The Encounter of Voyager 2 with Neptune’s Magnetosphere

Abstract

The particles and fields complement of instruments on the Voyager 2 spacecraft performed a comprehensive set of measurements during the encounter with the Neptune system on August 24 through 28, 1989. These included measurements of the magnetic field, plasma (10 ev ≤ E ≤ 6 keV), energetic and high energy particles (22 keV ≤ E ≤ 5 MeV), plasma waves (10 Hz to 50 kHz) and radio emissions (~ 20 to ~1300 kHz); additional information relating to UV emissions was provided by the ultraviolet spectrometer. The preliminary results of these measurements are reviewed in this paper and may be summarized as follows:

• (a) The planetary magnetic field outside ~ 4 RN may be described by an offset (~ 0.55 RN), tilted (47°), dipole of moment 0.133 Gauss-R_N ³; inside that distance the field is dominated by higher order terms.

• (b) Plasma densities were generally low (~10^-3 cm^-3), except at magnetic equatorial crossings when densities up to ~ 1 cm^-3 were seen; best fits (but not unique) to corotating Maxwellians are obtained for two components, H⁺ and N⁺.

• (c) Energetic ions (≥ 28 keV) and electrons (≥ 22 keV) were seen throughout the magnetosphere, while higher energy (≥200 keV) particles (equivalent to Van Allen belts at Earth) were confined inside the orbital radius of Triton. Spectrally soft electron and ion enhancements during spacecraft passage through the polar cap region suggest possible auroral precipitation.

• (d) A variety of plasma wave emissions were seen, including chorus, hiss, electron cyclotron waves, and upper hybrid resonance in the inner magnetosphere.

• (e) Radio wave bursts in the range 100 to 1300 kHz, narrowband and strongly polarized, have enabled determination of the planetary rotation period as 16.11 ± 0.05 hours; smooth emissions in the range 20 to 865 kHz were also observed.

• (f) Weak auroral emissions in H Lyman β (1025Å) have been tentatively identified on the nightside of Neptune with total radiated power of 5 × 10⁷ W. The magnetic field orientation with respect to the rotation axis was such that the spacecraft entered a “pole-on” magnetosphere, fortuitously sampling the cusp region of an outer planet for the first time. The measured flux of soft electrons and ions over the polar region of ~ 5 × 10^-4 erg/cm² sec results in an estimated power input of ~ 3 × 10⁷ W, i.e. substantially less than that at other planets. Possible reasons for these differences are discussed.