A study has been made of electron density and total electron content in tubes of force near the knee in magnetospheric ionization. It was based on whistler observations made at Eights, Antarctica, in July and August 1963, under conditions of steady, moderate geomagnetic agitation (Kp = 2–4). During the period 0100–0400 LT, the electron density near the equatorial plane at a geocentric distance of 4 RE drops a factor of 30–100 within a distance of less than 0.15 RE. The corresponding change in tube content above 1 cm2 at 1000 km is a factor of about 10 within less than a degree of latitude. The afternoon profiles are generally similar to the postmidnight results. In the afternoon the profiles are often well defined outside the knee, falling smoothly to about 1 el/cm3 at 7 RE. Both the equatorial and tube content profiles show a definite repeatability between observations at a given local time and under conditions of moderate, steady geomagnetic agitation. Diurnal effects are evident, with amplitude differing from point to point on the profiles. The problem of the field-line distribution of ionization has been studied with a view to minimizing error in the profiles. Experimental and theoretical support has been found for using a diffusive-equilibrium distribution along the field lines inside the knee and a ‘collisionless’ model, behaving approximately as Ne ∝ R−4, along the field lines outside the knee. A study of error shows that the uncertainty in individual points on the knee profiles is everywhere less than a factor of ±2, and on the content profiles less than a factor of ±1.5.
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