The Lunar Wake: MFI Observations and Interpretation


A new view of the solar wind-moon interaction is now provided by the high resolution instrumentaiton onboard the WIND spacecraft. The spacecraft has made many lunar swingbys (over a dozen) since its launch in November 1994. Early studies of the lunar wake region in the 1960's and 1970's emphasized the magnetohydrodynamics (MHD) to explain many of the observed features of the lunar plasma environment measurements. However, the WIND observations demonstate that plasma kinetic effects are also important to consider in the solar wind-moon interaction.

The MFI experiment on WIND has contributed greatly to advancing this understanding of the interaction. Particular emphasis has been on the WIND wake transit occurring on 27 December 1994, when the spacecraft flew within 7 Rl of the body. During this passage, the magnetometer detected a distinct quasi-DC magnetic pertubation associated with boundary currents at the wake flanks [Owen et al., 1996], and upstream ULF magnetic waves and electron outflows on field-lines connect to the wake. These are both discussed below.

Although the MFI investigators emphasize magnetic measurements, their sceintific approach has been "holisitc" in nature. With cooperation with other WIND investigation teams, a larger picture of the wake dynamics has revealed itself.

The Quasi-DC Magnetic Signature of the Lunar Wake

List of WIND Lunar Encounters

ULF Magnetic Precusors: Evidence for Electron Wings

Due to the enhanced resolution of the WIND magnetometer and electron instruement, new WIND observations now show that the moon's influence in the solar wind extends over a much wider region, well beyond the extent of the trailing ion wake. Specifcally, The WIND magnetic fied instruent (MFI), particle experiment (SWE) and and WAVES radio experiment detected substantial and significant distrubances on field lines upsteam, but directly connected to the lunar wake [Farrell et al., 1996, Farrell et al., 1997; Bale et al., 1997; Bale, 1997].

The magnetometer detected a ULF magnetic disutrubance for almost an hour preceding the actual wake encounter. Intense magnetic precursor activtiy was previously reported by Ness and Schatten [1969] using the magnetometer on Explorer-35. Unfortunately, electron distribution data was unavailable to the Explorer-35 scientiest. Figure 1 shows the location where the activity was observed, and Figure 2 shows a ULF magnetic spectrogram of the actual disturbance near 1 Hz.

Graphic showing where in the lunar system the activity was observed

Magnetic Spectrogram of the actual disturbance

A most interesting observation was the correlation of outflowing electrons of keV energies directly from the wake at the same time as the ULF wave activity was observed. The electron distribution, as derived by the WIND SWE experiment, had an enhanced energetic tail in the antisunward direction. Bale et al. [1997] report that both Langmuir wave and broadband electrostatic noise increased dramaticaly during magnetic connection and electron outflowing periods. As described by Farrell et al. [1996], the outflowing electron anisotropy could be a potential ULF magnetic wave driver down at 2 Hz, with resonance occuring with electrons near 1 keV. Bale et al., [1997] indicates that such electron flows must occur to give rise to the enhanced Langmuir and electrostatic noise observed by WIND/WAVES.

Initially, this upstream disturbances consisting of electron outflows, ULF magnetic wave [Farrell et al, 1996], Langmiur wave, and broadband electrostatic noise (BEN) [Bale et al., 1997] was considered analogous to activity observed upstream of planetary bow shocks, that found in the foreshock regions. Consequently, the region was considered as a sort of "forewake" for preconditioning the incoming particles to the advent of an upcoming and substantial wake [Farrell et al., 1997].

However, the true nature of this region was best defined by the models of Bale et al. [1997] who demonstrated that the region appears more like an electron "wing" structure propagating at large angles on field lines connected and upstream of the wake. Essentially, the region possesses a depletion in electrons, giving rise to a net positive potential that is capable of drawing in electron from exterior locations.

Obviously, given the copius plamsa wave and anomalous particle distributions, the electron wings observed by WIND possess a kinetic nature. In fact, the electron wings are not predicted or observed in MHD simulations of the lunar wake carried out in the late 60's [Spreiter et al., 1970].

Why was this highly extended wing system not identified previously? Primarily, the reason resides in the fact that Explorer-35 flew no radio, could not obtain particle distributions and had a 100 time less magentometer sensitivy. As a consequence, the spacecraft flew through these regions but did not contain the modern day instrumatation to detect the "weaker" electron wings. ULF magnetic disturbances upstream of the moon were observed by Explorer 35 [Ness and Schatten, 1969], which were certainly indicative of the wings, but these signals were detected only within 4 Rl of the body. Beyond this point, the signals were just too weak for detection by the onboard magnetometer. Thus, the detection of a highly extended and flared wing system propagating as far as 10 Rl upstream could not be made or even fully envisioned in the 1970's. Since no real lunar missions occured in the 80's and early 90's, its now the WIND measurements that allow for an update of the physics associate with a large unmagetized body in a plasma stream.

