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.