Photoionization rates in the night-time E- and F-region ionosphere∗

Quantitative estimates of ionization sources that maintain the night-time E- and F-region ionosphere are given. Starlight (stellar continuum radiation in the spectral inverval 911-1026 Å) and resonance scattering of solar Ly- β into the night sector are the most important sources in the E-region and are capable of maintaining observable electron densities of order (1-4) × 10 ³ cm ^-3. Starlight ionization rates have substantial variations (factors of 2-4) with latitude and time of year since the brightest stars in the night sky occur in the southern Milky Way and Orion regions. In the lower F-region the major O ⁺ source in the equatorial ionosphere is 910 Å radiation from the O ⁺ recombination in the F2-region, whereas in the extratropical ionosphere interplanetary 584 Å radiation only exceeds resonance scattering of solar 584 and 304 Å radiation as the dominant O ⁺ source during the month of December.