The morphology and temporal variations of the Van Allen radiation belt, October 1959 to December 1960

Abstract

The time variations of intensity in the ‘horn’ of the outer Van Allen radiation belt as measured by the omnidirectional GM counter 302 in Explorer 7 were investigated for the period October 1959 to December 1960. A consistent empirical relationship between intensity and scalar magnetic field B was derived for different values of the magnetic shell parameter L that made it possible to ‘correct’ for dependency of intensity on B and thus to examine the true time variations at any fixed L. The correlation between counting rates in the outer zone over North America and Australia at the same value of L improved decidedly after correction for B. The changes in the corrected intensity for several L values between 2.5 and 4.7 earth radii revealed some large variations (greater than a factor of 100) prominent only between L = 2.5 and L = 3.2, and some fluctuations by a factor of 10 only at the larger L values. The temporal variations of intensity between L = 2.5 and 3.5 over North America are similar to those between L = 1.8 and 2.5 over South Africa. The changes in intensity tend to be correlated negatively with U, the geomagnetic equatorial ring current field, for L>3.4, and positively for L<3.4. Lmax, the value of L at which the maximum intensity occurs, tends to decrease with increasing U, the correlation coefficient between Lmax and log U being −0.7. During the main phase of large magnetic storms the intensity for L>3.5 is generally less, and for L<3.5 it often tends to be greater, than before the storms. For 2.5≤L≤3.5 nearly all the observations over Australia, and about half those over North America, were at B values equal to or greater than those at sea level over the region of South Africa for the same L values. Consequently, all the particles observed under these conditions over Australia and North America are lost as they drift over the region of South Africa. The observed intensities over Australia, in particular, measure the outflux of particles from the outer radiation belt in a time interval less than the longitudinal drift period. This result implies a very high rate of replenishment of energetic electrons in the outer belt.