Plasma wave measurements in the magnetosphere of Uranus (original) (raw)

Plasma wave observations at Neptune

Advances in Space Research, 1992

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 pV/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 RN including the northern polar region.

A study of Uranus' bow shock motions using Langmuir waves

Journal of Geophysical Research, 1996

During the Voyager 2 flyby of Uranus, strong electron plasma oscillations (Langmuir waves) were detected by the plasma wave instrument in the 1.78-kHz channel on January 23-24, 1986, prior to the inbound bow shock crossing. Langmuir waves are excited by energetic electrons streaming away from the bow shock. The goal of this work is to estimate the location and motion of Uranus' bow shock using Langmuir wave data, together with the spacecraft positions and the measured interplanetary magnetic field. The following three remote sensing analyses were performed: the basic remote sensing method, the lag time method, and the trace-back method. Because the interplanetary magnetic field was highly variable, the first analysis encountered difficulties in obtaining a realistic estimation of Uranus' bow shock motion. In the lag time method developed here, time lags due to the solar wind's finite convection speed are taken into account when calculating the shock's standoff distance. In the new trace-back method, limits on the standoff distance are obtained as a function of time by reconstructing electron paths. Most of the results produced by the latter two analyses are consistent with predictions based on the standard theoretical model and the measured solar wind plasma parameters. Differences between our calculations and the theoretical model are discussed.

Low-frequency waves in the solar wind near Neptune

Geophysical Research Letters, 1991

Plasma and magnetic field observations from the Voyager 2 spacecraft when it was outbound from Neptune reveal low-frequency waves in the solar wind which are clearly associated with the planet. The waves have frequencies below the proton cyclotron frequency fc•,, which is about 10 -3 Hz during the periods waves are observed. The waves are present when the interplanetary magnetic field is oriented such that the spacecraft is connected to the bow shock by the magnetic field lines. We have identified the waves to be Alfvtnic waves propagating at-140 ø to the ambient magnetic field and away from the bow shock. As at the other planets, these downstream waves are thought to be generated in the upstream region, where energetic protons created near the nose of the bow shock excite waves as they stream along solar wind magnetic field lines. Res., 89, 9159, 1984. Zhang, M., et al., Alfv6n waves and associated energetic ions downstream of Uranus, J. Geophys. Res. 96, 1647, !991.

Whistler mode waves in the Jovian magnetosheath

Journal of Geophysical Research, 1994

During the Ulysses flyby of Jupiter in February, 1992, the spacecraft traversed the Jovian magnetosheath for a few hours during the inbound pass and for a few days during the outbound pass. Burst-1ike electromagnetic waves at frequencies of N 0.1 -0.4 of the local electron cyclotron frequency have been observed by the unified radio and plasma wave (URAP) cxpcrirncnt. The waves were more often observed in the regions which were probably the outer or middle magnetosheath, especially near the bow shock, and rarely seen in the magnetosphere/magn ctosheath boundary layer. The propagation angle of the waves are estimated by comparing the measurements of the wave electric and magnetic fields on the spin plane with the corresponding values calculated using the cold plasma dispersion relation under local field and plasma conditions. It is found that the waves may propagate obliquely with wave aylglcs between * 30°--50°. These waves are likely to blc the whistler mode waves which are excited by suprathermal electrons with a few hundred eV and a slight anisotropy (T' /Tll N 1.1 -1.5). They are probably similar in nature to the lion roars observed in th.c Ea,rth's magnetosheath. Signature of coupling between the mirror mode and the whistler mode have also been observed. 'l'he plasma conditions which favor the excitation of the whistler mode instability during the wave events exist as observed by the plasma cxpcrimcnt of Ulysses.

Ulysses observations of whistler waves at interplanetary shocks and in the solar wind

