Photoelectrons and Lunar Limb Shocks

Abstract

It is suggested that boundary conditions for solar wind/lunar limb interactions are active. The ‘whole-Moon’ limb does not evoke a shock cone because warm (≃ 13 eV electron⁻¹) solar wind electrons are replaced by cool (< 2 eV electron⁻¹) photoelectrons that are ejected from the generally smooth areas of the lunar terminator illuminated at glancing angles by the Sun. A localized volume of low thermal pressure is created in the solar wind by these cool photoelectrons. The solar wind expands into this volume without shock production. Directly illuminated highland areas exchange hot photoelectrons (< 20 eV electron⁻¹) for warm solar wind electrons. The hot electrons generate a localized pressure increase (Δp) in the adjacent solar wind flow which evokes a shock streamer in the solar wind. Shock streamers are identifiable by a coincident increase in the magnitude (ΔB ~ Δp) of the solar wind magnetic field. Shock occurrence is controlled by lunar topography, solar activity in the hard ultraviolet (> 20 eV), solar wind electron density and thermal velocity, and the density of the solar wind magnetic field.