Unlike Earth, whose magnetic field deflects much of the incoming solar wind, the surface of the Moon absorbs most of the particles that the Sun sends its way. This creates a void behind the Moon that gradually refills with plasma, forming a cone-shaped “lunar wake.” However, the processes that govern refilling remain unclear, although models suggest they probably include a mixture of kinetic effects and effects particular to the dynamics of electrically conducting fluids.
Zhang et al. now provide a detailed characterization of the lunar wake and its relationship to the direction and strength of the solar wind. The researchers used 2 years’ worth of data from the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) mission to characterize many physical properties of the wake, including its magnetic properties, ion and electron densities, temperatures, pressures, and flow while simultaneously monitoring the solar wind.
The scientists find that the lunar wake trails behind the Moon out to a distance at least 12 times its radius. The edges of the wake generate density waves that form disturbances that propagate both outward and inward like the wake of a boat. This process can mostly be explained by known principles about the flow of plasma. In contrast, they find that kinetic effects most likely explain the midwake maximum in ion and electron temperatures, which may result from high-energy particles refilling the wake faster than their low-energy counterparts.
Notably, the researchers find that the angle between the direction of the solar wind and the orientation of the interplanetary magnetic field also influences the shape and character of the lunar wake, making it more ringlike for angles close to parallel and flatter for more perpendicular configurations. The scientists also find that the interplanetary magnetic field itself bulges toward the Moon inside the lunar wake, although they cannot yet identify the mechanism behind this observation. (Journal of Geophysical Research: Space Physics, doi:10.1002/2014JA020111, 2014)
—Julia Rosen, Freelance Writer
Citation: Rosen, J. (2015), Satellite data yield detailed picture of the lunar wake, Eos, 96, doi:10.1o29/2015EO025751. Published on 9 March 2015.