Characteristics of a plasma sheath in a magnetic dipole field: Implications to the solar wind interaction with the lunar magnetic anomalies

The solar wind interaction with the lunar surface, especially in regions of crustal magnetic anomalies, remains of great interest for in situ plasma measurements. Small-scale laboratory experiments cannot reproduce the conditions near the lunar surface, but provide a unique opportunity to identify and examine several of the physical processes. We study plasma interaction with a magnetic dipole field at an insulating surface in order to understand the effect of crustal magnetic anomalies on the solar wind-lunar surface interaction. In our experiments, electrons are magnetized with gyroradii r smaller than distances from the surface d (r < d) but ions remain unmagnetized with r > d. The measured potential distribution shows a non-monotonic sheath above the surface and variations on the surface along the axis of the dipole field. The surface near the center of the dipole is charged more positively by ions as the electrons are magnetically shielded away. A potential minimum is found in the shielding region between the surface and the bulk plasma due to collisional and magnetic mirror trapping effects. Potential variations on the surface are the result of the inhomogeneity of the dipolar field, showing an enhancement of the electric field at the cusps. Enhanced electric fields in the regions of magnetic anomalies on the lunar surface may enhance the transport of small-sized charged dust particles, possibly explaining the formation of the lunar swirls.