Production and loss of electrons in the quiet daytime D region of the ionosphere

TLDR: In this paper, the authors show that the presence of a steep ledge in electron density at an altitude that lies between 80 and 90 km, in the vicinity of the mesopause, cannot be related to a change in the primary production rate of electrons.

Abstract: Rocket probe measurements of the concentration of free electrons in the undisturbed daytime D region typically show the presence of a steep ledge in electron density at an altitude that lies between 80 and 90 km, in the vicinity of the mesopause. This paper is chiefly concerned with the interpretation of this feature. It is shown that the ledge cannot be related to a change in the primary production rate of electrons, and calculations based on existing knowledge of negative-ion reactions indicate that its presence is not likely to be related to negative-ion formation. We conclude that the ledge probably owes its existence to a sharp change in electron-positive-ion recombination coefficient, coinciding in altitude with the sharp change in nature of the dominant positive-ion species from molecular ions above the ledge to water vapor cluster ions below. The small electron densities below the ledge can be readily explained if the complex cluster ions have recombination coefficients of the order of 50 times larger than those of simple molecular ions at mesospheric temperatures. Such large recombination coefficients may be related to a new mode of recombination available to complex cluster ions, but not to the relatively simple molecular or atomic ions.