Energization of Cold Lunar-Origin Ions in Earth's Magnetotail Lobes: Kaguya Observations and Test-Particle Simulations

Abstract

Low-energy ion data (E < 25 keV/q) acquired by the Kaguya spacecraft were used to study ions originating near the lunar surface when the Moon was in the terrestrial magnetotail lobe. We focus on three intervals during which Kaguya detected these ions at different altitudes: ∼100 km on 15 October 2008, ∼50 km on 9 April 2009, and a descent from ∼60 to ∼30 km on 9 May 2009. In each interval, the strongly enhanced fluxes of lunar-origin ions exhibited a band structure spanning less than 100 eV/q to ∼1,000 eV/q, with pitch angles from 30° to 140°. To understand where and how cold lunar-origin ions (<1 eV) are energized to the observed energy levels, we conducted test-particle simulations. The simulations indicate that these ions can reach energies of ∼300 eV through multiple reflections from the lunar photoelectron sheath, and are further accelerated by the motional electric field, increasing their energies from several hundred eV to ∼1,000 eV, consistent with Kaguya observations.