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Mercury's Magnetospheric Magnetic Field After the First Two MESSENGER

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Mercury's Magnetospheric Magnetic Field After the First Two MESSENGER Icarus 209 (2010) 23–39 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus Mercury’s magnetospheric magnetic field after the first two MESSENGER flybys Igor I. Alexeev a,*, Elena S. Belenkaya a, James A. Slavin b, Haje Korth c, Brian J. Anderson c, Daniel N. Baker d, Scott A. Boardsen b, Catherine L. Johnson e, Michael E. Purucker b, Menelaos Sarantos b, Sean C. Solomon f a Scobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Leninskie Gory, 119992 Moscow, Russia b Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA c The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA d Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309-7814, USA e Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC, Canada V6T 1Z4 f Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA article info abstract Article history: The ‘‘paraboloid” model of Mercury’s magnetospheric magnetic field is used to determine the best-fit Received 13 October 2009 magnetospheric current system and internal dipole parameters from magnetic field measurements taken Revised 15 January 2010 during the first and second MESSENGER flybys of Mercury on 14 January and 6 October 2008. Together Accepted 25 January 2010 with magnetic field measurements taken during the Mariner 10 flybys on 29 March 1974 and 16 March Available online 4 February 2010 1975, there exist three low-latitude traversals separated in longitude and one high-latitude encounter. 3 From our model formulation and fitting procedure a Mercury dipole moment of 196 nT Á R (where RM Keywords: M is Mercury’s radius) was determined. The dipole is offset from Mercury’s center by 405 km in the north- Mercury ward direction. The dipole inclination to Mercury’s rotation axis is relatively small, 4°, with an eastern Magnetospheres Magnetic fields longitude of 193° for the dipole northern pole. Our model is based on the a priori assumption that the dipole position and the moment orientation and strength do not change in time. The root mean square (rms) deviation between the Mariner 10 and MESSENGER magnetic field measurements and the predic- tions of our model for all four flybys is 10.7 nT. For each magnetic field component the rms residual is 6 nT or about 1.5% of the maximum measured magnetic field, 400 nT. This level of agreement is pos- sible only because the magnetospheric current system parameters have been determined separately for each flyby. The magnetospheric stand-off distance, the distance from the planet’s center to the inner edge of the tail current sheet, the tail lobe magnetic flux, and the displacement of the tail current sheet relative to the Mercury solar-magnetospheric equatorial plane have been determined independently for each flyby. The magnetic flux in the tail lobes varied from 3.8 to 5.9 MWb; the subsolar magnetopause stand-off distance from 1.28 to 1.43 RM; and the distance to the inner edge of the current sheet from 1.23 to 1.32 RM. The differences in the current systems between the first and second MESSENGER flybys are attributed to the effects of strong magnetic reconnection driven by southward interplanetary mag- netic field during the latter flyby. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction et al., 1974). The higher solar wind pressure and the stronger mag- nitude of the interplanetary magnetic field (IMF) in the inner heli- Magnetic field measurements in the vicinity of Mercury to date osphere combined with the relatively weak planetary field result in have been obtained in the course of four spacecraft flybys: two a magnetosphere about Mercury whose size is only about 6% that Mariner 10 flybys on 29 March 1974 (M10 I) and 16 March 1975 of Earth’s magnetosphere (Slavin and Holzer, 1979a). The forma- (M10 III), respectively, and two flybys by the MErcury Surface, tion of the magnetosphere is associated with the flow of currents, Space ENvironment, GEochemistry, and Ranging (MESSENGER) which carry a magnetic field contribution that is superposed on the spacecraft (Solomon et al., 2001) on 14 January 2008 and 6 October planetary magnetic field. These currents may vary between differ- 2008 (M1 and M2, respectively). These measurements revealed the ent passes or even during a single pass due to the short reconfigu- presence of an internal magnetic field that at Mercury’s surface is ration time of Mercury’s magnetosphere, which is on the order of a about 100 times smaller than Earth’s surface magnetic field (Ness few minutes (e.g., Slavin et al., 2007). To account for these contri- butions and to refine the estimate for the planetary moment, a magnetospheric magnetic field model is required. The model * Corresponding author. presented in Alexeev et al. (2008), used previously to analyze E-mail address: [email protected] (I.I. Alexeev). 