Nighttime topside ionic composition and temperature over Arecibo, Puerto Rico

Abstract

The ionic composition and temperature of the nighttime ionosphere over Arecibo, Puerto Rico, have been deduced from Thomson (incoherent) scatter spectra obtained with the radar at the Arecibo Ionospheric Observatory on seventeen nights between August 1965 and August 1966. At no time was more than 20% He+ detected at any height. In general, the transition from O+ to H+ was low (475 km) and abrupt in the winter and higher (675 km) and more gradual in the summer. Typically, the transition level (height where %O+ = %H+) lowered progressively during the summer nights, but showed a post-sunset increase during the winter nights. Analysis of this post-sunset increase indicates that it was due to a general increase in the height of the topside ionosphere, which occurred too rapidly for chemical equilibrium to be established. The resulting time constants imply a charge-exchange cross section of less than 9 × 10−16 ev cm−2 sec−1, a value smaller than previously assumed. The data for one winter night were analyzed in detail. An estimate of the downward heat flux agrees in magnitude and time variation with previous theoretical results on the cooling of the protonosphere. The composition profiles seem to imply a downward H+ flux comparable with estimates of the theoretical limit. However, observations at other times of the year suggest that other (unknown) effects may complicate the interpretation of composition profiles.