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

Abstract

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.