The position of the knee in the density of magnetospheric ionization was measured on a high time-resolution basis using whistlers recorded during July and part of August 1963. (The knee is an abrupt decrease in magnetospheric ionization density, frequently observed at field lines with an equatorial radius of about 4 RE.) The data were obtained at Eights (64°S dipole latitude) and Byrd (70°S dipole latitude) in the Antarctic. The whistler results and results from other experiments confirm that the knee is a regular feature of the magnetosphere. For conditions of steady, moderate geomagnetic agitation (Kp = 2–4), the diurnal variation in geocentric equatorial range to the knee is remarkably repeatable. It is characterized by (1) a slow inward movement of the knee on the nightside, covering about 1.5 RE in 10 hours; (2) a slight outward movement on the dayside covering about 0.5 RE and (3) a rapid outward shift in the late afternoon covering about 1 RE in 1 hour. During periods of changing magnetic activity, the knee position changes with at most a few hours' delay, moving inward with increasing magnetic activity. The results from Eights and Byrd may be generalized to describe a three-dimensional model of thermal ionization in the magnetosphere involving a dense (∼100 el/cm3) inner region and a tenuous (∼1 el/cm3) outer region separated by a sharp field-aligned boundary, the plasmapause. During the postmidnight hours, the inward motion of the knee involves a corresponding inward motion of the ionization just inside the plasmapause. The rapid outward shift of the knee near 1800 LT does not involve an outward plasma motion, but instead involves the presence of a region of ‘new’ high-density (∼100 el/cm3) plasma in the equatorial range of about 4–5 RE. Preliminary evidence shows that, at least in the period 0000–1700 LT, the ionization inside the plasmapause rotates at approximately the angular velocity of the earth.