The determination of D-region electron densities from observations of cross modulations

Abstract

Theoretical expressions for the relationship between cross modulation and electron densities and collision frequencies of the D-region are considered. It is found that expressions such as that derived by Bailey and Martin (1934a, b, 1935) from the classical magneto-ionic theory are approximations which are not sufficiently accurate for interpreting cross modulation observations. On the other hand expressions based on Boltzmann's equation and the recent laboratory measurements by Phelps and Pack (1960) of the electron neutral collision frequency predict cross modulation curves which fit the observations fairly well. These new expressions for cross modulation are used along with a trial-and-error method to decide D-region electron densities. It is found that rapid changes in the D-region occur around sunrise, but that there are no striking changes during the rest of the day. Around sunrise a nearly uniform layer extending from about 60 to 80 km is created almost simultaneously at all heights. The implication of this result for theories of the production of D-region ionization are discussed.