DIRECT MEASUREMENTS OF THE HELIUM PHOTOIONIZATION RATE FROM SOUNDING ROCKETS

A method of directly measuring the absolute helium photoionization rat e coefficient independent of detailed solar EUV spectra and photoioniz ation cross sections has been demonstrated. Three such measurements we re obtained using windowless rare-gas ionization cells operated in an optically thin mode. The cells were launched on Nike/Terrier boosted B lack 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 1 42 (10(-22) W m(-2) Hz(-1)), respectively. The top of the atmosphere s olar EUV flux values inferred from the measured ionization rates were found to be consistent with absolute flux measurements obtained simult aneously with a neon ionization cell operated in the optically thick m ode.