Minimagnetospheres above the Lunar Surface and the Formation of Lunar Swirls

In this paper we present in situ satellite data, theory, and laboratory validation that show how small-scale collisionless shocks and minimagnetospheres can form on the electron inertial scale length. The resulting retardation and deflection of the solar wind ions could be responsible for the unusual “lunar swirl” patterns seen on the surface of the Moon.

Figure 1

The Reiner Gamma formation (7.4°N, 300.9°E) is an example of a lunar swirl. Pictured here on the left-hand side of the image. Reiner Gamma is named after the Reiner impact crater shown for comparison on the right. The crater is 117 km to the east and has diameter of 30 km with a depth of 2.6 km. By contrast, the unusual diffuse swirling of the formation and concentric oval shape has fluidlike wisps that extend further to the east and west. Its distinctive lighter color stands out against the flat, dark surface of Oceanus Procellarum. Unlike crater ejecta, the shape of the formation appears unrelated to any topographic structures that would account for its presence. Image courtesy of NASA.

Figure 2

A sketch of the generic scenario of a miniature magnetic field emerging from the lunar surface and interacting with the solar wind. Flow is from left to right.

Figure 3

Photographs, with graphical labels overlaid, of the supersonic plasma stream being deflected by two different-strength magnets. (a) $23\times 20\mathrm{mm}$, 0.45 T on axis. (b) $10\times 3\mathrm{mm}$, 0.30 T on axis. External horizontal magnetic field is 0.03 T. A field-aligned current can be seen connecting the cusp region to the southern poles in both cases.

Figure 4

In situ Langmuir probe plots of the plasma density and floating potential ahead of the larger target magnet ($23\times 20\mathrm{mm}$, 0.45 T). Data from Ref. 26.

Figure 5

A plot of the measured ion density (black dots) and electric potential (green triangles) along the axis of the experiment in the vertical plane. The plasma flow is from left to right. Data from Ref. 26.