Asymmetric Lunar Magnetic Perturbations Produced by Reflected Solar Wind Particles

Abstract

Magnetic perturbations characterize the solar wind interaction of the Moon. The solar wind plasma absorption on the dayside surface produces large-scale field perturbations behind, i.e., the field enhancement in the central wake and reduction on the wake boundary. The solar wind repellence over local lunar magnetic anomalies (LMAs) leads to small-scale magnetic compressions ahead. In this study, the magnetic perturbations around the Moon are examined by using the observations from a near-Moon satellite mission, the Lunar Prospector, and they exhibit a clear left–right asymmetry in a coordinate system related to the solar wind convection electric field ( ). The magnetic field is observed to enhance before the left terminator that points to, while on the opposite side, it is not. The test particle simulations show that can divert the particles reflected over the LMAs to the left and then the solar wind pickup of these particles leads to the field enhancement observed before the left terminator. Behind the lunar terminator, the wake field reduction is also asymmetric. On the left, the field reduction is more remarkable and located closer to the central wake. The denser plasma, consisting of the background as well as the reflected solar wind particles, may produce a stronger diamagnetic current and thus more significant field reduction there. The asymmetric plasma and magnetic perturbations associated with the reflected particles may be a common and nonnegligible element during the solar wind interaction of a small-scale magnetic field, such as that of an asteroid or a comet.

A Meandering Lunar Wake Produced by the Pickup of Reflected Solar‐Wind Ions

“…This asymmetric magnetic enhancements have been reported by Zhang et al. (2020). Also because of the large gyro‐radii, the reflected ions are easy to bypass the Moon and penetrate into the downstream lunar wake (Nishino et al., 2009).…”

“…This type of magnetic enhancement prefers to occur near the terminator which the solar-wind electric field points to. This asymmetric magnetic enhancements have been reported by Zhang et al (2020). Also because of the large gyro-radii, the reflected ions are easy to bypass the Moon and penetrate into the downstream lunar wake (Nishino et al, 2009).…”

“…Figure 2a shows the XY plane projection of the spherical distribution of the magnetic perturbations, | ⃗ |∕| ⃗ IMF, | − 1 , around the Moon, where ⃗ is observed by LP and ⃗ , is the corresponding undisturbed IMF obtained from the OMNI data set. Three typical characteristics exhibit: the magnetic field enhancement near the terminator as discussed by Zhang et al (2020); the field decrease in the expansion regions; and the field enhancement in the central wake. All the three types of perturbations are surprisingly biased clockwise in Figure 2a and are asymmetric about the X axis.…”

“…Two coordinate systems (CSs) are adopted here to analyze the reflected ion motion-associated phenomena (Figure 1). One is the "Lunar Solar-wind-Electric field" (LSE) CS (Zhang et al, 2020), in which the effects arising from the solar-wind electric field |) to remove the effect on the wake length from the different solar-wind velocity, where A second CS, named as "BE," is also adopted, in which the Y axis is still against ⇀ ( 𝐴𝐴 𝐴 𝐴𝐴𝐵𝐵𝐵𝐵 = 𝐴 𝐴𝐴𝐿𝐿𝐿𝐿𝐵𝐵 ), the Z axis, however, is fixed along the background…”

“…Two coordinate systems (CSs) are adopted here to analyze the reflected ion motion‐associated phenomena (Figure 1). One is the “Lunar Solar‐wind‐Electric field” (LSE) CS (Zhang et al., 2020), in which the effects arising from the solar‐wind electric field $${\stackrel{\rightharpoonup }{E}}_{\text{SW}}=-{\stackrel{\rightharpoonup }{V}}_{\text{SW}}\times {\stackrel{\rightharpoonup }{B}}_{\text{IMF}}$$ can be well described, where $${\stackrel{\rightharpoonup }{B}}_{\text{IMF}}$$ is the IMF. In this CS, the Y axis $${\hat{y}}_{LSE}$$ is always against $${\stackrel{\rightharpoonup }{E}}_{\text{SW}}$$, the X axis $${\hat{x}}_{\text{LSE}}$$ points always against the instantaneous solar‐wind velocity $${\stackrel{\rightharpoonup }{V}}_{SW}$$, and the Z axis completes the right‐hand orthogonal set ($${\hat{z}}_{\text{LSE}}={\hat{x}}_{\text{LSE}}\times {\hat{y}}_{\text{LSE}}$$).…”