Cislunar electron content as determined by radar group delay measurements

An earlier report described an experimental radar technique for measuring differential group delay to the moon with an accuracy of 10 μsec and presented the results of 5 measurements. These data, when combined with other radar work, showed large differences in total cislunar electron content above 1000 km in the solar and antisolar directions. It was suggested that this difference was most likely due to a high-density (several hundred electrons per cubic centimeter) plasma wake of the earth formed by the solar wind and extending at least as far as the moon. Since this interpretation is not supported by other evidence, the experiment was refined and then performed during 55 lunar meridian transits in July and August 1965. These data have been compared with ionospheric contents deduced from Faraday rotation on 136-Mhz signals from the Syncom satellite and with protonospheric content calculated using Explorer 22 1000-km Langmuir probe data. The radar data support the initial results concerning the solar-antisolar asymmetry, but the new data for the computation of electron content between 1000 km and the plasmapause (at about 4 RE) indicate that the difference is due to plasma in this relatively unexplored region of the magnetosphere. Thus it is concluded that, within the ±30 electrons cm−3 accuracy of the combined measurement, the electron number density in the geomagnetic tail is not markedly different from that of interplanetary space.