Plasma Wave Instrument Research Articles

Narrow-band kilometric radio emission as observed by the Galileo plasma wave instrument

Jovian narrow-band kilometric (nKOM) radio emissions have been well-studied, and are believed to have source regions near the outer edge of the Io torus. Recent investigations have indicated that these emissions increase in intensity associated with magnetic storm-like behavior in the Jovian magnetosphere. The orbit of Galileo affords the opportunity to pass close to the suggested source region near each perijove. We have examined the plasma wave spectra of nKOM on all orbital passes, and we present three cases in some detail. The nKOM emission is seen to increase in intensity at the outer edge of the plasma torus near the edge of the magnetic equator/current sheet crossings. Near perijove, and also associated with magnetic equator crossings, intense bursty (probably electrostatic) and narrow-banded emissions similar to upper hybrid emission seen at Earth are observed. We postulate that these emissions are related to the source of the nKOM emission.

Theoretical interpretation of Jupiter's multibanded whistler mode emission

Three distinct bands of whistler mode emissions were detected by the Voyager 1 and Voyager 2 plasma wave instruments during the encounter with Jupiter. The broadband whistler mode hiss below 1 kHz and two narrowbanded, high‐frequency emissions near 10 kHz were identified as whistler mode chorus and half electron cyclotron emission. In the present analysis the entire electron velocity space distribution is modeled by a superposition of three anisotropic components, two kappa distributions of different spectral index, and a drifting Maxwellian distribution. Applying a linear instability analysis on this specific distribution and providing the basic plasma parameters from spacecraft observations yields a highly accurate picture of the structure of the observed discrete Jovian whistler emission spectrum. It is concluded that the generation of the hiss structure can be interpreted to be a consequence of resonating suprathermal non‐Maxwellian electrons, whereas the chorus is generated by a nearly bi‐Maxwellian population. The half electron cyclotron emission can be associated with the interaction of a drifting, anisotropic component of keV electrons. The analysis of the main emission bands of a specific full spectrum can serve as representative pattern for the interpretation of various whistler mode events in planetary magnetospheres.

Statistical study of auroral kilometric radiation fine structure striations observed by Polar

We have conducted a statistical survey of a semirandom sample of the auroral kilometric radiation (AKR) data observed by the plasma wave instrument wideband receiver on board the Polar spacecraft. We have determined that AKR fine structure patterns with very narrowband, negative drifting striations occur in approximately 6% of the high‐resolution wideband spectrograms when AKR is present. Positive sloping striations are also observed, but at a much lower rate. More than 8200 AKR stripes have been scaled. The stripes are predominantly found in the 40‐ to 215‐kHz frequency range and have a frequency extent of about 4 kHz and a duration of usually less than 2 s. The majority of the stripes have drift rates between −8 and −2 kHz/s, with a peak in the distribution between −6 and −4 kHz/s. There is also a much smaller group of striations with positive drift rates of up to about 5 or 6 kHz/s. We have further investigated the change of drift rate with frequency. Almost all striations are observed in the lowest two frequency bands of the wideband receiver (ƒ< 215 kHz). There is an increase in the statistical drift rate with increasing frequency. The statistical slope of the striations increases with frequency from about −4.4 kHz/s at 75 kHz to about −5.7 kHz/s at 170 kHz. This frequency dependence of the drift rate is consistent, under certain conditions, with a production mechanism stimulated by an upward propagating electromagnetic ion cyclotron wave, as had been suggested earlier. However, such a changing drift rate is also compatible with a stimulated source region that propagates upward along the magnetic field line at the velocity of an ion beam accelerated by a local, upward directed electric field, as is typically observed in the auroral region. An explanation for this association is not apparent at this time.

Cassini and Wind stereoscopic observations of Jovian nonthermal radio emissions: Measurement of beam widths

During two intervals in 1999, simultaneous observations of Jupiter's decametric and hectometric radio emissions were made with the Cassini radio and plasma wave instrument (RPWS) and the radio and plasma wave instrument (WAVES) on the Wind spacecraft in Earth orbit. During January the Jovian longitude difference between the two spacecraft was about 5°, whereas for the August–September Earth flyby of Cassini, the angle ranged from 0° to about 2.5° (the Jovicentric latitudinal difference was <0.3° during both intervals). With these separations the instantaneous widths of the walls of the hollow conical radiation beams of some of the decametric arcs were measured by cross correlating dynamic spectra. The results suggest that the typical width is approximately 1.5° ±0.5°. The conical beams seem to move at Io's revolution rate for Io‐controlled arcs. Additionally, some of the nonarc hectometric wavelength emissions show some properties of both wide and very narrow beam widths.

