Natural mini-magnetospheres on the Moon

Mini-magnetospheres above the Lunar Surface and the Formation of Lunar Swirls

R.A. Bamford1,*, B. Kellett1, W.J. Bradford1, C. Norberg23, K.J. Gibson3, I.A. Crawford4, L. Silva5, L. Gargaté5, R. Bingham6,1

  1. RAL Space, STFC, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, OX11 0QX, U.K.
  2. Swedish Institute of Space Physics, Box 812, SE-981 28 Kiruna, Sweden
  3. York Plasma Institute, Department of Physics, University of York, Heslington, York, YO10 5DD, United Kingdom
  4. Dept of Earth and Planetary Sciences, Birkbeck College, London, United Kingdom
  5. GoLP/Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, 1049-001 Lisboa Portugal.
  6. University of Strathclyde, Glasgow, Scotland, UK.

Abstract

Fig. 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.

In this paper we present in situ satellite data, theory, and laboratory validation that show how small-scale collisionless shocks and mini-magnetospheres 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.

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