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Magnetic Field Direction and Lunar Swirl Morphology: Insights from Airy And JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, E10012, doi:10.1029/2012JE004165, 2012 Magnetic field direction and lunar swirl morphology: Insights from Airy and Reiner Gamma Doug Hemingway1 and Ian Garrick-Bethell1,2 Received 21 June 2012; revised 23 August 2012; accepted 30 August 2012; published 30 October 2012. [1] Many of the Moon’s crustal magnetic anomalies are accompanied by high albedo features known as swirls. A leading hypothesis suggests that swirls are formed where crustal magnetic anomalies, acting as mini magnetospheres, shield portions of the surface from the darkening effects of solar wind ion bombardment, thereby leaving patches that appear bright compared with their surroundings. If this hypothesis is correct, then magnetic field direction should influence swirl morphology. Using Lunar Prospector magnetometer data and Clementine reflectance mosaics, we find evidence that bright regions correspond with dominantly horizontal magnetic fields at Reiner Gamma and that vertical magnetic fields are associated with the intraswirl dark lane at Airy. We use a genetic search algorithm to model the distributions of magnetic source material at both anomalies, and we show that source models constrained by the observed albedo pattern (i.e., strongly horizontal surface fields in bright areas, vertical surface fields in dark lanes) produce fields that are consistent with the Lunar Prospector magnetometer measurements. These findings support the solar wind deflection hypothesis and may help to explain not only the general form of swirls, but also the finer aspects of their morphology. Our source models may also be used to make quantitative predictions of the near surface magnetic field, which must ultimately be tested with very low altitude spacecraft measurements. If our predictions are correct, our models could have implications for the structure of the underlying magnetic material and the nature of the magnetizing field. Citation: Hemingway, D., and I. Garrick-Bethell (2012), Magnetic field direction and lunar swirl morphology: Insights from Airy and Reiner Gamma, J. Geophys. Res., 117, E10012, doi:10.1029/2012JE004165. 1. Introduction [Garrick-Bethell et al., 2011], and are depleted in hydroxyl molecules [Kramer et al., 2011]. Swirls appear to overprint 1.1. Background local topography, having no detectable topographic or tex- [2] Although the Moon does not now possess a global tural expression of their own [Neish et al., 2011]. So far not magnetic field [Ness et al., 1967], remanent crustal magne- identified anywhere else in the solar system, swirls are tization has been identified on the surface and several stable unique natural laboratories where space weathering and regional magnetic fields have been detected from orbit [Dyal crustal magnetism intersect. As such, their study could help et al., 1970; Coleman et al., 1972; Lin, 1979; Lin et al., 1998; address important questions in lunar science including the Hood et al., 2001]. Curiously, many, but not all of these Moon’s dynamo history [Garrick-Bethell et al., 2009; Dwyer crustal magnetic anomalies are accompanied by sinuous et al., 2011; Le Bars et al., 2011], the relative influences of patterns of anomalously high surface reflectance known as solar wind and micrometeoroid bombardment on space swirls [Hood et al., 1979; Hood and Williams, 1989; weathering [Hapke, 2001; Vernazza et al., 2009], and the Richmond et al., 2005; Blewett et al., 2011]. production and distribution of water over the lunar surface [3] Compared with their surroundings, swirls are optically [Pieters et al., 2009; Kramer et al., 2011]. immature, exhibit spectrally distinct space weathering trends [4] Electrostatic migration of dust has been proposed as a possible mechanism for swirl formation [Garrick-Bethell et al., 2011], as have meteoroid or comet impacts [Schultz 1Earth and Planetary Sciences, University of California, Santa Cruz, and Srnka, 1980; Pinet et al., 2000; Starukhina and California, USA. Shkuratov, 2004]. Another model suggests that crustal mag- 2School of Space Research, Kyung Hee University, Yongin, South netic fields act as mini magnetospheres, deflecting the solar Korea. wind and protecting portions of the surface from the optical Corresponding author: D. Hemingway, Earth and Planetary Sciences, maturation and darkening effects of proton bombardment University of California, 1156 High St., Santa Cruz, CA 95064, USA. [Hood and Schubert, 1980; Hood and Williams, 1989]. This ([email protected]) paper aims to make and test predictions based on this solar ©2012. American Geophysical Union. All Rights Reserved. wind deflection model for swirl formation. 0148-0227/12/2012JE004165 E10012 1of19 E10012 HEMINGWAY AND GARRICK-BETHELL: MAGNETIC FIELD DIRECTION AT LUNAR SWIRLS E10012 Figure 1. Magnetic field lines due to a single dipole located at the origin and oriented either (a) vertically or (b) horizontally in the plane of the page. (c and d) Profiles of the horizontal component of the magnetic field shown at the various altitudes represented by dashed lines in Figures 1a and 1b. In both cases, the dipo- lar source has a magnetic moment of 1012 Am2. 