D. D. Barbosa

Study of dust in the vicinity of dione using the Voyager 1 Plasma Wave Instrument

The flyby of Voyager 1 at Saturn yielded the detection of a large variety of plasma waves, for example, chorus, hiss, and electron cyclotron harmonics. Just before the outbound equator crossing, the Voyager 1 plasma wave instrument detected a strong, well-defined low-frequency enhancement in signal levels. Initially, it was thought that this enhancement was due to plasma waves, but more recently it was suggested that dust impacts might be at least partial contributors. In this report we present evidence that dust impacts are partly responsible for the low-frequency enhancement. A new method of analysis which relies mainly on the 16-channel spectrum analyzer has been used to derive the dust impact rate. The available wideband waveform observations (which have been used previously to study dust impacts) were useful for calibrating the impact rate from the spectrum analyzer data. The mass and hence size of the dust particles were also obtained by analyzing the response of the plasma wave spectrum analyzer. The results show that the region sampled by Voyager 1 is populated by dust particles that have rms masses of up to few times 10−11 g and sizes of up to a few microns. The dust particle number density is of the order of 10−3 m−3. The optical depth of the region sampled by the spacecraft is approximately 10−6. The particle population is centered at 2470(±150) km south of the equatorial plane and has a north-south FWHM (full-width, half-maximum) thickness of 4130(±450) km. The dust may be part of the E ring or a localized ringlet associated with Dione.

Plasma wave observations at Neptune

Abstract During the Voyager 2 flyby of Neptune, the plasma wave instrument detected many familiar phenomena. These include radio emissions, electron plasma oscillations in the solar wind upstream of the bow shock, electrostatic turbulence at the bow shock, electrostatic electron cyclotron waves and upper hybrid resonance (UHR) waves, whistler mode noise, and dust impacts. The radio emissions occur in a broad continuum-like spectrum extending from about 5 kHz to above 50 kHz, and are emitted in a disk-like beam along the magnetic equatorial plane. The radio emissions are believed to be generated by mode conversion from UHR waves at the magnetic equator. The inner magnetosphere has relatively low plasma wave intensities, generally less than 100 μV/m. At the ring plane crossing, many small micron-sized dust particles were detected striking the spacecraft. The maximum impact rate was about 280 impacts per second at the inbound ring plane crossing, and about 110 impacts per second at the outbound ring plane crossing. Most of the particles are concentrated in a dense disk, about one thousand km thick, near the equatorial plane. A broader, more tenuous distribution of dust also extends along the entire trajectory inside of 6 R N including the northern polar region.

Heavy ion dynamics and auroral arc formation in the Jovian magnetosphere

Abstract This paper gives a brief review of some of the current controversial issues surrounding the Jovian aurora. In particular, the manner of its excitation be it that of electron or heavy ion precipitation is examined critically in the context of proposed models for magnetospheric dynamics, particle energization, and auroral energy input. A model for the X-ray aurora based on bremsstrahlung by a primary electron beam and its ionization secondaries is highlighted and the connection to the outward magnetospheric transport of heavy ion plasma from the satellite Io is made.

Electrostatic wave excitation in planetary magnetospheres : application to Neptune

Voyager 2 observations of electrostatic electron and ion cyclotron harmonic waves in Neptunes magnetosphere are addressed. A model of electron Bernstein modes generated by a loss cone distribution of superthermal electrons is scaled to Neptune parameters and a comparison of theory with the observed electron flux shows good agreement. A model of proton Bernstein modes generated by a ring distribution of Tritonogenic nitrogen ions is also investigated and satisfactory agreement with the data are obtained compatible with known properties of the magnetosphere. The success of the model in accounting for electrostatic emissions observed by Voyager over a wide range of sampled parameters recommends its general applicability to planetary magnetospheres.