Mercury's magnetosphere and magnetotail revisited

Abstract

Magnetic observations which are not complicated by currents of trapped plasma are a good test of geomagnetopause and geomagnetotail predictions. Recent attempts to model the Hermean magnetospheric field based on a planet‐centered magnetic multipole field with a quadrupole moment in addition to the planetary dipole field or a dipole field linearly displaced from planet center and no quadrupole moment have produced reasonably good fits to the Mercury magnetic field measurements. In this work we find a better fit for a dipole displacement from the planet center by making use of an improved representation of the magnetic field in the magnetotail, where many of the Mercury measurements were made. The rms deviation of the data was reduced from 10. or 11. γ to 9.3 γ by employing this new tail field representation. Also, by making use of this new tail field representation, we find a best fit for a dipole displacement of −0.0285 RM (earlier, 0.026 RM) toward the dawn in the magnetic equatorial plane and 0.17 RM (earlier, 0.189 RM (earlier, 0.189 RM) northward along the magnetic dipole axis, where RM is the planet radius. Thus with only minor adjustments in the displacement vector of the dipole from the planet center we achieve a measurable improvement in the fit of the data by using the improved magnetotail field representation.

Mercury’s magnetospheric magnetic field after the first two MESSENGER flybys

“…The dipole moment compares well to results from previous analyses (e.g., Anderson et al, 2008Anderson et al, , 2010 and is similar to estimates obtained by other methods (e.g., Slavin et al, 2009a). The northward dipole offset reported by Bergan and Engle (1981), 0.17 R M , is very close to our estimate, 0.166 R M (see Section 8). Similar to the case for M2, the B y component is weakly dependent on the magnetospheric contributions, and indeed these data can be used to define dipole moment (see Fig.…”

Planetary Magnetism

“…This result is rather similar to the Mariner 10 models (Bergan and Engle, 1981;Ng and Beard, 1979) characterized by an aligned dipole (with a moment of 180-207 nT-R m 3 ) offset by 0.17-0.19R m northward along the z-axis.…”

“…An explicit model of the external field allows one to extend analyses through regions of local currents (e.g., tail current sheet), accommodating nonpotential fields more readily. A number of these models (Bergan and Engle, 1981;Jackson and Beard, 1977;Ng and Beard, 1979;Whang, 1977) were applied to M10 observations and allowed for the possibility of internal field through degree 2 or 3 but only in the zonal harmonic (m ¼ 0) coefficient, for example, g 2 0 and g 3 0 .…”

Paraboloid model of Mercury's magnetosphere

“…At the beginning of section 3.1 we described the eight input model parameters which should be fixed before such calculations are performed (Ψ is taken to be zero). Following Bergan and Engle [1981], we also allow for a dipole offset from origin as a possible feature of the intrinsic planet's magnetic field. The dipole offset, Δ z d and Δ y d , in the terminator plane (x = 0) represent two additional parameters.…”