FIELD AND PLASMA IN LUNAR WAKE

A theory is presented to explain the observed variations of the magnetic field and plasma in the vicinity of the moon. Under the guiding-center approximation, solutions for the plasma flow near the moon are obtained from the kinetic equation. The creation of a plasma cavity in the core region of the lunar shadow disturbs the interplanetary magnetic field. Maxwell's equations are used to study perturbations of the magnetic field in the lunar wake. The acceleration drift current, which was omitted from the earlier work, is included in the present theory in the calculation of the total electric current in the lunar wake. Numerical solutions of Maxwell's equations are obtained. When the interplanetary magnetic field lines penetrate into the lunar body, the sudden change in magnetic permeability disturbs the magnetic field at the lunar limbs. Propagations of this disturbance with magnetoacoustic speed form a Mach cone downstream, which is sometimes observed as the exterior increase of field magnitude in the lunar penumbra. Perturbations of the magnetic field are restricted to the region inside the Mach cone; the region outside remains undisturbed. The numerical results agree extremely well with experimental data from the Explorer-35 spacecraft. © 1969 The American Physical Society.