Hot Plasma and Energetic Particles in Neptune's Magnetosphere

¹ Applied Physics Laboratory, The Johns Hopkins University, Laurel, MD 20707-6099
² University of Kansas, Lawrence, KS 66044
³ Max-Planck-Institut für Aeronomic, Postfach 60, 3411 Lindau, Federal Republic of Germany
⁴ University of Maryland, College Park, MD 20742
⁵ AT&T Bell Laboratories, Murray Hill, NJ 07974
⁶ University of Iowa, Iowa City IA 52242

The low-energy charged particle (LECP) instrument on Voyager 2 measured within the magnetosphere of Neptune energetic electrons (22 kiloelectron volts E 20 megaelectron volts) and ions (28 keV E 150 MeV) in several energy channels, including compositional information at higher (0.5 MeV per nucleon) energies, using an array of solid-state detectors in various configurations. The results obtained so far may be summarized as follows: (i) A variety of intensity, spectral, and anisotropy features suggest that the satellite Triton is important in controlling the outer regions of the Neptunian magnetosphere. These features include the absence of higher energy (150 keV) ions or electrons outside 14.4 R_N (where R_N = radius of Neptune), a relative peak in the spectral index of low-energy electrons at Triton's radial distance, and a change of the proton spectrum from a power law with 3.8 outside, to a hot Maxwellian (kT [unknown] 55 keV) inside the satellite's orbit. (ii) Intensities decrease sharply at all energies near the time of closest approach, the decreases being most extended in time at the highest energies, reminiscent of a spacecraft's traversal of Earth's polar regions at low altitudes; simultaneously, several spikes of spectrally soft electrons and protons were seen (power input 5 x 10^-4 ergs cm^-2 s^-1) suggestive of auroral processes at Neptune. (iii) Composition measurements revealed the presence of H, H₂, and He⁴, with relative abundances of 1300:1:0.1, suggesting a Neptunian ionospheric source for the trapped particle population. (iv) Plasma pressures at E 28 keV are maximum at the magnetic equator with 0.2, suggestive of a relatively empty magnetosphere, similar to that of Uranus. (v) A potential signature of satellite 1989N1 was seen, both inbound and outbound; other possible signatures of the moons and rings are evident in the data but cannot be positively identified in the absence of an accurate magnetic-field model close to the planet. Other results indude the absence of upstream ion increases or energetic neutrals [particle intensity (j) < 2.8 x 10^-3 cm^-2 s^-1 keV^-1 near 35 keV, at 40 R_N] implying an upper limit to the volume-averaged atomic H density at R 6 R_N of 20 cm^-3; and an estimate of the rate of darkening of methane ice at the location of 1989N1 ranging from 10⁵ years (1-micrometer depth) to 2 x 10⁶ years (10-micrometers depth). Finally, the electron fluxes at the orbit of Triton represent a power input of 10⁹ W into its atmosphere, apparently accounting for the observed ultraviolet auroral emission; by contrast, the precipitating electron (>22 keV) input on Neptune is 3 x 10⁷ W, surprisingly small when compared to energy input into the atmosphere of Jupiter, Saturn, and Uranus.