AMBIPOLAR DIFFUSION AND MOTION OF ION CLOUDS IN EARTHS MAGNETIC FIELD

It is shown that the behaviour of an ion cloud at altitudes above about 95 km may depend strongly on its initial configuration, and particularly whether it is closely magnetic field aligned. From consideration of planar ionized irregularities it is found that diffusion across the field lines is ion-controlled unless the irregularities are closely field aligned (to within 1° in the E-region, and less in the F-region). In the latter case the transverse diffusion is electron controlled (above 95 km) and hence considerably inhibited by the magnetic field. This result is also confirmed by numerical computations for the case of cylindrical irregularities. Somewhat similar results are found for the effects of neutral winds and electric fields. For instance it is shown that, in the E-region, ion clouds will move with the neutral wind unless they are highly field-aligned when they will remain fixed to the magnetic field lines. An electric field E, at right angles to the magnetic field B, causes an irregularity to move in the direction of E Λ B; non field-aligned ones move with the Hall drift velocity of the ions, while field-aligned irregularities move with the Hall drift of the electrons. These velocities are approximately equal in the F-region but differ greatly in the E-region. This is proposed as the explanation for the large East-West motions (~1 km sec-1) of field-aligned irregularities observed in the radio-aurora, and requires a meridional electric field of order 50 mV/m. Observations of radio meteor trains, made at a frequency of 501 MHz, are described. These reveal a significant effect of the geomagnetic field at altitudes above 92 km, both on the rate and duration of the echoes, when the aerial beam makes a normal intersection with the magnetic field lines at these altitudes. The experimental results are compared with the theoretical predictions for diffusion in the geomagnetic field. © 1969.