Contraction of the plasmasphere during geomagnetically disturbed periods

Direct measurements of the thermal positive ions of hydrogen and helium have been obtained from positive ion mass spectrometers aboard the Orbiting Geophysical Observatories 1 and 3. Observations made during 1965 and 1966 show distributions of H+ and He+ extending to altitudes as great as 40,000 kilometers, corresponding to a magnetospheric coordinate of L = 8. The outer boundary of the plasmasphere is characterized by an abrupt decrease in the ion concentration. This boundary or plasmapause, defined by the reduction of H+ concentration to 5 × 10° ions/cm³ or less, is often quite sharp, with decreases in ion concentration of as much as an order of magnitude occurring within 250 kilometers. The position of the plasmapause is observed to move inward and outward from the earth in an inverse correlation with the planetary magnetic activity index Kp, indicating significant large-scale expansion and contraction of the plasmasphere during periods of agitated magnetospheric conditions. The apparent correlation between measurements of the hydrogen ion boundary and the ‘knee’ whistler evidence of the plasmapause suggests that the mechanism responsible for the depletion of the ionization is effective along the lines of the magnetic field, extending well into the earth's inner atmosphere, to 1000 kilometers and below.