Journal of Geophysical Research, 1996

This study of whistler wave emission observed by the Ulysses Unified Radio and Plasma Wave (URAP) experiment between 1 and 5 AU is a complement to previous studies of whistler waves observed by the Helios spacecraft between 0.3 and 1 AU. The Helios spacecraft continuously detected a background of whistlers close to the Sun, and this background was found to decrease in intensity with larger heliocentric distance. Ulysses plasma wave observations confirm this trend. Within a heliocentric distance of approximately 2 AU, whistler waves are routinely observed. Beyond about 3 AU the waves are usually observed only downstream of interplanetary shocks. Moreover, whistler waves are routinely observed within about 2 AU at all heliographic latitudes of the Ulysses trajectory (-80 ø to +80ø). The combined observations from the Helios and Ulysses spacecraft suggest that whistler emission is always present in the solar wind, although at larger heliocentric distances the wave amplitudes are often below the thresholds of the URAP instrument. Observations throughout the first 5 years of the Ulysses mission show a clear correlation of whistler emission intensity with magnetic field strength, or gyrofrequency, such that increases in wave intensities coincide with increases in gyrofrequency. This correlation is especially evident in observations of interplanetary shocks and high-speed streams. A possible cause of this correlation is increased whistler wave growth due to enhanced electron temperature anisotropies in regions of compressed magnetic field. A shift of the background whistler spectrum as a function of gyrofrequency could account for the observed decrease in whistler amplitudes with increasing heliocentric distance. 1. Introduction Studies have shown that magnetic wave activity at frequencies between the ion and electron cyclotron frequencies is prevalent in the solar wind. A ubiquitous background of waves continuously present in the solar wind was detected by the Helios spacecraft [Neubauer et al., 1977a, b; Beinroth and Neubauer, 1981; Denskat et al., 1983] between 0.3 and 1 AU. This background was found to increase by more than an order of magnitude over this distance range as the spacecraft approached the Sun. It was also found that the maximum observed wave frequency decreased with increasing heliocentric distance. The waves were observed up to a frequency of 470 Hz near 0.5 AU, but near 1 AU the maximum observed Paper number 96JA00548. 0148-0227/96/96JA-00548 $09.00 frequency was about 220 Hz. This whistler wave background was enhanced by the passage of interplanetary shocks and high-speed streams. Discrete bursts or wave enhancements of minutes or less, called "events," were also found, usually associated with magnetic structures such as directional and rotational discontinuities and magnetic holes. Further analyses of wave enhancements at interplanetary shocks and highspeed streams at 1 AU have been reported by Gurnett et al. [1979], Kennel et al. [1982], and Coroniti et al. [1982]. Preliminary studies of whistler waves observed at interplanetary shocks by the Ulysses spacecraft beyond 1 AU have been published [Lengyel-Frey et al., 1992, 1994]. Observed magnetic to electric field amplitude ratios are similar to the computed index of refraction for whistler waves propagating parallel to the ambient magnetic field [Gurnett et al., 1979; Rodriguez and Gurnett, 1975; Coroniti et al., 1982], or at oblique angles [Lengyel-Frey et al., 1994], which is consistent with the whistler interpretation of these waves. In this paper we report Ulysses plasma wave observations of whistler waves at interplanetary shocks as well as in the solar wind with no shock association. We determine how whistler activity changes with heliocentric distance and latitude. We also discuss how Ulysses observations compare with Helios studies of whistler waves within 1 AU and offer an interpretation for the changes seen in whistler emission at interplanetary shocks and in the large-scale solar wind. 27,555 27,556

ARTEMIS Observations of Plasma Waves in Laminar and Perturbed Interplanetary Shocks

The Astrophysical Journal, 2021

The “Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun” mission provides a unique opportunity to study the structure of interplanetary shocks and the associated generation of plasma waves with frequencies between ∼50 and 8000 Hz due to its long duration electric and magnetic field burst waveform captures. We compare wave properties and occurrence rates at 11 quasi-perpendicular interplanetary shocks with burst data within 10 minutes (∼3200 proton gyroradii upstream, ∼1900 downstream) of the shock ramp. A perturbed shock is defined as possessing a large amplitude whistler precursor in the quasi-static magnetic field with an amplitude greater than 1/3 the difference between the upstream and downstream average magnetic field magnitudes; laminar shocks lack these large precursors and have a smooth, step function-like transition. In addition to wave modes previously observed, including ion acoustic, whistler, and electrostatic solitary ...

Intense plasma wave emissions associated with Saturn's moon Rhea

Geophysical Research Letters, 2011

[1] Measurements by the Cassini spacecraft during a close flyby of Saturn's moon Rhea on March 2, 2010, show the presence of intense plasma waves in the magnetic flux tube connected to the surface of the moon. Three types of waves were observed, (1) bursty electrostatic waves near the electron plasma frequency, (2) intense whistler-mode emissions below one half of the electron cyclotron frequency, and (3) broadband electrostatic waves at frequencies well below the ion plasma frequency. The waves near the electron plasma frequency are believed to be driven by a low energy (∼35 eV) electron beam accelerated away from Rhea. Their bursty structure is believed to be due to a nonlinear process similar to the threewave interaction that occurs for Langmuir waves in the solar wind. The whistler-mode emissions are propagating toward Rhea and are shown to be generated by the loss-cone anisotropy (at parallel cyclotron resonance energies around 230 eV) caused by absorption of electrons at the surface of the moon. Scattering by these whistler-mode waves may be able to explain previously reported depletions of energetic electrons in the vicinity of the moon. The low-frequency waves may play a role in nonlinear three-wave interactions with the bursty electrostatic waves.

A survey of low frequency waves at Jupiter: The Ulysses encounter

Journal of Geophysical Research: Space Physics, 1993

We report the results of a survey of low‐frequency (LF) plasma waves detected during the Ulysses Jupiter flyby. In the Jovian foreshock, two predominant wave periods are detected: 102‐s and 5‐s, as measured in the spacecraft frame. The 102‐s waves are highly nonlinear propagate at large angles to (typically 50°), are steepened, and sometimes have attached whistler packets. For the interval analyzed the 102‐s waves had mixed right‐and left‐hand polarizations. We argue that these are all consistent with being right‐hand magnetosonic waves in the solar wind frame. The 102‐s waves with attached whistlers are similar to cometary waves. The trailing portions are linearly polarized and the whistler portions circularly polarized with amplitudes decreasing linearly with time. The emissions are generated by ∼2‐keV protons flowing from the Jovian bow shock/magnetosheath into the upstream region. The instability is the ion beam instability. Higher Z ions were considered as a source of the waves...