Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p44a..04a&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P44A-04
Physics
[5440] Planetary Sciences: Solid Surface Planets / Magnetic Fields And Magnetism, [5443] Planetary Sciences: Solid Surface Planets / Magnetospheres
Scientific paper
On 18 March 2011, the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft was inserted into a near-polar orbit about Mercury with a periapsis altitude of 200 km, an inclination of 82.5°, an apoapsis altitude of 15,300 km, and nominal orbit period of 12 hours. Magnetometer (MAG) data acquired since 23 March provide multiple circuits in solar local time and planetary longitude and yield extensive coverage of the planetary magnetic field sufficient to resolve the dominant magnetic fields of internal and external origin. Plasma pressures exceeding the magnetic pressure are commonly found within ±30° latitude of the equator and complicate solutions for the planetary field that use conventional spherical harmonic analysis. However, because the planetary field constrains the locations of external currents (e.g., the magnetopause and tail currents) to be symmetric about the magnetic equator, the location of that equator can be identified from the geometry of the magnetic field without the need to correct for local plasma pressures and external currents. We identify Mercury's magnetic equator from the orbital positions at which the cylindrical radial magnetic field component vanishes and find that the magnetic equator is offset north of the geographic equator by 484 ± 11 km. With this offset for the dipole we then analyze the tilt, position, and intensity of the best-fit dipole moment and find that the global planetary field is best represented as a southward-directed dipole, centered on the spin axis, tilted from that axis by less than 2.5°, and having a moment of 195 ± 10 nT-RM3, where RM is Mercury's radius. Mercury's axially symmetric but equatorially asymmetric field may imply lateral variations in heat flow at the planet's core-mantle boundary. This solution provides the basis for defining Mercury-solar-magnetospheric coordinates used to order observations of Mercury's magnetosphere, constructing a model for the magnetopause and tail current systems, and detrending magnetic field data to study higher-order structure in the internal and external field. An overview is also presented of results for signatures of the cusps, plasma sheet, field-aligned currents, and higher-order planetary field signatures.
Anderson Benjamin J.
Johnson Clifton L.
Korth Haje
McNutt Ralph L.
Purucker Michael E.
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