Direct Measurement of Cosmic-Ray Energy Loss in the Heliosphere Using Electron-Capture-Decay Isotopes

Details Direct Measurement of Cosmic-Ray Energy Loss in the Heliosphere Using Electron-Capture-Decay Isotopes Direct Measurement of Cosmic-Ray Energy Loss in the Heliosphere Using Electron-Capture-Decay Isotopes

May 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004aps..apr.j7002s&link_type=abstract

American Physical Society, April Meeting, 2004, May 1-4, 2004, Denver, Colorado April 2004, MEETING ID: APR04, abstract #J7.002

Physics

Scientific paper

We report a direct measurement of the value of the solar modulation parameter φ . This parameter characterizes the energy loss of cosmic rays as they diffuse in the interplanetary magnetic field embedded in the expanding solar wind. Using the Cosmic Ray Isotope Spectrometer (CRIS) on NASA's Advanced Composition Explorer (ACE), we report on the energy-dependence of the electron-capture decay of the secondary isotopes ^49V and ^51Cr to ^49Ti and ^51V, respectively. At the highest energies observed by CRIS, where electron attachment is unlikely, ^49V and ^51Cr are essentially stable; at lower energies the timescale for electron attachment is comparable to that for fragmentation or escape from the Galaxy and substantial decay does occur. The energy-dependence of the daughter/parent ratios ^49Ti/^49V and ^51V/^51Cr during periods of solar minimum and maximum conditions demonstrate that φ is about 350 - 400 MV during solar minimum and about 800 - 1100 MV at solar maximum. These values of φ are in good agreement with the values inferred from comparison of the observed elemental energy spectra with models of cosmic-ray propagation in the Galaxy and heliosphere.

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