Open AccessCosmic ray energy loss in the heliosphere: Direct evidence from electron‐capture‐decay secondary isotopes
Author(s) -
Niebur S. M.,
Scott L. M.,
Wiedenbeck M. E.,
Binns W. R.,
Christian E. R.,
Cummings A. C.,
Davis A. J.,
George J. S.,
Hink P. L.,
Israel M. H.,
Leske R. A.,
Mewaldt R. A.,
Stone E. C.,
von Rosenvinge T. T.,
Yanasak N. E.
Publication year - 2003
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2003ja009876
Subject(s) - physics , cosmic ray , heliosphere , solar wind , nuclide , electron capture , isotope , electron , atomic physics , interstellar medium , astrophysics , nuclear physics , plasma , galaxy
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 49 V and 51 Cr can decay to 49 Ti and 51 V, 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 49 V and 51 Cr are essentially stable. At lower energies, attachment and decay do occur. Comparison of the energy dependence of the daughter/parent ratios 49 Ti/ 49 V and 51 V/ 51 Cr 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.