D. A. Gurnett

Galileo evidence for rapid interchange transport in the Io torus

Anomalous plasma signatures were detected by the Galileo particles and fields instruments during the initial transit through the Io torus. These unusual events are characterized by abrupt changes in the magnetic field, enhanced levels of broadband low frequency electromagnetic waves and a pronounced change in both the flux and pitch angle anisotropy of energetic particles. Here we present a coordinated study of one of the events which occurred near 6.03Rj just after 17:34 UT on December 7, 1995. The available data are consistent with the concept of rapid inward transport, and this is interpreted as the first evidence for the predicted interchange motions in the region exterior to the orbit of I o. Theoretical arguments indicate that the interchanging flux tube is characterized by substantially reduced plasma density, a spatial scale comparable to 10 3 km, and an inward radial velocity comparable to 102 km/s.

The Galileo plasma wave investigation

The purpose of the Galileo plasma wave investigation is to study plasma waves and radio emissions in the magnetosphere of Jupiter. The plasma wave instrument uses an electric dipole antenna to detect electric fields, and two search coil magnetic antennas to detect magnetic fields. The frequency range covered is 5 Hz to 5.6 MHz for electric fields and 5 Hz to 160 kHz for magnetic fields. Low time-resolution survey spectrums are provided by three on-board spectrum analyzers. In the normal mode of operation the frequency resolution is about 10%, and the time resolution for a complete set of electric and magnetic field measurements is 37.33 s. High time-resolution spectrums are provided by a wideband receiver. The wideband receiver provides waveform measurements over bandwidths of 1, 10, and 80 kHz. These measurements can be either transmitted to the ground in real time, or stored on the spacecraft tape recorder. On the ground the waveforms are Fourier transformed and displayed as frequency-time spectrogams. Compared to previous measurements at Jupiter this instrument has several new capabilities. These new capabilities include (1) both electric and magnetic field measurements to distinguish electrostatic and electromagnetic waves, (2) direction finding measurements to determine source locations, and (3) increased bandwidth for the wideband measurements.

The ionosphere of Ganymede

Abstract We consider the distribution of plasma density in the atmosphere of Ganymede and present electron density profiles inferred from data of the Plasma Wave instrument of Galileo. To study the question of ionospheric plasma, we present a zero-dimensional local rate equation model of the source and loss functions and the atomic and molecular processes we assume to be taking place. We conclude from the model that Ganymede has a bound ionosphere composed mainly of molecular oxygen ions in the polar regions and of atomic oxygen ions at low latitudes and that protons are absent everywhere. This implies that the plasma observed to be flowing out along the open flux tubes connected to the polar cap is composed of ions of atomic oxygen. We predict that Ganymede is surrounded by a corona of hot oxygen atoms. The model neutral atmosphere has a composition similar to that of the ionosphere and is exospheric everywhere. Our calculated neutral column density is consistent with values of Ganymede ultraviolet auroral brightness observed by means of the Hubble Space Telescope.

Fine structure of Langmuir waves observed upstream of the bow shock at Venus

Highly structured Langmuir waves, also known as electron plasma oscillations, have been observed in the foreshock of Venus using the plasma wave experiment on the Galileo spacecraft during the gravity assist flyby on February 10, 1990. The Galileo wideband sampling system provides digital electric field waveform measurements at sampling rates up to 201,600 samples per second, much higher than any previous instrument of this type. The main Langmuir wave emission band occurs near the local electron plasma frequency, which was approximately 43 kHz. The Langmuir waves are observed to shift above and below the plasma frequency, sometimes by as much as 20 kHz. The shifts in frequency are closely correlated with the downstream distance from the tangent field line, implying that the shifts are controlled by the electron beam velocity. Considerable fine structure is also evident, with timescales as short as 0.15 ms, corresponding to spatial scales of a few tens of Debye lengths. The frequency spectrum often consists of beat-type waveforms, with beat frequencies ranging from 0.2 to 7 kHz, and in a few cases, isolated wave packets. The peak electric field strengths are approximately 1 mV/m. These field strengths are too small for strongly nonlinear processes to be important. The beat-type waveforms are suggestive of a parametric decay process.