Automatic Waveform Selection method for Electrostatic Solitary Waves

The present paper introduces the Automatic Waveform Selection (AWS) method for identifying the Electrostatic Solitary Waves (ESW) observed by Plasma Wave Instrument (PWI) onboard the Geotail spacecraft. Since the proposed method is based on the bit-pattern comparison, it does not require a lot of computer resources and the process speed is much faster than other pattern matching method using the First Fourier Transformation or the Wavelet transformation. We define the ESW index using the proposed AWS method. The ESW index allows us to perform statistical analyses by calculating it from a huge amount of Geotail data sets. We describe the detailed procedure of this new method for identifying the ESW waveforms and evaluate its efficiency. Further, we show one example of the statistical analyses conducted by using the AWS method, and discuss the result consulting the plasma electron measurements.

Plasma densities in the vicinity of Callisto from Galileo plasma wave observations

The Galileo spacecraft has made seven close flybys of Jupiter's moon Callisto. During the closest of these (C22), which approached to within 535 km of the surface, the plasma wave instrument detected a very clear upper hybrid emission as the spacecraft passed near the moon. The peak electron density indicated by the upper hybrid resonance emission was 400 cm−3, almost one‐thousand times the electron density in the magnetosphere of Jupiter at the orbit of Callisto. These observations indicate that Callisto is probably surrounded by a dense ionospheric‐like plasma.

Radio wave scattering in the outer heliosphere

The Voyager 1 and 2 plasma wave instruments have observed low‐frequency (1.5–4 kHz) radio waves apparently generated near the heliopause. The waves are found in two distinct bands. Power at the lower frequencies centered on 1.78 kHz shows no modulation when the spacecraft is rolled about the Earth‐spacecraft line, indicating that the radiation is isotropic. Power in the higher band centered on 3.11 kHz shows roll modulation as high as 60%, indicating that the source of the radiation is quite compact, subtending an angle <1 rad at the spacecraft. Simple estimates of the scattering of 3.11 kHz radiation from electron density fluctuations indicate that, if the radiation had originated from the distance of the heliopause, the scattering would be so large that no roll modulation should be observed. Here we show that these earlier scattering estimates were too high because they ignored the latitude variation of scattering and the inner scale of the electron density fluctuation spectrum. With these two effects properly included, the expected scattering is consistent with the observations and the postulate that the radiation originates from near the nose of the heliosphere.

On the perpendicular scale of electron phase‐space holes

The perpendicular scale of electron phase‐space holes is investigated using electric field data from the Polar Plasma Wave Instrument. We show that the electron phase‐space holes are roughly spherical for Ωe/ωp > 1, and become more oblate (with the perpendicular scale larger than the parallel scale) with decreasing Ωe/ωp. A scaling argument based upon electron gyrokinetic theory is proposed as a possible explanation for the observed scaling. The data indicate that the ratio of the parallel dimension (L∥) to the perpendicular dimension (L⟂) is such that L∥/L⟂ ≃ (1 + ρe²/λD²)−1/2. Our results provide a connection between the Geotail measurements in the deep magnetotail, where Ωe/ωp ≪ 1, and the FAST measurements in the low altitude auroral zone, where Ωe/ωp ≫ 1.

Galileo plasma wave observations of iogenic hydrogen

The Galileo plasma wave instrument has detected intense electromagnetic wave emissions approximately centered on the second and fourth harmonics of the local proton gyrofrequency during the close equatorial flyby of Io on 7 December 1995. Their frequencies suggest these emissions are likely generated locally by an instability driven by non thermal protons. Given that this process occurs close to Io, we suggest that hydrogen-bearing compounds, escaping from Io, are broken up/ionized near this moon, thereby releasing protons. Newly-created protons are thus injected in the Jovian corotating plasma with the corotation velocity, leading to the formation of a ring in velocity space. Several electromagnetic wave–particle instabilities can be driven by a ring of newborn protons. Given that the corotating plasma is sub-Alfvénic relative to Io, the magnetosonic mode cannot be destabilized by this proton ring. The full dispersion relation is studied using the WHAMP program ( Rönmark, 1982. Rep. 179. Kiruna Geophys. Inst., Kiruna, Sweden) as well as a new algorithm that allows us to fit the distribution function of newborn protons in a more realistic way. This improvement in the ring model is necessary to explain the relative narrowness of the observed spectral peaks. The measured E/ B ratio is also used to identify the relevant instability and wave mode: this mode results from the coupling between the ion Bernstein and the ion cyclotron mode (IBCW). To our knowledge this mode has not yet been studied. From the instability threshold an estimate of the density of newborn protons around Io is thus given; at about 2 Io radii from the surface and 40°W longitude from the sub-Jupiter meridian, this density is found to be ≥0.5% of the local plasma density (∼4000 cm −3), namely ≥20 cm −3. Assuming a stationary pickup process and a r − n distribution of pickup protons within several Io radii of Io’s wake, this implies that more than 10 26 protons/s are created around Io. The ultimate origin of these protons is an open issue.

