Minimagnetospheres above the Lunar Surface and the Formation of Lunar Swirls

Abstract

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.Miniature magnetospheres have been found to exist above the lunar surface [1] and are closely related to features known as "lunar swirls" [2]. Mini-magnetospheres exhibit features that are characteristic of normal planetary magnetospheres namely a collisionless shock. Here we show that it is the electric field associated with the small scale collisionless shock that is responsible for deflecting the incoming solar wind around the minimagnetosphere. These ions impacting the lunar surface resulting in changes to the appearance of the albedo of the lunar "soil" [2]. The form of these swirl patterns therefore, must be dictated by the shapes of the collisionless shock.Collisionless shocks are a classic phenomena in plasma physics, ubiquitous in many space and astrophysical scenarios [3]. Well known examples of collisionless shocks exist in the heliosphere, where the shock is formed by the solar wind interacting with a magnetised planet. What is a surprise is the size of the mini-magnetospheres, of the order of several 100 km; orders of magnitude smaller than the planetary versions. Results from various lunar survey missions have built up a good picture of these collisionless shocks.These collisionless shocks have a characteristic structure in which the ions are reflected from a rather narrow layer, of the order of the electron skin depth c/ω pe (where c is the speed of light and ω pe is the electron plasma frequency), by an electrostatic field that is a consequence of the magnetised electrons and unmagnetised ions. The narrow discontinuity in the shock structure produces a specular reflected ion component with a velocity equal to or greater than the incoming solar wind velocity. The reflected ions from a counter-propagating component to the solar wind flow that form the magnetic foot region, which extends about an ion Larmor orbit upstream from the shock. This occurs when the Mach number (the ratio of flow velocity to Alfvén velocity) is of the order 3 or less.We have carried out laboratory experiments using a plasma wind tunnel, to investigate mini-magnetospheres

3d Pic Simulations of Collisionless Shocks at Lunar Magnetic Anomalies and Their Role in Forming Lunar Swirls

“…The electric field responsible for the scattering can be seen to be omnidirectionally pointing outward, regardless of the magnetic field orientation. This is because it is proportional to the gradient in the magnetic field intensity, | |  B 2 , not | | B , in accordance with theoretical expectations (Bamford et al 2012). The projection onto the x−y or surface plane shows the electric field intensity at the lunar surface.…”

“…The simulations quantifiably confirm the satellite (Halekas et al 2014) findings and the theoretical predictions (Bamford et al 2012), namely that a miniature collisionless shock can be responsible for all the observations (Lin et al 1998;Huixian et al 2005;Wieser et al 2009Wieser et al , 2010Futaana et al 2010;Hashimoto et al 2010;Lue et al 2011;Wang et al 2012;Halekas et al 2014;Yokota et al 2014). Here it is confirmed that the interaction or boundary layer can form well above (i.e., kilometers rather than meters) the lunar surface (depending upon conditions) and need not be a photoelectricsheath (Garrick-Bethell et al 2011) restricted to a few meters above the surface.…”

“…The barrier that forms results in responsive inductive currents and corresponding magnetic fields. The thickness of the barrier is of the order of the electron skin depth wc pe , as theoretically predicted by Bamford et al (2012). The small-scale plasma instabilities, waves, and turbulence that are formed provide the means by which the proton and electron particle distributions become non-thermal and exchange energy through Landau damping (Bingham 1993).…”

“…This occurs not just at the lunar surface but throughout the mini-magnetosphere boundary. In order to model this scale of interaction, these criteria must be maintained between lunar, laboratory (Bamford et al 2008(Bamford et al , 2012, and computer simulation,. Only a full plasma kinetic code (Dawson 1983;Fonseca et al 2002) can capture these characteristics.…”

“…These can be compared to the analytical expressions (Bamford et al 2012) and observational data, e.g., Halekas et al (2014), for mini-magnetospheres. Figure 3 shows the results of the simulation of the solar wind plasma impacting a localized crustal magnetic field structure.…”