Book Description

Published studies of ionospheric motions directly associated with geomagnetic variations have postulated three different models of the mechanism by which the interaction occurs. All the models predict relative amplitudes which agree with the observations within a few orders of magnitude, and their authors were unable to choose between them with any degree of certainty. The purpose of this communication is to report some new observations, and to discuss their bearing upon the three models. It is concluded that only one of the mechanisms is ordinarily compatible with the data, i.e. interaction with hydromagnetic waves at a height where the velocity of the waves is influenced by the ion-neutral particle collision frequency. The data and theory suggest that the interaction between the ionosphere and the fluctuating geomagnetic field which gives rise to the observed Doppler shifts at 2.7 and 4.6 MHz usually occurs in the daytime at heights between 135 and 300 km and somewhat lower down at night. During the daytime the hydromagnetic EM mode of propagation may be the Left circularly polarized mode propagating parallel to the geomagnetic field. At night, however, and particularly in the case of Pt disturbances, the evidence supports the hypothesis that the principal hydromagnetic mode of propagation is the E-O, fast HM, or modified Alfven wave propagating perpendicular to the geomagnetic field. (Author).