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Published 2003 | Published
Book Section - Chapter Open

Direct Evidence of Energy-Loss in Electron-Capture-Decay Secondary Isotopes in the Heliosphere


We report direct evidence of the energy loss of galactic cosmic rays as they diffuse in magnetic irregularities expanding with the solar wind. Using the Cosmic Ray Isotope Spectrometer (CRIS) on the Advanced Composition Explorer (ACE), we report on the energy-dependence of the electron-capture decay of the secondary isotopes ^(49)V and ^(51)Cr. At the highest energies observed by CRIS, where electron attachment is unlikely, ^(49)V and ^(51)Cr are essentially stable; at lower energies the timescale for electron attachment is much shorter and substantial decay does occur. Comparing the energy dependence of the daughter/parent ratios ^(49)Ti/^(49)V and ^(51)V/^(51)Cr during periods of solar minimum and maximum demonstrates that the solar modulation parameter φ is about 400 to 700 MV higher during solar maximum than at minimum. Absolute values of φ inferred from these electron-capture-isotope data agree well with values inferred from comparison of the observed elemental energy spectra with model calculations.

Additional Information

Copyright Universal Academy Press Inc. This research was supported by NASA grants NAG5-6912 and NAG5-12929 to the California Institute of Technology and by related grants to the Jet Propulsion Laboratory, the Goddard Space Flight Center, and Washington University.

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August 19, 2023
October 23, 2023