Voyager II Plasma Observations at Uranus and Neptune.

Details Voyager II Plasma Observations at Uranus and Neptune. Voyager II Plasma Observations at Uranus and Neptune.

Jun 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991phdt........23z&link_type=abstract

Thesis (PH.D.)--MASSACHUSETTS INSTITUTE OF TECHNOLOGY, 1991.Source: Dissertation Abstracts International, Volume: 53-02, Secti

Computer Science

3

Scientific paper

In the regions upstream of the Uranian and Neptunian bow shocks, low-frequency MHD waves are observed whenever the interplanetary magnetic field is oriented such that the field lines that pass through the spacecraft intersect the bow shocks. The waves appear to propagate outward from the bow shocks; at Uranus the propagation is in the direction along the ambient magnetic field lines and at Neptune against the field lines. The magnetic field and velocity components transverse to the background field are strongly anticorrelated for the Uranus waves and correlated for the Neptune waves, consistent with the interpretation that these waves are Alfven waves; there is also a correlation between the plasma density and magnetic field strength, indicating that the Alfven waves are mixed with traces of fast-mode waves. Theory suggests that these waves at both planets have a common origin: they are generated in the upstream region by a resonant instability with a proton beam streaming along the magnetic field lines; the solar wind subsequently carries these waves downstream to the spacecraft location. The waves at Uranus are associated with the presence of energetic (>28 keV) ions. These ions appear two days after the start of the wave activity and occur thereafter whenever the Alfven waves occur. The ions are argued to originate in the Uranian magnetosphere, but pitch-angle scattering in the upstream region is required to bring them downstream to the spacecraft location. At Neptune Voyager's post-encounter trajectory allows us to distinguish the directions of the magnetic field in the magnetosheath produced by the draping of the typical interplanetary field and by alignment with the magnetotail. Changes of the field from the draped to the magnetotail-aligned direction accompanied by decreases of plasma velocity, density, and temperature, observed at intervals of approximately a Neptunian day, are consistent with the assumption that Voyager repeatedly encountered a plasma mantle region extending well into the magnetosheath and modulated by the rotation of the planet's magnetic dipole. At Uranus periodic decreases in the plasma density, velocity and temperature in the outbound magnetosheath can be interpreted in the same way. The thesis also describes electron data obtained during the Neptune encounter. The data indicate that the escape of charged particles from Triton and the local ionization of atoms in the neutral torus originating from Triton are the major plasma sources. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617 -253-5668; Fax 617-253-1690.) (Abstract shortened with permission of school.).

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