Perturbations of the interplanetary magnetic field by the lunar wake


  • N. F. Ness,

  • K. W. Behannon,

  • H. E. Taylor,

  • Y. C. Whang


Detailed measurements of the interplanetary magnetic field in the vicinity of the moon have been performed on the Explorer 35 selenocentric spacecraft launched July 1967. The observational evidence fails to reveal the existence of a lunar bow shock wave. A pseudo-magnetosphere as suggested by Gold and interpreted from Luna 10 measurements is not observed. The interplanetary magnetic field appears to be convected through the lunar body without a large-scale distortion of its direction or magnitude. Perturbations as much as 30% of the average magnitude are noted in the solar plasma umbra and penumbra. As the satellite passes through the leeward flow, an alternating pattern of magnitude increases and decreases is observed in the penumbra, while generally only an increase is observed in the umbra. Using a theoretical model of plasma flow due to Whang, a first-order solution of the perturbed interplanetary magnetic field is compared with observations. It is concluded that the perturbations can be partially explained on the basis of the magnetization, gradient, and curvature currents induced in the disturbed solar plasma flow. The umbral increase and the innermost penumbral decrease are consistent with the first-order theory, and it is suggested that a higher-order approximation is required to explain the newly detected penumbral increases and additional penumbral fluctuations.