1.2. Predicted Influence of Magnetic Field Direction the bright swirls should be dominantly horizontal and that [5] The solar wind deflection model suggests that solar away from swirls and in the intraswirl dark lanes, the fields wind ions are magnetically deflected due to the Lorentz may be either closer to vertical or too weak to offer the sur- force. Neglecting the induced electric field, the cross product face any protection from solar wind darkening. of particle velocity and magnetic field in the Lorentz Law [6] Because magnetic field strength decreases rapidly with means the magnetic deflection force is maximized when distance from the source, swirl morphology should depend particle velocity is perpendicular to the magnetic field and mainly on the very low altitude structure of the magnetic zero when it is parallel. Hence, if the high albedo of swirls is field. The structure of the field at higher altitudes, where field the result of inhibited space weathering due to magnetic strength is weaker, has less influence on solar wind deflec- deflection of solar wind ions, magnetic field direction should tion. Unfortunately, spacecraft observations are typically influence swirl morphology. While the solar wind incidence limited to higher altitudes and therefore do not directly cap- angle varies, the ion flux at the surface and thus any dark- ture the structure of the near-surface magnetic field. It is ening effects will be greatest when the ion trajectories are therefore important to understand the way in which the field vertical. We therefore predict that portions of the crust that patterns change with observation altitude, including the role are shielded by dominantly horizontal magnetic fields should that is played by the direction of magnetization. Figure 1 receive maximum protection from the solar wind while por- illustrates the magnetic field due to a single dipole that is tions of the crust associated with vertically oriented magnetic either vertically oriented (Figures 1a and 1c) or horizontally fields should experience protection only at low solar wind oriented (Figures 1b and 1d). Figures 1a and 1b illustrate incidence angles, when darkening effects would be minimal magnetic field lines as seen in vertical cross section while anyway. This suggests that the magnetic fields directly over Figures 1c and 1d show profiles of the horizontal component of the magnetic field as observed along the dashed lines 2of19 E10012 HEMINGWAY AND GARRICK-BETHELL: MAGNETIC FIELD DIRECTION AT LUNAR SWIRLS E10012 shown in Figures 1a and 1b (in both cases, the model dipole the minimally processed magnetometer measurements rather is placed at the origin and arbitrarily assigned a magnetic than to a derived field model. While we focus here on direct moment of 1012 Am2). Directly over the vertically oriented mapping, we did compare our maps against maps we derived dipole, field lines are vertical at any altitude (Figure 1a). from the Purucker and Nicholas [2010] model coefficients. Peaks in the horizontal field strength appear to either side and We found that in the latter, crustal anomaly fields appear are separated by a distance equal to the observation altitude morphologically similar to the same features in our maps, but (Figure 1c). Directly over the horizontally oriented dipole, are typically broader in horizontal extent and exhibit lower field lines are horizontal at any altitude (Figure 1b). As the field strength. observer moves along an axis parallel to the dipole direction, [9] Each of our maps incorporates data from a sequence of horizontal field strength decreases, reaching zero (i.e., verti- consecutive orbits that cross the anomaly of interest. The cal field lines) at a horizontal distance of 1/√2 times the polar orbiting spacecraft obtained global coverage twice each observation altitude, before temporarily increasing again lunation, with consecutive orbits spaced 1 degree in lon- slightly (Figure 1d). Appendix A gives a quantitative treat- gitude (30 km at the equator) and with along-track mea- ment of the way in which magnetic fields vary with position surements recorded at 5-s intervals, corresponding to 8km relative to the source. spacing in latitude. In order to capture the undisturbed signal [7] Our study uses Lunar Prospector magnetometer data of the crustal magnetic anomalies, avoiding times when the collected at altitudes of 18 km and higher. While we cannot field is distorted by the impinging solar wind [Kurata et al., measure the magnetic field structure below these altitudes, 2005; Purucker, 2008], we use only those magnetometer we can distinguish between vertically and horizontally ori- measurements collected when the Moon was protected from ented magnetizations (we assume that strong crustal mag- solar wind while passing through the Earth’s magnetotail netic anomalies are approximately dipolar). This means we (excluding times when the field was disturbed by the plasma can select anomalies exhibiting orientations that allow us to sheet) or when the Moon was in the solar wind but the test our predictions.
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