The total electron content of the ionosphere often responds in a dramatic way to increases in geomagnetic activity. By monitoring the VHF signals from the geostationary satellite ATS 3, it has been possible to study in detail the very pronounced increases in total content often found during the afternoon hours on the day of the commencement of a magnetic storm. Comparisons with magnetic field data show that the enhancements in electron content coincide with increases in the total magnetic field. This simultaneity suggests that, when the magnetosphere is compressed during the initial phase of a storm, the ionization stored in the magnetic tubes of force may be dumped into the topside of the F region. Such a depletion of the protonosphere is in agreement with whistler measurements, which indicate that a contraction of the plasmasphere occurs during periods of increased magnetic activity.

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A study of the power density spectrum of ionospheric scintillation of a radio star and satellites, at a subauroral location, has revealed the following: (1) Scintillations generally display a ‘pink’ noise spectrum, with almost uniform spectral density at frequencies below ∼0.01 Hz, and decreasing spectral density at frequencies above this value. (2) At frequencies ƒ ≳ 0.01 Hz, the power spectrum varies as ƒ−n, with n being typically 2.7 (i.e., a decrease of 8 db per octave). (3) The width of the spectrum displays a diurnal variation such that, on the average, higher scintillation frequencies are observed at night than during the day. These results have been examined in the context of existing theories involving electrodynamic or hydromagnetic processes, in an attempt to establish the source of the ionospheric irregularities. Although many theories suggest that hydromagnetic waves are responsible for the observed irregularities, they leave the precise nature of the interaction in doubt. It is noted that the spectrum of geomagnetic micropulsations is in substantial agreement with the scintillation power spectrum, and, therefore, a theory is developed tying the two phenomena together. A model is proposed in which horizontally ducted hydromagnetic waves produce irregularities in the F region and geomagnetic micropulsations, causing both to have the same power law spectrum.