measure of geomagnetic storm intensity that has been computed according to a uniformly applied algorithm continuously since 1932. It has become a standard against which space physicists like to correlate all manner of space weather variables. Geomagnetic conditions can be considered stormy when the Kp index exceeds 5, which happened 6.2 percent of the time during solar cycle 22. During class 1 SPEs (those capable of satisfying criterion 1 for an SPE event), however, Kp exceeded 5 nearly four times as often: that is, if an SPE is in progress, there is about a 24 percent chance that a geomagnetic storm is also occurring and that the SPE zones are dilated. For the most intense cases, the probability of a geoeffective storm coinciding with an SPE is still higher.
Since SPE zones widen during geomagnetic storms and Kp measures storm intensity, one can estimate SPE-zone width from observed Kp values. (There is, however, no database of SPE-zone widths that would allow us to eliminate the Kp proxy for them.) A Kp proxy for SPE-zone size can be obtained from an empirical relation that specifies in terms of Kp the angular size of a circle defined by the equatorward edge of the auroral oval, which is a rough indicator of the width of the SPE zones.9 The equatorward edge of the auroral oval delimits the volume of the magnetosphere in direct plasma contact with the magnetotail, a volume into which energetic SPE particles can considerably (though not always completely) penetrate.
Figure 2.3 shows 3-hour averages of the directional dose rate from particles with energies above 30 MeV (which penetrate all parts of a space suit) recorded in 1989, a year around the maximum of solar cycle 22, during which several intense SPEs occurred. The figure shows that the most intense fluxes tended to occur when the SPE zones were wide, although the tendency was not always observed. For example, there were high fluxes (above 103 particles cm–2s–1sr–1) when the width of the SPE zones was less than 30 degrees. These cases illustrate that the solar energetic particles get to Earth before the shock that brings the magnetic storm that opens the SPE zones. Nonetheless, a tendency for high fluxes to occur when SPE zones are wide is discernable. The box marks a danger