Positive ions in the middle atmosphere during sunrise conditions

Two subsonic Gerdien condenser experiments were recently conducted at White Sands Missile Range, New Mexico, to study upper atmospheric ionization processes during morning twilight conditions. The same instrument was flown on 15 July 1975 at 0618 MST (chi = 75 deg) and 26 September 1975 at 0600 MST (chi = 90 deg). Electrical conductivity data from two subsonic blunt probe experiments (9 June 1971 at 0809 MST (chi = 53 deg) and 28 July 1971 at 0705 (chi = 68 deg) are also included in this study. At 30 km, the positive ion conductivity data for the four flights are generally in good agreement, thus indicating no particular discrepancies due to differences in launch dates or measurement techniques. Above 60 km, the buildup in conductivity with respect to the launch times presumably reflects an increase in positive ion number density associated with solar ultraviolet ionization. The most noticeable buildup, however, occurs between 35 and 60 km where the positive ion conductivity values increase during the early morning period by as much as an order of magnitude (between 45 and 50 km). The Gerdien condenser measurements indicate that this buildup in conductivity is related to an increase in positive ion mobility, thus suggesting the presence during the early morning hours of a solar dependent process for forming smaller, more mobile positive ions.