The magnetospheric plasmapause and the electron density trough at the alouette i orbit

Abstract

The location of the magnetospheric plasmapause or ‘knee’ and the position of the electron density minimum or ‘trough’ at the orbit of the Alouette I satellite near 1000 km have been derived from whistler data and from topside ionograms respectively. A statistical study of the dependence of both knee and trough positions on the degree of magnetic disturbance and on local time during the Summer of 1963 reveals that, in general, within the statistical scatter, the geomagnetic field lines on which each occur are the same. During the local night, the equatorial radial distance to the centre of the plasmapause is described by the relation: R p = 5.64 − (0.78 ± 0.12) √ K p Earth radii, whereas the field line through the centre of the electron density trough is best described by the equation: L T = 5.64 − (1.09 ± 0.22) √ K p . It is inferred that the plasmapause in the magnetosphere and the trough near the exospheric base are related phenomena, and that a sharp decrease of plasma density occurs beyond a particular field line, the position of which depends on local time and on the degree of geomagnetic activity. For magnetically quiet conditions ( K p = 0), this line of force corresponds to L≅ 5.6 near midnight, whereas for moderate magnetic activity ( K p = 3) its L value is approximately 4. The radial distance at which this field line crosses the equatorial plane decreases at a rate of approximately 0.1 Earth radii/hr throughout the local night. The geomagnetic latitude at which the field line concerned reaches an altitude of 1000 km is approximately 57° at 02.00 LT for K p ≅ 3, and decreases at a rate of 1.8° ± 0.4° per unit increase of K p .