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Laboratory for Atmospheric and Space Physics

ACR Solar Cycle dependence

September 24, 2012

The principal source of high energy (>10 MeV/nuc) heavy ions with Z>=3 in Earth’s radiation belts is anomalous cosmic rays (ACRs), an interplanetary component of particles that originates from interstellar atoms that have been accelerated at the solar wind termination shock. Interplanetary ACRs become trapped when they are stripped of their remaining electrons in passing through the upper atmosphere. Measurements with the MAST instrument on SAMPEX over the past six years have revealed a relatively narrow radiation belt at L~2, composed of trapped C, N, O, Ne, and Ar whose intensity increased by a factor of 3 to 4 during the approach to solar minimum; remaining at more than 100 times the intensity in interplanetary space. In May, 1998, the interplanetary ACR intensity measured by SAMPEX and ACE suddenly decreased by a factor of ~3 as the effects of the onset of solar cycle 23 began to set in. This provided an opportunity to see if the flux of trapped ACRs would also decay once its source was suddenly decreased. The results were surprising – the flux of trapped ACRs also suddenly decreased by a similar factor – much sooner than expected.

During the next few years the interplanetary ACR intensity is expected to decrease by a factor of ~100. It will be interesting to see if the ACR radiation belt disappears below detection limits, or levels off at some lower intensity. Continued correlation between the interplanetary and trapped ACR intensities and composition, combined with studies of the evolving pitch-angle distribution, will have important implications for our understanding of the trapping process, its efficiency, and lifetime of trapped heavy ions in the magnetosphere.


Contributed by R. A. Mewaldt and R. S. Selesnick, Caltech & Aerospace Corp.

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