Ganymede: A New Radio Source

Observations by the Galileo plasma wave receiver during the first two flybys of Ganymede revealed that this Jovian moon is the source of narrowband electromagnetic radio waves, making it the only satellite in the solar system known to generate non-thermal radio emissions. The emissions are the result of mode-coupling from electrostatic electron cyclotron emissions near the upper hybrid resonance frequency, similar to non-thermal continuum radiation found at the known magnetized planets.

The whistler-mode bow wave of an asteroid

The Galileo spacecraft has flown by two asteroids, Gaspra in 1990 and Ida in 1993. In both cases the magnetometer detected magnetic field perturbations that are believed to be produced by an interaction of the asteroid with the solar wind. Kivelson et al. have proposed that these perturbations are caused by whistler-mode waves excited by the solar wind flow around the asteroid. This paper presents an analysis of whistler-mode waves generated by the interaction of a small object with the solar wind. Three cases are considered, with the solar wind magnetic field (1) parallel, (2) perpendicular, and (3) at an arbitrary angle to the velocity vector. Using the Cerenkov condition and the quasi-longitudinal approximation for the whistler mode, the angular limits of the wave pattern are determined. For the parallel case the waves are confined to the upstream region, and for the perpendicular case the waves are confined to two wedge-shaped regions extending downstream from the magnetic field line through the object. For intermediate magnetic field orientations the accessibility region maintains the wedge-shaped configuration, with the leading edge of the wedge roughly following the direction of the magnetic field. The outer boundary of the accessibility region is a caustic surface, along which large field amplitudes are expected, similar to the bow wave of a ship. In all cases the interaction involves a characteristic wavelength, λc = ƒcc²/(ƒp²V), that is comparable to the size of an asteroid.

A multi‐instrument study of a Jovian magnetospheric disturbance

Using observations from different Galileo experiments (plasma wave system, magnetometer and energetic particle detector), we analyze a strong magnetospheric disturbance that occurs on day 311 of 1996 as Galileo was close to Jupiter (less than 15 Jovian radii). This perturbation is characterized by multiple injections of energetic particles in the inner magnetosphere and has been described as a possible analog of the terrestrial magnetic storm by Mauk et al. [1999]. We show here that it also corresponds to a large-scale magnetospheric perturbation similar to the “energetic events” described by Louarn et al., [1998, 2000]. It is associated with the development of a particular magnetic activity in the outermost part of the Io torus, over periods of 2–4 hours and in sectors of longitude with a typical 30°–80° longitudinal extension. At distances ranging from 10 to 13 Rj, the activity itself is characterized by the generation of low-frequency magnetic oscillations (18 min periodicity in the present case) that correlate with dispersionless energetic electron injections and modulations of the auroral radio flux. When they are observed a few hours after their formation, these injections present a weak energy-time dispersion and are still periodic. They then progressively mix and finally define a region of limited longitudinal extension where the density of energetic particles is particularly large. We show that this region corresponds to the source of the narrowband kilometric radiation (n-KOM). By combining remote sensing radio observations, in situ particle, and magnetic field measurements, we show that the active zone where the large scale disturbance initially develops most probably does not corotate and would even be almost fixed in local time. In the present case, the magnetospheric event is the consequence of two activations separated by a few hours. They occur in two separated longitude sectors and give rise to two different n-KOM sources. During the event, some 1012 W are transferred to the electron population. It is proposed that this set of phenomena is the manifestation of a sporadic dissipation of a part of the Io torus rotational energy and would be thus associated with the development of a large-scale instability in the external part of the Io torus.

Control of Jovian radio emission by Callisto

Galileo has been in orbit around Jupiter since December 1995 and a large database has been collected. We present the results of a survey of the plasma wave data for the frequency range 2.0 MHz to 5.6 MHz, the low frequency decametric (DAM) emissions. While the control of a portion of the radio emission by the moon Io is well known, and Ganymede control has been more recently indicated, we report that a small but significant portion of DAM emission is seen to be correlated with the orbital phase of Callisto. While the occurrence rate of emission controlled by Ganymede and Callisto is considerably less than for Io, the power levels can be nearly the same. We estimate the power of the Callisto-dependent emission to be ∼70% of the Io-dependent radio emission and about the same as the Ganymede-dependent radio emission. This result indicates an Alfven current system associated with Callisto, and thus a significant interaction of the magnetosphere of Callisto with that of Jupiter as is believed to exist for both Io and Ganymede.