WHEN A PLANET or satellite has a weak internal magnetic field or none at all, its interaction with the external plasma can be drastically different from that described in earlier chapters. Such interactions can vary widely, depending on whether or not the body has a significant atmosphere. The simplest case, which is an appropriate starting point for discussion, is that of a body similar to the earth's moon that has, at most, a negligibly thin atmosphere.
PLASMA INTERACTIONS WITH MOONLIKE BODIES
A body like our moon, composed of insulating material and submerged in a flowing plasma, simply absorbs the particles of the plasma that are incident on the body. Thus, the lunar soil contains a record of the composition and energy of the ancient solar wind. At the moon, a bow shock will not form upstream; there is virtually no obstacle as far as the oncoming flow is concerned. The magnetic field diffuses into the very weakly conducting outer layers at a rapid rate, so that it is barely perturbed from its upstream orientation. The most notable features are associated with the plasma-absorption wake left in the plasma behind the body.
Whereas the rest of the interaction is essentially the same as previously described for other bodies, regardless of the properties of the incident plasma flow, and regardless of the orientation of the ambient magnetic field, the wake structure will depend on the upstream variables. If the body's magnetic field is zero, and the flow speed is high compared with the thermal velocity, the wake will persist to large distances; but if the flow is slow relative to the thermal speed, thermal motions perpendicular to the flow direction can refill the empty space within a short distance downstream of the body. The introduction of a magnetic field can either inhibit the refilling of the wake, if the field is nearly parallel to the upstream flow, or have a minimal effect, if it is perpendicular. The cartoons in Figure 8.1 show two hypotheti- cal configurations for the lunar wake in the solar wind for these two magnetic-field orientations.