Direct accretion of ³He and ³H from cosmic rays

Abstract

The importance of the direct accretion of cosmic ray ³He and ³H is investigated in light of recently reported spacecraft measurements of the cosmic ray fluxes of these isotopes. The relevant solar and galactic cosmic ray data for solar cycle 20 are briefly reviewed, and average accretion rates are calculated for ³He, 4He, and ³H by using these data and including the shielding effects of the geomagnetic field. These calculations indicate that, for energy thresholds of ∼1 Mev/nucleon, the average cosmic ray accretion of ³He, 4He, and ³H is small compared to other sources of these isotopes. With the data presently available, it is not possible to ascertain whether accretion of these isotopes is significant at energies below 1 Mev/nucleon. However, extrapolations from the existing measurements suggest that direct accretion of solar cosmic rays should not be excluded as a possible source of terrestrial ³He and ³H. For instance, if the solar flare ³He spectra reported at ∼10 Mev/nucleon can be extrapolated to energies as low as 10 kev/nucleon, the average accretion of solar ³He may be as great as ∼1 atom cm−2 sec−1, comparable to terrestrial ³He production by cosmic ray interaction in the atmosphere, precipitation from the solar wind, or the degassing of the earth. Possible measurements of atmospheric ³He fluctuations following intense solar events are briefly considered in the context of current mass spectrometer techniques.