Atmospheric helium and geomagnetic field reversals

The problem of the earth's helium budget is examined in the light of recent work on the interaction of the solar wind with nonmagnetic planets. It is proposed that the dominant mode of helium (4He) loss is ion pumping by the solar wind during geomagnetic field reversals, when the earth's magnetic field is very small. The interaction of the solar wind with the earth's upper atmosphere during such a period is found to involve the formation of a bow shock. The penetration altitude of the shock-heated solar plasma is calculated to be about 700 km, and ionization rates above this level are estimated for a cascade ionization (electron avalanche) process to average 109 ions cm−2 sec−1. The calculated ionization rates and the capacity of the solar wind to remove ionized helium (4He) from the upper atmosphere during geomagnetic dipole reversals are sufficient to yield a secular equilibrium over geologic time scales. The upward transport of helium from the lower atmosphere under these conditions is found to be adequate to sustain the proposed loss rate. The implications of the differential ion pumping for other constituents are outlined for the earth's atmosphere. A pumping rate for Venus is inferred, which is sufficient to remove the hydrogen associated with about 30 atm of water from the upper Cytherian atmosphere during geologic time. The terrestrial ³He budget is strongly affected both by this mechanism and by enhanced production by cosmic rays during the depression of the geomagnetic field before and after a reversal.