Kinetic Models of the Wake Dynamics

Given the large size of moon and the fact that Explorer-35 emphasize magnetic lunar wake measurements, the simulations of the moon's interaction with the solar wind emphaized the magnetohydrodynamic (MHD) aspects. For example, Spreiter et al. [1970] performed a detail MHD simulation demonstrating that a large Alfvenic wake should extend behind the moon. However, there was considerable discussion at the time that particle or "kinetic" effects might also be a dominate process in the wake dynamics (see the "Introduction" to Spreiter et al. [1970] . Thus, early in the study of the lunar wake, there was some interesting debate over the approach to be used in modeling the plasma.

The WIND observations suggest the the wake dynamics is far more kinetic in nature. For example, upstream electron flows, and wave activity in many modes are all occuring in the electron wings [Farrell et al., 1996; Bale et al., 1997]. Strong BEN and ion beams are observed in the central lunar wake [Kellogg et al., 1996, Ogilvie et al, 1996; Bale et al., 1997a,b; Farrell et al., 1997]. Thus, it is becoming quite clear that the dynamics of the wake cannot be fully understood via MHD simulation alone.

As a consequence, we initiated a very simple kinetic simulations in order to understand the fundamental aspects of kinetic plasma expansion into a void. Ogilvie et al. [1996] analytically-derived the density of a plasma expanding into the vacuum region, and their one-dimensional solution compared favorably with many elements of the the SWE observations. Thus, an obvious first set is to simulate plasma expansion into a void, in this case using a one-dimensional electrostatic PIC code. This self-consistent solution can be compared with the Ogilvie et al. [1996] result.

Unfortunetly, two-dimensional fully electromagnetic PIC codes of size comparable to the moon and extended wake would be so large (requiring 10's of millions of particles and run times on the order of 10's of thousands of plasma periods) that they are beyond the realm of possiblity. Also, convection by the solar wind would require particles to constantly be recycled. However, by analyzing part of the problem, and realizing the limitations, comparison of modeled and observed kinetic effects can be made.

For example, to simulate the one-dimensional analytical calculation of plasma expansion in Ogilvie et al [1996], we recently ran a simple one-dimensional electrostatic PIC code using 20000 electrons and ions spread over 500 Debye lengths, but with a central void of particles of 125 Deble lengths in size. The simulation is periodic, and thus is terminated at a time when wake related particles appear to cross the boundary.

Figure 3 shows the results of the ion distibution in configuration and velocity space for a series of simulated times. At the start time, the ion void is obvious and distinct. However, after 80 plasma periods, ion beams are forming at the wake edges, and are propagating into the wake as counterstreams separeated by approxiamtely 3 ion thermal speeds. Associated with the ion beam formation is a strong electrostatic field formed by ambipolar effects. Such ion beams were observed by the WIND SWE experiment. After about 300 plasma periods, the counterstreaming ion beams become disrupted due to an ion beam instabilty; this effectively creating a local enahcned electric field in the central wake region that retards the beam movement. The resulting disrupted flow gives rise to a local density maximum in the central wake, and initiates the processes of filling in the wake.

Graphic showing results of the ion distribution

This simple example demonstrates the potential of even a simple kinetic model to lunar wake dynamics. Because of the new shift in paradgme from an MHD to kinetic wake nature, even simple kinetic models currently available make interesting applications to the new lunar wake observations by WIND.

Figure 4 shows the ion and electron simulated particle density, n, as a function of simulation time. Movement of plasma and forces are along to Y direction only. The 1D slice of plasma being simulated is actually in the frame of refence moving with the solar wind, which is asusmed in the simulation to be 25 time the ion thermal speed. Thus, the desnity vesus time plot (n(Y) vs. t), can be converted to a plot of density in both Y and X, (n(Y) vs X= V-convection * t). In essence, when we examine a wake crossection in time, we are also examining the crossections at different locations along the assumed time-stationary tail. Thus, we obtain a two-dimensional picture of the lunar wake.

Graphic showing ion and electron simulated particle density

Note that the kinetic picture of the wake suggests a disturbance that extends out beyond 20 Rl, and the wake proceeds to become replensihed via the electrostatic instabilty that creates ion beam disruption and a local maximim in density in the central region. As the wake becomes replenished from the central region outward, low density streamers flow tailward as the last vestiges of the moon-induced density depletion.


Bale SD et al. submitted, Geophys. Res. Lett., 1997.

Bale SD, submitted, Geophys. Res. Lett., 1997.

Farrell WM et al., Geophys. Res. Lett., 23, 1271, 1996.

Farrell WM et al., Geophys. Res. Lett., 24, 1135, 1997.

Kellogg et al Geophys. Res. Lett., 23, 1267, 1996.

Ness, N. F. and K. H. Schatten, , J. Geophys. Res., 74, 6425, 1969.

Owen C.J. e t al., Geophys. Res. Lett., 23, 1263, 1996.

Ogilvie KW et al., Geophys. Res. Lett, 23, 1255, 1996.

Spreiter, JR et al., Cosmic electrodynamics, 1, 5, 1970.

For other references see the WIND MFI Bibliography page.

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