The escape of helium from the Earth's atmosphere

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Abstract
The rate of influx of helium 4 from the earth's crust into the atmosphere is about 2 × 106 atoms cm−2 sec−1. The average rate of thermal escape averaged over the last solar cycle is 6 × 104 atoms cm−2 sec−1. The present abundance of helium in the atmosphere would accumulate in about two million years. Either some nonthermal mechanism permits at present a greater rate of escape of helium or the rate of escape has varied in time. This paper is concerned with an examination of possible changes in the escape rate due to variations in the earth's magnetic field. Problems associated with the heat balance in the upper atmosphere are reviewed. Statistical analysis of drag on satellites and solar decimeter flux establishes that the 27‐day variation in atmospheric density results from fluctuations in the intensity of the solar wind. It is considered likely that a heat source over and beyond that provided by ultraviolet radiation is required to account for both the day‐night variations in upper atmosphere densities and the 27‐day variation. Various mechanisms by which energy density in the solar wind can be transferred into the upper atmosphere are considered. It is suggested that, if the additional heat source is indeed due to the solar plasma, historical fluctuations in the earth's magnetic field could result in a greater escape rate for helium. The establishment and identification of the additional heat source is necessary for a full understanding of the escape of helium. The terrestrial economy of helium 3 is reviewed. The contribution by solar cosmic rays to the abundance of helium 3 remains a major problem. The rate of influx of helium 3, about 10 atoms cm−2 sec−1, is greater than the escape rate averaged over the last solar cycle, about 4 atoms cm−2 sec−1, if solar cosmic rays contain about 10 per cent helium 3.

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