Energetic electrons in the midlatitude nighttime E-region

Nike Apache 14.439 was launched from Wallops Island at 0003 EST on 1 November 1972, a very disturbed night (Kp = 8). A Geiger counter in the payload detected electrons (>70 keV) with a maximum flux of 1086 ± 26 cm^-2 sec^-1 ster^-1. The height-averaged ionization rate in the upper E-region is calculated from the measured electron density profile and has a value of 35 cm^-3 sec^-1. The ionization rate on this and four other nights is found to be strongly dependent on geomagnetic activity and is well represented by q = 0.14 exp (0.71 Kp) cm^-3 sec^-1 for values of Kp between 0+ and 8. It is suggested that energetic electrons are the principal source of ionization at midlatitudes in the upper E-region near midnight, even under rather quiet geomagnetic conditions.