Cosmic ray energy loss in the heliosphere: Direct evidence from electron-capture-decay secondary isotopes

S. M. Niebur, L. M. Scott, M. E. Wiedenbeck, W. R. Binns, E. R. Christian, A. C. Cummings, A. Davis, J. S. George, P. L. Hink, M. H. Israel, R. A. Leske, R. A. Mewaldt, E. C. Stone, T. T. Von Rosenvinge, N. E. Yanasak

Research output: Contribution to journalArticlepeer-review

16 Scopus citations


Measurements by the Cosmic Ray Isotope Spectrometer (CRIS) on the Advanced Composition Explorer (ACE) spacecraft provide direct evidence that galactic cosmic rays lose energy as a result of their interactions with magnetic fields expanding with the solar wind. The secondary isotopes 49V and 51Cr can decay to 49Ti and 51V, respectively, only by electron capture. The observed abundances of these isotopes are directly related to the probability of attaching an electron from the interstellar medium; this probability decreases strongly with increasing energy around a few hundred MeV/nucleon. At the highest energies observed by CRIS, electron attachment on these nuclides is very unlikely, and thus 49V and 51Cr are essentially stable. At lower energies, attachment and decay do occur. Comparison of the energy dependence of the daughter/parent ratios 49Ti/49V and 51V/51Cr during solar minimum and solar maximum conditions confirms that increased energy loss occurs during solar maximum. This analysis indicates an increase in the modulation parameter Φ of about 400 to 700 MV corresponding to an increase in average energy loss for these elements of about 200 to 300 MeV/nucleon.

Original languageEnglish (US)
Article number8033
JournalJournal of Geophysical Research: Space Physics
Issue numberA10
StatePublished - Oct 2003
Externally publishedYes


  • Adiabatic energy loss
  • Cosmic ray isotopes
  • Electron-capture decay
  • Solar modulation

ASJC Scopus subject areas

  • Geophysics
  • Space and Planetary Science


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