Twilight helium 10,830-A calculations and observations. [in upper atmosphere

Calculations have been made of the 10,830-A column emission rates from metastable He(2/sup 3/S) atoms in the upper atmosphere, and these have been compared with other calculations and observations. The dawn/dusk intensity ratio is accounted for mainly by changes in the ambient thermal electron concentration for the observations in Peru and Brazil and by changes in plasmasphere opacity to conjugate point photoelectrons for the New Mexico and to some extent the Brazilian data. The large enhancement of the New Mexico intensities at local solar zenith angle 100degree by a factor of 10 or more in winter is due partly to the presence at that time of conjugate point photoelectrons, which increase the excitation rate up to 3 times that due to the local photoelectrons alone. The change in ambient helium concentration from summer to winter by a factor of 3 or more accounts for the remainder of the enhancements. There is a discrepancy by a factor of 6 in absolute intensities, which could be removed by a rather improbable conjunction of errors all acting in the right direction. Alternatively, increasing the photoelectron flux calculated by a factor of 2 and decreasing by an order of magnitude the recently measured crossmore » section for loss of He(2/sup 3/S) by Penning ionization on atomic oxygen (Cook et al., 1974) would remove the discrepancy. (AIP)« less

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