The Moon has a thin atmosphere/exosphere and no global magnetic field. Earlier measurements back to Apollo era have inferred the absence of a global magnetic field at the Moon by the absence of a standing shock wave in the solar wind upstream from the Moon as well as at the limb of the Moon. Rather, limits on the scale sizes of the lunar fields were indicated from both the Apollo surface measurements and orbiter observations. These small-scale magnetic fields, called magnetic anomalies, are regions of crustal magnetization distributed over the lunar surface. Recent in situ measurements show that these local magnetic fields can be quite strong even at 20–30 km altitude, and range in size from less than a km to hundreds of km. Studies have shown that the lunar magnetic anomalies may have strong influence on the solar wind flow.

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.

NLSI’s CCLDAS team studied the plasma interaction with a magnetic dipole field over an insulating surface for the case of the magnetized electrons and unmagnetized ions. The emergence of non-monotonic sheaths and complex potential variations along the dipole axis were identified. The potential on the surface in the dipole center was found to be slightly more positive than the bulk plasma potential due to the ion charging, while the electrons remained shielded away.

The experiments showed that both electron-electron and electron-neutral collisions can cause the electrons to migrate into the shielding region. A potential minimum was thus formed when these electrons were trapped between the symmetrical cusps due to the magnetic mirror effects. The potential variations on the surface were a consequence of the inhomogeneous dipolar field, which displayed an electric field enhancement at the cusps. The laboratory results showed that the charge separation could occur at the lunar surface in the regions where the solar wind interacts with the smallscale magnetic anomalies, leading the surface to charge to large positive values in order to reflect a fraction of the incoming protons. At the Moon, a non-monotonic sheath may also form due to the photoelectrons and secondary electrons trapped in the electron-shielding region. The enhanced electric fields in such regions may increase dust transport, possibly explaining lunar swirl formation.

You can read the entire paper, published today online in the Journal of Geophysical Research for Space, here:
Wang, X., M. Horányi, and S. Robertson (2012), Characteristics of a plasma sheath in a magnetic dipole field: Implications to the solar wind interaction

Posted by: Soderman/NLSI Staff
Source: NLSI Team

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