MINI-MAGNETOSPHERIC PHENOMENA AT THE DESCARTES, AIRY AND REINER-GAMMA LUNAR MAGNETIC ANOMALIES. Michael Nayak1, Ian Garrick-Bethell2 and Doug Hemingway3, 1Department of Earth and Planetary Sciences, University of California, Santa Cruz (mnayak@ucsc.edu), 2,3Department of Earth and Planetary Sciences, University of California, Santa Cruz (igarrick@ucsc.edu, djheming@ucsc.edu)

Introduction: The existence of mini-

magnetospheres has been proposed as a formation mechanism for lunar swirls [1], enigmatic high-albedo surface markings co-located with magnetic anomalies, making small-scale magnetic field interactions with the solar wind of interest. We compare magnetic field measurements in the solar wind to source magnetiza-tion models constructed from observations in the lunar wake and Earth’s magnetotail. We find three anoma-lies (Descartes, Airy and Reiner-Gamma) which show evidence for field intensification in the solar wind. When the magnetic field in the solar wind is signifi-cantly higher than predicted in wake/tail, the flight path may be below and close to the magnetopause boundary.

Theory: When encountering a magnetic field, ions and electrons deflect in opposite directions, creating a current. This current enhances the magnetic field at the interface between the field and incident plasma (mag-netopause) and further opposes solar wind ion/electron impingement. Such enhancements have been noted as early as data from Apollo subsatellites [3]–[5]. Further, the deceleration of the solar wind from supersonic to subsonic velocities by the presence of a magnetosphere sets up a standing bow shock upstream; moving down-stream across a shock is characterized by a decrease in velocity, increase in density and increase in magnetic field. Increases in the Earth’s magnetic field have been noted just inside the subsolar point [6] up to a factor of f = 2.44 [7]. Therefore, magnetic field intensification is indicative of mini-magnetosphere-like activity, with lines of force concentrating toward the magnetopause. Figure 1 shows a cartoon that depicts this behavior. Increases of f = 2 to f = 3 have been documented on the Moon, suggesting that fully formed collisionless shocks could exist [8, 9]. Plasma deceleration and den-sity increases have been noted from Apollo 12 surface readings [3, 10]. MHD simulations show field line concentration toward the magnetopause [11], which was used to propose the possibility of a mini-magnetosphere at Airy [12]. This work presents addi-tional evidence at Airy and other locations that appears to support this hypothesis.

Methods: We use spacecraft magnetometer data from the Lunar Prospector (LP-MAG) and SELENE/Kaguya (K-MAG) missions for magnetic field measurements and data from the Advanced Com-position Explorer (ACE) mission to determine incident solar wind azimuth (AZ= -90°, westward; AZ=90°,

eastward), elevation (EL=0°, horizontal; EL=90°, ver-tical) and dynamic pressure (Psw) for solar wind ions outbound to the Moon.

Figure 1. Cartoon depiction of field line intensification at a

lunar mini-magnetosphere.

Higher order moments of lunar magnetic anomalies are significant to the size and shape of mini-magnetospheres and the deflection of solar wind [11], therefore we use multipole models to model all mag-netic anomalies. At Reiner-Gamma and Airy, the struc-ture of the source material is informed by the local albedo pattern [13]. At Descartes the model is generat-ed by placing multiple dipoles at centers of magnetiza-tion on total field contour maps and applying inversion techniques [14]. RMS error in a least-squares sense is minimized to determine the best fit between the model and LP/K-MAG data. We compare magnetic meas-urements in the solar wind to source magnetization models constructed from LP/K-MAG observations in the wake/tail. This allows for comparisons at different spacecraft altitudes. The relative strength of the wake/tail representative model compared to spacecraft observations is then used to infer field line intensifica-tion, reported as a percentage of the model magnetic field intensity at the corresponding altitude. A particu-lar puzzle is whether or not fields observed in the solar wind represent the Interplanetary Magnetic Field (IMF) compressed by the anomaly, or the distorted crustal field inside the magnetosphere. Other work by the authors addresses this concern by examining the change in direction of the fields between wake/tail and in-wind and concludes that the fields observed are in fact crustal in nature [14].

Results: At Reiner-Gamma, we compare observa-tions in the solar wind at 28 and 58 km (Figure 2). Inci-dent solar wind angles and Psw are nearly identical, eliminating the angle as a confounding effect. Magne-tometer observations are significantly stronger than the predicted model strength at the higher altitude case

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(137% vs. 295% intensification at 28 vs 58 km), im-plying a closer proximity to the acting magnetopause.

Figure 2. Reiner-Gamma: Azimuth (AZ) and elevation (EL) of incident solar wind for (a) LP-MAG day 40; (c) K-MAG day 21; (b, d) are matching models to (a, c) respectively. Units of magnetic field are nT, white dots are locations of magnetometer measurements.

Figure 3. Airy: Azimuth and elevation of incident solar wind for (a) LP-MAG day 35; (c) K-MAG day 16; (b, d) matching models to (a, c) respectively. Units of magnetic field are nT, white dots are locations of magnetometer measurements.

We note the same at Airy between 27 and 59 km

(Figure 3; 113% vs 284% intensification, respectively), and at Descartes, between 29 and 60 km km (Figure 4; 101% vs 471% intensification, respectively); at similar

Psw, the field is highly intensified at higher altitude datasets compared to the model.

Figure 4. Descartes: Azimuth and elevation of incident solar wind for (a) LP-MAG day 116; (c) K-MAG day 15; (b, d) matching models to (a, c) respectively. Units of magnetic field are nT, white dots are locations of magnetometer meas-urements.

Conclusions: Analysis of field line intensification in the solar wind at lunar magnetic anomalies provides supporting evidence to earlier work proposing mini-magnetosphere-like properties at Reiner-Gamma [15] and Airy [12] and presents new evidence supporting a mini-magnetosphere over Descartes. Implications of the lateral distortion of the crustal field is discussed by the authors in [14]. Based on the altitude at which in-tensification is observed (Figure 2-4), we can place lower bounds on the effective magnetopause altitude m given acting Psw: At Airy, m ≥ 59 km, at Reiner-Gamma, m ≥ 58 km, at Descartes, m ≥ 61 km, for Psw ≤ 2.25 nPa.

References: [1] Hood and Williams (1989) LPSC. [2] Bamford et al. (2012) Phys. Rev. Lett., vol. 109, no. 8. [3] Clay et al. (1975) J. Geophys. Res., vol. 80, no. 13. [4] Lin (1998). Science, vol. 281, no. 5382. [5] Siscoe et al (1969), J. Geophys. Res., vol. 74, no. 1, pp. 59–69. [6] Crooker et al (1982), J. Geophys. Res., vol. 87, no. A12, pp. 10407–10412. [7] Petrinec and Russell (1993), Sol. Terr. Predict. IV. [8] Halekas et al (2008), Adv. Sp. Res., vol. 41, no. 8. [9] Halekas et al (2008), Planet. Space Sci., vol. 56, no. 7. [10] Sharp et al, Moon, 1973. [11] Harnett & Winglee (2003), J. Geophys. Res., vol. 108, no. A2. [12] Richmond et al (2008), LPSC. [13] Hemingway & Garrick-Bethell (2012), J. Geophys. Res., vol. 117, no. E10. [14] Nayak et al, SM51F-4330, AGU Fall Meeting, 2014. [15] Kurata et al (2005), Geophys. Res. Lett., vol. 32, no. 24.

1926.pdf46th Lunar and Planetary Science Conference (2015)