2017 ARTEMIS SCIENCE NUGGETS
Solar wind interactions with small-scale magnetic fields at the Moon
by Jasper Halekas
University of Iowa
Introduction
Though most people think of the Moon as having no magnetic field, small patches of magnetized crust dot the lunar surface, with scale sizes and strengths many orders of magnitude smaller than most planetary magnetic fields. At most times, these tiny magnetic fields hardly disturb the flow of the solar wind, with their effects as minor as the flapping of a butterfly's wings in a strong breeze. However, under rare circumstances, they can behave more like a supersonic jet, creating a strong and macroscopic perturbation that ARTEMIS can observe from lunar orbit. A series of papers [Halekas et al., 2014, 2017] has utilized two-point measurements from ARTEMIS to determine whether these structures could in fact represent shock waves analogous to those formed by a supersonic jet plane, and to determine under what conditions they can form.
Results
As shown in Fig. 1 [Halekas et al., 2014], ARTEMIS at some times observes structures near lunar magnetic fields that share many of the features of collisionless shocks, including compression and deflection of the plasma flow, ion and electron heating, dissipation in the form of plasma waves, and mass flux across a boundary. The observation of such strong and macroscopic signatures appears somewhat surprising, given the fact that the lunar magnetic fields, with scale sizes of at most hundreds of km, are rather small considered to typical plasma scales such as the convected ion gyroradius (~500-2000 km) and the ion inertial length (~50-200 km).
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| Figure 1. ARTEMIS observations of a lunar flyby, showing (a) ESA measurements of ion energy spectra near the Moon in differential energy flux [eV/(eV cm2 s sr)], (b) electron energy spectra in the same units, (c) FFTs of electric field fluctuations from EFI, (d) FFTs of magnetic field fluctuations from SCM, (e) onboard electron temperature moments in magnetic field-aligned coordinates, (f) onboard density moments in the undisturbed upstream flow (nu) and near the Moon (nd), (g) change in the ion (solid) and electron (dash-dot-dash) onboard velocity moments near the Moon, with upstream Alfvén velocity for comparison, (h) magnetic field components and magnitudes measured by FGM in the undisturbed upstream flow (Bu) and near the Moon (Bd), and (i) an illustration of the conservation of momentum flux across the discontinuity, with electron and ion thermal pressure, normal dynamic pressure, magnetic pressure associated with the component perpendicular to the normal, and the total of these pressure terms. |
Offering a possible solution to this conundrum, Fatemi et al. [2014] simulated the interaction of a population of reflected solar wind protons with the incoming solar wind in a global hybrid simulation, showing that the effects of the reflected protons spreading out from the reflection point can in effect increase the scale of the interaction, leading to a macroscopic signature with many similarities to that observed by ARTEMIS. In the Fatemi et al. [2014] simulation, the strongest compressional feature formed on the flank of the Moon where the –v x B motional electric field pointed toward the lunar surface. There are some reasons to suspect that this geometry should be favored. Under this geometry the motional electric field exerts a Moonward force on the reflected protons. In order to conserve linear momentum, the incoming solar wind must therefore experience an outward force, plausibly leading to deflection and compression of the solar wind.
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| Figure 2. The top two panels show the vector velocity deflections for each identified event, with the vector length corresponding to the magnitude of the peak velocity deflection. The bottom two panels show the average degree of alignment of the observed velocity deflection Δv with the upstream motional electric field E, with a point for each event located at the time of maximum velocity deflection. In the bottom panels, squares represent steady IMF events, diamonds represent variable IMF events, and pluses indicate steepened ULF waves. Pluses inside of squares or diamonds indicate ULF precursors for coherent events. |
Intriguingly, both of the two events discussed in Halekas et al. [2014] also shared this geometry. To determine whether this geometry is favored, in a follow-up study we conducted a statistical investigation of all of the strong compressional interactions observed by the two ARTEMIS probes during their mission to date [Halekas et al., 2017]. Fig. 2 shows an overview of the results of this study. Intriguingly, regardless of the geometry of the motional electric field, the observed velocity deflection is always outward from the Moon, possibly due to the effects of pressure gradient and/or J x B forces. Our results show that compressional events occur more frequently when the motional electric field points toward the surface, anti-parallel to the velocity deflection (purple symbols); however, many events also occur under the opposite geometry where the motional electric field points away from the surface, parallel to the velocity deflection (green symbols).
Though strong compressional events occur under both under the expected anti-parallel geometry and the harder to explain parallel geometry, different solar wind conditions favor the occurrence of the two different types of events, as shown in Fig. 3. Anti-parallel events occur under conditions with lower magnetic field, higher convected ion gyro-radius, and higher Alfvén Mach number, while parallel events occur under conditions with higher magnetic field, lower convected ion gyro-radius and lower Alfvén Mach number. These differences presumably arise due to the balance of forces responsible for deflecting the solar wind, but we do not yet understand all the details of why the events have the characteristics that they do.
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| Figure 3. Distribution of upstream solar wind parameters during compressional events observed by ARTEMIS (same symbols and color code as Fig. 2) compared to the overall statistical distribution of solar wind parameters observed by ARTEMIS over its entire mission, for the same range of lunar phases (grey-scale 2-d frequency distributions). We define cone angle as the angle between the IMF and the SSE X-axis, and clock angle as the angle from the Y-axis in the Y-Z plane, with positive angles representing positive Z. |
Conclusion
The Moon represents a unique laboratory for investigating the interaction of the solar wind with very small-scale magnetic fields. The structures that occasionally form near these fields have many of the characteristics of collisionless shocks. If they are indeed shocks, they may represent the smallest collisionless shock waves in the solar system. Regardless, they appear to represent a delicate balance, given that they only form under rare circumstances, and that different solar wind conditions favor the occurrence of distinctly different interactions. The results of these two studies represent a rich chance for plasma theorists and modelers to investigate just how these unique structures form, and how their properties and occurrence depend on the ambient conditions.
References
Fatemi, S., M. Holmström, Y. Futaana, C. Lue, M. R. Collier, S. Barabash, and G. Stenberg (2014), Effects of protons reflected by lunar crustal magnetic fields on the global lunar plasma environment, J. Geophys. Res. Space Physics, 119, 6095–6105, doi:10.1002/2014JA019900.Halekas, J. S., A. R. Poppe, J. P. McFadden, V. Angelopoulos, K.-H. Glassmeier, and D. A. Brain (2014), Evidence for small-scale collisionless shocks at the Moon from ARTEMIS, Geophys. Res. Lett., 41, 7436–7443, doi:10.1002/2014GL061973.
Halekas, J. S., A. R. Poppe, C. Lue, W. M. Farrell, and J. P. McFadden (2017), Distribution and solar wind control of compressional solar wind-magnetic anomaly interactions observed at the Moon by ARTEMIS, J. Geophys. Res. Space Physics, 122, doi:10.1002/2017JA023931.
Biographical Note
Jasper Halekas is an associate professor in the Department of Physics and Astronomy at the University of Iowa. His research focuses on using charged particle measurements to study solar wind interactions with planets, moons, and the interplanetary medium. For more details, see http://www.physics.uiowa.edu/~jhalekas/index.html
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu