Dependence of the lunar wake on solar wind plasma characteristics

Simultaneous measurements in cislunar space of the characteristics of the solar wind plasma as observed by Explorer 34, and the perturbed magnetic field in the lunar wake as detected by Explorer 35, were performed during July-October 1967. The plasma parameter βi for the ions is found to be more important in determining the magnitude of the umbral positive and penumbral negative anomalies than the direction of the interplanetary magnetic field. A quantitative comparison of these observations with the lunar wake theory of Whang indicates that the solar wind electrons must contribute even more significantly than the ions to the effective β for the plasma. Using the lunar wake as a solar wind sock for the electrons yields βe ∼ 2βi, so that on the average the temperature of the electrons is indirectly determined to be Te/Ti ≈ 2 and the net effective β of the plasma is ∼3βi.