The Magnetic Field of Mercury

Abstract

The magnetic field strength of Mercury at the planet's surface is approximately 1% that of Earth's surface field. This comparatively low field strength presents a number of challenges, both theoretically to understand how it is generated and observationally to distinguish the internal field from that due to the solar wind interaction. Conversely, the small field also means that Mercury offers an important opportunity to advance our understanding both of planetary magnetic field generation and magnetosphere-solar wind interactions. The observations from the Mariner 10 magnetometer in 1974 and 1975, and the MESSENGER Magnetometer and plasma instruments during the probe's first two flybys of Mercury on 14 January and 6 October 2008, provide the basis for our current knowledge of the internal field. The external field arising from the interaction of the magnetosphere with the solar wind is more prominent near Mercury than for any other magnetized planet in the Solar System, and particular attention is therefore paid to indications in the observations of deficiencies in 308 B.J. Anderson et al.our understanding of the external field. The second MESSENGER flyby occurred over the opposite hemisphere from the other flybys, and these newest data constrain the tilt of the planetary moment from the planet's spin axis to be less than 5°. Considered as a dipole field, the moment is in the range 240 to 270 nT-R 3 M , where R M is Mercury's radius. Multipole solutions for the planetary field yield a smaller dipole term, 180 to 220 nT-R 3 M , and higherorder terms that together yield an equatorial surface field from 250 to 290 nT. From the spatial distribution of the fit residuals, the equatorial data are seen to reflect a weaker northward field and a strongly radial field, neither of which can be explained by a centered-dipole matched to the field measured near the pole by Mariner 10. This disparity is a major factor controlling the higher-order terms in the multipole solutions. The residuals are not largest close to the planet, and when considered in magnetospheric coordinates the residuals indicate the presence of a cross-tail current extending to within 0.5R M altitude on the nightside. A near-tail current with a density of 0.1 µA/m 2 could account for the low field intensities recorded near the equator. In addition, the MESSENGER flybys include the first plasma observations from Mercury and demonstrate that solar wind plasma is present at low altitudes, below 500 km. Although we can be confident in the dipole-only moment estimates, the data in hand remain subject to ambiguities for distinguishing internal from external contributions. The anticipated observations from orbit at Mercury, first from MESSENGER beginning in March 2011 and later from the dual-spacecraft BepiColombo mission, will be essential to elucidate the higher-order structure in the magnetic field of Mercury that will reveal the telltale signatures of the physics responsible for its generation.

A modelling approach to infer the solar wind dynamic pressure from magnetic field observations inside Mercury’s magnetosphere

“…The M1 flyby has been studied before (e.g. Slavin et al 2008;Anderson et al 2010Anderson et al , 2011aRaines et al 2011) and compared with hybrid simulations (Müller et al 2012). The D113 and D182 orbits have been compared with simulations (Richer et al 2012;Herčík et al 2016) and compiled into a statistical analysis of Mercury's magnetosphere (Winslow et al 2013).…”

“…Later, The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft (Solomon et al 2001) provided a more accurate determination of the internal magnetic field of Mercury using two flybys in 2008, known as M1 and M2 (e.g. Anderson et al 2008Anderson et al , 2010, and nearly four years of magnetic field observations around Mercury from early 2011 until 2015 (e.g. Anderson et al 2011aAnderson et al , 2012Johnson & Hauck 2016).…”

Intense solar near‐relativistic electron events at 0.3 AU

“…Winslow et al, 2013]). The structure of the Mercury's magnetosphere has been described elsewhere [e.g., Anderson et al, 2007Anderson et al, , 2010Anderson et al, , 2011Slavin et al, 2012], and we refer the reader to these articles for details.…”

A new look at the planet Mercury

“…5 As with Earth's internal field, Mercury's vector dipole moment points toward the planet's south geographic pole. The strength of the dipole moment, however, is a factor of about 1000 less than that of Earth.…”