Abstract: Direct observations of the magnetic field of Mercury and its magnetosphere, formed by the interaction of the solar wind, were performed twice by the Mariner 10 spacecraft, in March 1974 and again in March 1975. From these data, it is clear that there exists an intrinsic magnetic field of the planet, sufficiently strong at present to deflect the solar wind flow around the planet and to form a detached bow shock-wave in the super-Alfvénic solar wind. Four methods have been used to analyze the magnetic field data and derive quantitative values for the description of the planetary field: (i). Comparison of bow shock and magnetopause relative positions at Mercury to those at the Earth (cf. Slavin and Holzer, 1979). (ii). Direct spherical harmonic analysis of the data. (iii). Modelling of the magnetosphere by an image dipole and infinite 2-D current sheet in addition to the planetary field. (iv). Scaling of a mathematical model for the terrestrial magnetosphere. The results obtained yield dipole moments /fr(g10)2+(h11)2+(g11)2 ranging from 2.4–5.1 × 1022gauss cm3, with the lower values associated with certain models using partial quadrupole (g20) and octapole (g30) terms to improve the least-squares fit of models to observations. Because the data set is incomplete, in the mathematical sense, no unique representation of the planetary field multipolar representation can be derived by method (ii). The use of only one of the five quadrupole moment terms and one of the eight octapole moment terms corresponds to a displacement of the dipole along its axis. These terms, used in methods (iii) and (iv), yield equivalent offsets of the dipole by approximately 0.2 RM. The selection of only those higher-order terms possessing axial symmetry cannot be justified. Thus, the large offset may reflect the limitations of the models used to represent the external current systems. Because of the relatively short radial excursion of the data, the g20 and g30 terms can also be spatially aliased with the g10 term. Analyses by method (ii) of subsets of data from the third encounter, taken near closest approach, yield a convergent series of dipole moment values which are believed to best represent the intrinsic planetary field. These provide a mean moment of 330 (±18)γRM3 = 4.8 × 1022 gauss cm3 at a tilt angle of 14 ± 5° and a longitude of 148 ± 21°. This means that the surface field of Mercury is about 1% that of the Earth, while the moment is 6 × 10−4 that of the Earth. The polarity sense is the same as the Earth's. The origin of the field cannot be uniquely determined. It may be due to an active dynamo, a remanent magnetic field or a combination of both. Consideration of remanence as the source leads to some difficulties, because there is insufficient definitive knowledge of planetary interior structure and thermal state to eliminate this source. Success in attempting to explain the field as due to an active dynamo has encouraged these efforts. Therefore, Mercury may join the Earth and Jupiter as an example of a planet possessing an internal fluid region with a convecting motion which regeneratively maintains the magnetic field. The sources of convective energy may be radiogenic decay and heat release, gravitational settling and differentiation or precessional torques.
Publication Year: 1979
Publication Date: 1979-11-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 26
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