Plasma Observations Near Neptune: Initial Results from Voyager 2

Abstract

The plasma science experiment on Voyager 2 made observations of the plasma environment in Neptune's magnetosphere and in the surrounding solar wind. Because of the large tilt of the magnetic dipole and fortuitous timing, Voyager entered Neptune's magnetosphere through the cusp region, the first cusp observations at an outer planet. Thus the transition from the magnetosheath to the magnetosphere observed by Voyager 2 was not sharp but rather appeared as a gradual decrease in plasma density and temperature. The maximum plasma density observed in the magnetosphere is inferred to be 1.4 per cubic centimeter (the exact value depends on the composition), the smallest observed by Voyager in any magnetosphere. The plasma has at least two components; light ions (mass, 1 to 5) and heavy ions (mass, 10 to 40), but more precise species identification is not yet available. Most of the plasma is concentrated in a plasma sheet or plasma torus and near closest approach to the planet. A likely source of the heavy ions is Triton's atmosphere or ionosphere, whereas the light ions probably escape from Neptune. The large tilt of Neptune's magnetic dipole produces a dynamic magnetosphere that changes configuration every 16 hours as the planet rotates.

Global MHD simulations of Neptune's magnetosphere

“…Voyager 2 remained inside the magnetosphere for more than 38 h and thus more than 2 Neptunian days. Compared to the other outer planets, Neptune's magnetosphere was found to be relatively empty [ Belcher et al , ] although Triton, with its atmosphere [ Broadfoot et al , ], may act as a source [ Richardson et al , ; Zhang et al , ]. However, the role of Triton as a plasma source in Neptune's magnetosphere is ultimately not yet known [ Masters et al , ].…”

Magnetopause structure and the role of reconnection at the outer planets

“…In contrast to Jupiter and Saturn the magnetospheres of Uranus and Neptune were found relatively "empty" during the Voyager 2 flyby [e.g., Cheng et al, 1987;Belcher et al, 1989]. Following this discovery, reconnection has been suggested as a likely mechanism to allow tailward escape of the plasma, but it could take place as outlined by Vasyliunas [1983] in the tail without necessarily being externally driven by the solar wind.…”

Planetary space weather: scientific aspects and future perspectives

“…Although this nonobservation does not rule out an active magnetosphere, it rules out processes similar to those associated with the aurora observed at Uranus. Whereas the plasma in the magnetosphere of Uranus has a relatively low density and is thought to be primarily of solar wind origin, at Neptune, the distribution of plasma is generally interpreted as indicating that Triton is a major source (Belcher et al 1989;Krimigis et al 1989;Mauk et al 1991Mauk et al , 1995Richardson et al 1991). Neptune's satellite Triton, the largest of the 14 known Neptunian moons, possesses a nitrogen and methane atmosphere with a surface pressure of 14 microbar and temperature of about 35.6 K, extending to 950 km above the surface.…”