Abstract Juno made a close flyby of Ganymede and flew through its magnetosphere on 7 June 2021, including an outbound crossing of Ganymede's upstream magnetopause. We present plasma and magnetic field observations near the upstream magnetopause from Juno's Jovian Auroral Distributions Experiment (JADE) and magnetometer. JADE observed enhanced electron fluxes, including field‐aligned electrons accelerated up to 2–3 keV/q, some having bidirectional pitch angle distributions, as Juno crossed Ganymede's magnetopause. Energy enhancements of cold protons and heavy ions originating from Ganymede were also observed on approach to the magnetopause. We interpret the presence of accelerated, field‐aligned electrons as indicating that magnetic reconnection is occurring on magnetic field lines that connect the spacecraft to Ganymede's magnetopause at that time. Counter‐streaming electrons observed on both sides of the magnetopause suggest the presence of multiple reconnection sites, both north and south of the spacecraft.
Abstract Quasiperiodic emissions are magnetospheric whistler mode waves at frequencies between about 0.5 and 4 kHz which exhibit a nearly periodic time modulation of the wave intensity. We use large data sets of events observed by the Van Allen Probes in the equatorial region at larger radial distances and by the low‐altitude DEMETER spacecraft. While Van Allen Probes observe the events at all local times and longitudes, DEMETER observations are limited nearly exclusively to the daytime and significantly less frequent at the longitudes of the South Atlantic Anomaly. Further, while the events observed by Van Allen Probes are smoothly distributed over seasons with only mild maxima in spring/autumn, DEMETER occurrence rate has a single pronounced minimum in July. The apparent inconsistency is explained by considering a nondipolar Earth's magnetic field and significant background wave intensities which in these cases prevent the quasiperiodic events from being identified in DEMETER data.
During the Voyager 2 flyby of Uranus the plasma wave and radio astronomy instruments detected a region of impulsive noise near the equatorial plane just inside the orbit of Miranda, at a radial distance of 4.51 R U . This noise is believed to be caused by micron‐sized particles hitting the spacecraft. Analysis of various coupling mechanisms shows that when a dust particle hits the spacecraft at a high velocity, the particle is instantly vaporized and ionized, thereby releasing a cloud of charged particles, some of which are collected by the antenna. The resulting voltage pulse is detected by the plasma wave instrument. Based on reasonable assumptions about the charge yield and collection efficiency of the antenna, the number density and mass of the particles can be estimated from the rate and amplitude of the voltage pulses. The analysis shows that the maximum number density of the particles is about 1.6 × 10 −3 m −3 , and the thickness of the impact region, based on a Gaussian fit, is 3480 km. The maximum number density occurs slightly after the ring plane crossing at a distance of about 280 km from the equatorial plane. The mass threshold for detecting the particles is estimated to be about 4.5 × 10 −10 g, and the rms mass of the particles is about 2.6 × 10 −9 g. For a density of a few grams per cubic centimeter the particles have radii of the order of a few microns. Possible sources for these particles include the rings, the small satellite 1985U1 discovered outside the ring system, or other unseen small bodies that lie between synchronous orbit (3.15 R U ) and 4.51 R U . If the particles are charged, electromagnetic forces produced by the rotating tilted dipole of Uranus may play a role in their transport and diffusion.
The Voyager 2 mission has provided the opportunity to explore the radio and plasma wave environments of Uranus and Neptune, nearly completing the exploratory phase of plasma processes in the solar system's planetary magnetospheres. At the same time increasingly sophisticated observations and theoretical techniques such as numerical simulations have allowed significant progress in our understanding of wave phenomena in the terrestrial magnetosphere. This report summarizes radio and plasma wave research in planetary magnetospheres during the interval 1987–1990.
The major momentum‐loading source in Saturn's magnetosphere, Enceladus, has been studied with seven Cassini flybys between 2005 and 2008. In this paper, we first use parameter tests with our 3‐D magnetohydrodynamic simulation to demonstrate and determine the sensitivity of the interaction to both electron impact rates and charge‐exchange rates. We also investigate the reasons behind our previous discovery that in the plume, within about two Enceladus radii of the plume's source, the momentum‐loading rates per unit ion and neutral density are orders of magnitude lower than at greater distances. We find that depletion of hot electrons and variations in charge‐exchange rates are two possible explanations for such a reduction of the momentum‐loading rates. Assisted by the Cassini observations, we use our understanding of the plasma interaction to determine the temporal variation of Enceladus' neutral plume, which is important in understanding its origin, as well as the geological evolution of this icy moon. We base our study on magnetometer observations during all seven flybys to present the first comparative analysis to all flybys in 2005 and 2008. It is found that the maximum variation in gas production rates is one third the largest rate. The plasma momentum‐loading rate ranges from 0.8 to 1.9 kg/s, which is consistent with previous studies.
Abstract Plasmaspheric hiss is one of the important plasma waves controlling radiation belt dynamics. Its spatiotemporal distribution and generation mechanism are presently the object of active research. We here give the first report on the shock‐induced disappearance of plasmaspheric hiss observed by the Van Allen Probes on 8 October 2013. This special event exhibits the dramatic variability of plasmaspheric hiss and provides a good opportunity to test its generation mechanisms. The origination of plasmaspheric hiss from plasmatrough chorus is suggested to be an appropriate prerequisite to explain this event. The shock increased the suprathermal electron fluxes, and then the enhanced Landau damping promptly prevented chorus waves from entering the plasmasphere. Subsequently, the shrinking magnetopause removed the source electrons for chorus, contributing significantly to the several‐hours‐long disappearance of plasmaspheric hiss.
We demonstrate that the acceleration of submicron dust originating at Enceladus by a reduced co-rotating E-field is capable of creating a dust pickup current perpendicular to the magnetic field with values ranging from 3 to 15 kA (depending upon the effective grain charge). Such a current represents a new contribution to the total pickup current in the region. As such, we suggest that dust pickup currents, along with ion and electron pickup currents, are all active within the plume.
Abstract We report observations of energetic electron butterfly distributions measured in a narrow range of Jupiter's magnetic latitudes by Juno during perijove 1. The electron butterfly distributions are characterized as clear electron flux peaks at 30–80° pitch angles, compared with the 90°‐peaked pitch angle distributions of the trapped electrons. Jupiter Energetic Particle Detector Instrument measurements during the close approach to Jupiter indicate a specific electron population with butterfly distributions formed between the main auroral oval and the radiation belt. The off‐90° flux peak is most clearly observed at tens of keV energies and gradually merges toward 90° pitch angle at higher energies. By projecting the observed electron pitch angle distributions along the magnetic field line, we found that the electron butterfly distributions are observed close to their source region. The particular electron distribution is possibly formed by parallel acceleration of electrons through Landau resonance with electrostatic waves.
