Intensity Variations in Outer Van Allen Radiation Belt

  1. James A. Van Allen
  1. S. E. Forbush,
  2. D. Venkatesan and
  3. C. E. Mcilwain

Published Online: 20 MAR 2013

DOI: 10.1029/SP037p0289

Cosmic Rays, the Sun and Geomagnetism: The Works of Scott E. Forbush

Cosmic Rays, the Sun and Geomagnetism: The Works of Scott E. Forbush

How to Cite

Forbush, S. E., Venkatesan, D. and Mcilwain, C. E. (1993) Intensity Variations in Outer Van Allen Radiation Belt, in Cosmic Rays, the Sun and Geomagnetism: The Works of Scott E. Forbush (ed J. A. Van Allen), American Geophysical Union, Washington, D. C.. doi: 10.1029/SP037p0289

Author Information

  1. Department of Physics and Astronomy, State University Of Iowa, Iowa City, Iowa

Publication History

  1. Published Online: 20 MAR 2013
  2. Published Print: 1 JAN 1993

ISBN Information

Print ISBN: 9780875908335

Online ISBN: 9781118667996



  • Cosmis ray variations;
  • Solar activity;
  • Geomagnetism


Using data from Explorer VII, the changes in the intensity of the outer Van Allen radiation belt were investigated in detail over the period October 26, 1959 to December 9, 1959. To relate the intensities to location in the belt, the parameter L was used. L is defined as a function of the integral invariant I and scalar magnetic field B, such that everywhere on the shell described by the motion of a trapped particle in the earth's magnetic field, L closely approximates the equatorial radius of the shell. For the period under study, the intensity variations at selected values of L were negatively correlated with geomagnetic activity. However, an analysis of data over a much longer period is required to determine whether this correlation is statistically significant.

For the period studied, the decrease in measured intensity during periods of magnetic storms (large ap) is consistent with the idea that some of the particles are ‘dumped' into the auroral and subauroral zones where they may partly contribute to the auroral zone currents that account for ap. In addition, the reduction in intensity generally occurred in the outer part of the zone. The low values of intensity, for L = 3.5, 4.1, and 4.7, occurred in the interval November 22 to December 9, at times when the equatorial ring current field, U (southward on the earth at the equator), was large. Thus, U cannot be due to the westward current due to the longitudinal drift of measured trapped particles in the region for which L is greater than 3.5, since a decrease in particle density therein would diminish the westward drift current with a consequent decrease in U, which is opposite to what is observed. This result indicates that the site of the ring current is either elsewhere, or that the principal contribution to it comes from drifting particles having energies below the detection thresholds of Explorer VII equipment. The phenomena were investigated over a longer period, using daily averages of the maximum counting rates. This procedure had serious limitations.