A simple simulation of a plasma void: Applications to Wind observations of the lunar wake

Abstract

Abstract. Previously, the formation of the lunar wake was considered from a magnetohydrodynamic perspective. However, recent Wind particle and field observations suggest the lunar wake may be formed by kinetic processes: those microphysical processes not considered in an MHD formalism. Unfortunately, a full multidimensional and selfconsistent kinetic simulation of the lunar wake is beyond current means. However, some elements of the kinetic structure can be simulated via a simple one-dimensional electrostatic particle-in-cell simulation. We present a self-consistent simulation of a cross-sectional element of a plasma void. Essentially, wakeward directed ion beams are formed at the flanks of the simulated void, consistent with the Wind observations of counterstreaming ion beams in the wake region. These wakeward directed beams are generated by ambipolar electric fields formed at the wake edges. Other structures observed by Wind are also seen in the simulation, including an electrostatically turbulent central wake region that causes the wake to fill-in and a rarefaction wave emanating outward from the wake. IntroductionThe Wind spacecraft was launched in November 1994 as part of the Global Geospace Science (GGS) fleet, and thereafter followed a double-lunar swingby orbit to allow for long periods of solar wind study. This orbit was primarily designed for lunar gravitational assist to boost the spacecraft to increasing perigee. However, the moon also became a target of opportunity for scientific study [1970]. Unfortunately, these early investigations had relatively low resolution magnetometers, and did not have simultaneous particle distribution and plasma wave measurements. As a consequence, there was an understandable deemphasis on plasma kinetic effects since the fundamental measurements of such processes where not available. However, some debate over an MHD versus particle/kinetic lunar wake did occur, as described in the introduction of Spreiter et al. [1970]. Unfortunately, high-resolution particle and electric wave measurements were unavailable to aid in their discussion. 23,653

Detailed structure and dynamics in particle-in-cell simulations of the lunar wake

“…The simulations by Farrell it et al [12], display some similarities with these higher resolution simulations. Rarefaction waves and a two-stream instability occur, which shall be discussed further in Sec.…”

“…Intriguingly, whereas Farrell et al [12] find that the ion dynamics dominate the interaction, the higher phase space resolutions of Birch and Chapman [13,14] established that the electron dynamics dominate the evolution of the wake. In this paper we present new results from these simulations, including detailed particle density and electric field descriptions, a comprehensive analysis of the electron instabilities occurring in the wake, and a detailed description of the structure and dynamics of electron phase space holes generated in the wake that are not found in previous simulations.…”

“…Simulations of this nature have previously been undertaken [12,13,14]. Intriguingly, whereas Farrell et al [12] find that the ion dynamics dominate the interaction, the higher phase space resolutions of Birch and Chapman [13,14] established that the electron dynamics dominate the evolution of the wake.…”

“…As in Birch and Chapman [13], ∼ 2500 particles per cell were used, compared with ∼ 80 particles per cell used by Farrell et al [12]. Figure 1 shows the geometry of the simulation.…”

Particle‐in‐cell simulations of the lunar wake with high phase space resolution

“…Once the instability has reached its saturated state, the beams will begin folding in phase space and eventually produce a monotone-decreasing distribution. A simple simulation of a plasma void [Farrell et al, 1998] produced a description of the lunar wake, We present results from a similarly geometrically sim-1D kinetic simulation of the lunar wake, but with ple 1 • significantly more particles per cell, thereby fully resolving both ion and electron phase space structure and dynamics for the first time. This enables a more complete comparison with the WIND observations, in particular including the full electron dynamics.…”