0019-1035/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2010.01.024 24 I.I. Alexeev et al. / Icarus 209 (2010) 23–39 observations from the Mariner 10 flybys, is extended in this work the M10 I, M1, and M2 flybys. These flyby trajectories were located in to include dipole-tilt effects. The new model incorporates the new- the nightside, near-equatorial magnetosphere, placing the space- ly available data from MESSENGER’s first two flybys to yield better craft within or in close proximity to the cross-tail current sheet. At estimates of Mercury’s magnetic field and to detail the effects of Earth, the inner edge of the tail current sheet, R2, can be usually found Mercury’s magnetospheric currents. at R2 = 0.7 R1 at local midnight, where R1 is the subsolar magneto- The Mariner 10 data have been extensively analyzed over the last pause distance, while at Mercury where the average R1 1.4 RM, three decades to infer Mercury’s internal field strength and configu- we typically have R2 1.0 RM. Throughout the M10 I pass, plasma- ration. Conducting a least-squares fit of the M10 I data to an offset sheet-type electron distributions were observed, with an increase tilted dipole, Ness et al. (1974) obtained a dipole moment of in temperature beginning near closest approach coincident with a 3 227 nT Á RM, where RM = 2439 km is Mercury’s radius, and a dipole series of intense energetic-particle events (Ogilvie et al., 1977; Chr- tilt angle of 10° relative to the planetary rotation axis. The dipole mo- iston, 1987). This observation implies that the inner edge of the Mer- ment was determined to be offset by 0.45 RM in the northward direc- cury tail current sheet was close to the planetary surface, so a scalar tion. This estimate of the central dipole moment was later revised to potential cannot be used for the external magnetic field representa- 3 the value of 350 nT Á RM (Ness et al., 1975). These authors used data tion along the M10 I, M1, and M2 trajectories. only from the inbound portion of M10 I in the determination of the magnetic moment, because substorm-like magnetospheric distur- 2. Methodology and fitting procedure bances were observed during the second half of that flyby (Siscoe et al., 1975). From the M10 III observations, Ness et al. (1976) deter- 3 In this paper we employ a ‘‘paraboloid” model of Mercury’s mined a dipole moment of 342 nT Á R , which is in close agreement M magnetosphere. The advantages of the paraboloid model are that with the revised estimate from M10 I above. Korth et al. (2004) ac- it is a robust treatment that reproduces the main features of solar counted for the contribution of the external magnetic field to the wind flow past a planetary dipole (Alexeev et al., 2003; Belenkaya, observations with a modified Tsyganenko 96 model and found 2009). We have successfully adapted the Earth magnetospheric resulting strengths of the dipole moment ranging between 3 3 model to the Jupiter and Saturn magnetospheres (Alexeev and 198 nT Á R and 348 nT Á R , consistent with findings from other M M Belenkaya, 2005; Alexeev et al., 2006). authors [see Engle (1997), in which the dipole moment estimates 3 3 We have elected not to employ spherical harmonic analysis to varied between 154 nT Á R (M10 I) and 182 nT Á R (M10 III)]. More M M find model parameters, because such analysis is most fruitful in sit- recently, Alexeev et al. (2008) introduced a new model of Mercury’s uations where observational points are well distributed in spherical magnetosphere that was then used to determine from observations geometry, whereas magnetic field observations at Mercury are to made during the two Mariner 10 flybys a Mercury magnetic dipole 3 date restricted to four spacecraft flyby trajectories. In this section, moment of 192 nT Á R , a value within the range of estimates from M we briefly describe the paraboloid model, which includes four terms previous models. This model ignored the dipole tilt angle and used that form the magnetospheric field: (1) the eccentric dipole; (2) both inbound and outbound observations from M10 I. The best fit magnetopause currents; (3) tail currents; and (4) the penetrated to the Mariner 10 measurements yielded a dipole offset of 0.18 RM IMF. The model formulation is detailed in Appendix. northward of the equatorial plane (Alexeev et al., 2008). The cause The determination of the internal dipole and its higher-order for the large spread in the reported estimates of the dipole term is terms from flyby data amounts to minimization of an objective the limited spatial coverage of the observations, which is insufficient function (such as the root mean square deviation between the data for separating the higher-order multipoles (Connerney and Ness, and the model) subject to the following key assumptions and/or 1988), and variable magnetic field contributions from the magneto- constraints: (a) the planetary dipole is assumed to be fixed in a spheric current systems (Slavin and Holzer, 1979b; Korth et al., planetographic coordinate system, i.e., it does not change in time 2004).
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