Evidence for Mini-Magnetospheres at four Lunar Magnetic Anomalies: Reiner-Gamma, Airy, Descartes and Crozier

Abstract:

Lunar swirls are enigmatic high-albedo surface markings co-located with magnetic anomalies. The existence of mini-magnetospheres has been proposed as a formation mechanism, making small-scale magnetic field interactions with the solar wind of interest. Using data from the Lunar Prospector, Clementine, and Advanced Composition Explorer missions, we develop three metrics for the identification of mini-magnetospheres: 1) presence of coherent magnetism at low altitude for magnetic field measurements taken in the solar wind; 2) directional field distortions that are correlated with changes in incident solar wind azimuth; 3) intensification of total field strength. These metrics are applied to four lunar magnetic anomalies with various reflectances and magnetic field strengths, ranging from fully developed swirls (Reiner-Gamma, Airy) to diffuse albedo patches which may or may not be swirls (Descartes, Crozier). Specifically, we compare magnetic field measurements in the solar wind to source magnetization models constructed from observations in the lunar wake and Earth’s magnetotail.

By applying these criteria, we confirm previous findings of magnetosphere-like phenomena at Reiner-Gamma. We also find evidence of these phenomena at Descartes and Airy, and propose that mini-magnetospheres may exist here. At Airy, very large upwind distortions are observed, comparable to the length scale of the anomaly itself. At Reiner-Gamma and Descartes, this distortion is significantly smaller, yet the average field strengths are higher, implying that the scale of distortion is linked to the anomaly’s field strength. Interestingly, at Crozier, the weakest anomaly considered, we do not observe this distortion. However, we do observe evidence of field intensification at high solar wind pressures (16 nPa).

While Descartes and Reiner-Gamma are among the strongest anomalies on the Moon, and both exhibit magnetospheric properties, only Reiner-Gamma shows a well-developed swirl pattern. Similarly, Airy and Crozier both exhibit properties of magnetospheres, yet only Airy is clearly a swirl. This implies that even if some mini-magnetospheric characteristics are observed, this is not sufficient to create swirl morphology; distribution of the source magnetization may play a critical role.