprise when on March 24, 1991, the Combined Release and Radiation Effects (CRRES) spacecraft observed a substantial energization and reorganization of the belt structure, including the energetic protons, when an exceptionally strong interplanetary shock hit Earth. A few qualitatively similar but weaker events have subsequently been found by retrospective analysis of old data. Thus, both major components of the radiation hazard faced by ISS—solar and radiation-belt particles—are strongly affected by the overall dynamics of the magnetosphere and particularly by the way in which the magnetosphere reacts to extreme events in the heliosphere.

1.3 METRICS OF RADIATION RISK

How one measures radiation depends on the application. For biological applications, the quantity of interest is the radiation dose absorbed by living tissue, for which the standard units are the gray (1 Gy = 1 joule of radiation energy absorbed per kilogram of tissue) and the centigray (0.01 Gy), also called a rad. For a given dose in these units, the biological effects vary with the type of radiation. A dose of energetic particles normally causes more



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