Contributions of the high-degree multipoles of Neptune's magnetic field: An Euler potentials approach

Details Contributions of the high-degree multipoles of Neptune's magnetic field: An Euler potentials approach Contributions of the high-degree multipoles of Neptune's magnetic field: An Euler potentials approach

Oct 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997jgr...10224393h&link_type=abstract

Journal of Geophysical Research, Volume 102, Issue A11, p. 24393-24402

Physics

8

Magnetospheric Physics: Current Systems, Planetology: Fluid Planets: Magnetic Fields And Magnetism, Planetology: Fluid Planets: Magnetospheres, Planetology: Solar System Objects: Neptune

Scientific paper

A new technique of calculating the Euler potentials α and β based on the geometrical relations between the magnetic field structure and Euler potentials [Huang and Yu, 1995] was used to determine the α and β coordinates of Neptune's magnetic field. The result is a mathematical definable coordinate system for a field of which the nondipole terms are nonnegligible. A framework is thus established for future studying of plasma dynamics such as particle drifts and plasma convection. The magnetic field model used is the Goddard Space Flight Center/Bartol Research Institute's I8E1 model. The full I8E1 model was used to investigate the significance of the higher-degree multipoles beyond the octupole (up to the eighth degree in the spherical harmonic expansions), particularly near the planet's surface. The results were compared to those of the O8 model, which is a simplified version of the I8E1. We found that the higher-degree terms do not change significantly the overall results of the O8. However, on the planet's surface, footprints of inner drift shells fall into a few concentrated areas, opening more regions into which no particle from these drift shells can precipitate. Close examination of these ``open'' regions indicates that they are isolated magnetic field systems that may trap particles locally. Our results entail new interpretation of the Voyager 2 UV spectrometer measurements on the planet's nightside surface emissions. Particle precipitation emissions are found to be concentrated on the areas into which the drift shells collapse. These areas of high concentration of drift shell footprints and emissions indicate that the strong emissions are due to high particle downflux into these regions of high magnetic field intensity. Our results suggest that magnetic moments higher than the octupole may constitute nonnegligible components of the magnetic field of planets that have high nondipole terms, such as Neptune and Uranus. For Neptune the high-degree terms should be included for studying plasma processes within two planetary radii.

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