Lunar Dayside Plasma in the Earth’s Magnetotail Lobes

Abstract

As introduced in Chap. 1, the lunar atmosphere is too tenuous to stand off the solar wind plasma and magnetic field by inducing currents in its ionosphere. Therefore, the Moon is often called an “airless” body and thought of as a passive plasma absorber. This chapter presents the dual-probe ARTEMIS observations in the Earth’s magnetotail lobes to demonstrate that plasma of lunar origin can be dominant and have a significant impact on the ambient plasma. The two-point measurements reveal that the plasma density on the lunar dayside sometimes becomes several times higher than the ambient lobe plasma density. Meanwhile, the electron pitch-angle distributions show ${\sim }90^{\circ }$ electron dropouts coexisting with the plasma of lunar origin, suggesting that the velocity distributions of lobe electrons are modified in association with the lunar plasma. The accurate electron measurements by ARTEMIS with knowledge of the spacecraft potential also suggest the existence of a high-energy tail population of lunar surface photoelectrons. The high-energy photoelectron emission results in large positive potentials on the dayside lunar surface in the tail lobes, accelerating lunar ions upward from the Moon.