Direct measurements of the helium photoionization rate from sounding rockets

A method of directly measuring the absolute helium photoionization rate coefficient independent of detailed solar EUV spectra and photoionization cross sections has been demonstrated. Three such measurements were obtained using windowless rare-gas ionization cells operated in an optically thin mode. The cells were launched on Nike/Terrier boosted Black Brant sounding rockets from the White Sands Missile Range in 1983, 1989, and 1990. The rate coefficients were determined for altitudes from 150 to 339 km and were extrapolated to give values of 11.1 x 10(-8) s(-1) +/- 11%, 21.3 x 10(-8) s(-1) +/- 16%, and 13.2 x 10(-8) s(-1) +/- 11% at the ''top'' of the Earth's atmosphere. The solar activity levels were characterized by absolute F-10.7 values of 119, 172, and 142 (10(-22) W m(-2) Hz(-1)), respectively. The top of the atmosphere solar EUV flux values inferred from the measured ionization rates were found to be consistent with absolute flux measurements obtained simultaneously with a neon ionization cell operated in the optically thick mode.