A model of the lightning discharge at Jupiter

Based on the lightning waveform and spectra observed by the Galileo Atmospheric Probe, we present a discharge, radiation, and propagation model of the Jovian lightning that explains the absence of detectable HF spherics along with the measurement of ELF whistler emissions by the Voyager radio and plasma wave instruments. We find that although there is signal attenuation in the ionosphere, the controlling factor in determining lightning detectability at Jupiter is the intrinsically slow nature of the lightning discharge itself. The proposed discharge model is not unique, but matches the observed spectrum which spans several frequency decades and provides a basis for comparing lightning discharge models for other planets.

Effectiveness of near‐grazing incidence reflection in creating the rotationally modulated lanes in the Jovian hectometric radio emission spectrum

The Galileo plasma wave instrument has identified a narrow (in frequency) attenuation band in the hectometric emission that varies in frequency with system III longitude. It is possible to model this emission band assuming a high‐latitude cyclotron source region with emission that is efficiently attenuated when the ray path is nearly tangent to an L shell that is close to the Io flux tube [Gurnett et al, 1998]. The data suggest that the mechanism for attenuating the emission is very efficient, with the ratio of attenuated to unattenuated emission I/I0 < 0.02, and not a strong function of frequency. In this paper we demonstrate that incoherent scattering alone cannot explain the attenuation lane, which does not preclude coherent scattering by uncertain processes. We find rather that the source of attenuation is consistent with near‐grazing incidence reflection of emission from an L shell that is near the Io flux tube (a caustic surface).

Local time dependence of Jovian radio emissions observed by Galileo

Galileo has been in orbit around Jupiter since December 1995. All the orbits are equatorial and elliptical, with apogees between 60 RJ–142 RJ and perigees from 8–12 RJ. Since orbit injection, the plasma wave instrument (PWS) has been collecting data over specific intervals of each of the orbits at all local times and a range of different radial distances. We present the results of a survey of the data for the frequency range 300 kHz to 5.6 MHz, which includes the hectometric (HOM) and low‐frequency decametric (DAM) emissions. The results indicate that both the HOM and DAM emission are more intense and occur more frequently in the midnight sector of Jupiter. This is in analogy to Earth and consistent with a magnetic substorm source for a portion of the radio emissions in this frequency range. Another peak in the power levels is observed on the Jovian dayside in the local time range 11 hrs.<LT<12 hrs. This peak does not have a terrestrial counterpart. We speculate that this dayside peak may be a result of sampling near perigee, but we cannot rule out the possibility that this is not the case.

Generation mechanism of ESW based on GEOTAIL plasma wave observation, plasma observation and particle simulation

Data of electrostatic waves and plasma particles in the deep magnetotail (X ∼ −209RE) respectively obtained by Plasma Wave Instrument and Comprehensive Plasma Instrument onboard the GEOTAIL spacecraft are presented. When the GEOTAIL spacecraft experienced multiple crossings of the plasma sheet boundary layer, broadband electrostatic noise (BEN) and Langmuir wave were observed alternatively. The dynamic frequency spectra of BEN are very bursty in time, and their waveforms are a series of electrostatic solitary waves (ESW). The LW is observed when an enhancement of an electron flux is found on a high‐energy tail of a relatively cold velocity distribution function of the major thermal electrons. The ESW, on the other hand, are observed in the presence of a hot thermal electron distribution function, in which electrons responsible for the ESW are embedded. These plasma conditions are in agreement with the ESW generation model based on particle simulations.

Wave and particle measurements for chorus emissions by GEOTAIL in the magnetosphere

Simultaneously measured wave and particle data by the GEOTAIL spacecraft for chorus emissions in the dayside outer magnetosphere are analyzed. While detailed spectral structures as well as propagation directions of chorus emissions are obtained from wave form observations by the Plasma Wave Instrument (PWI), linear growth rates of the emissions are directly computed from pitch-angle distributions of cyclotron resonant electrons measured every 0.375 sec by the Comprehensive Plasma Instrument (CPI). One-to-one comparisons between these wave and particle data show that the measured profiles of resonant electrons are likely to represent distribution functions after the pitch angle diffusion by the simultaneously observed chorus emissions, rather than those responsible for generating the emissions.

GEOTAIL observation of electron cyclotron harmonic waves near the dayside magnetopause

We have been studying electron cyclotron harmonic (ECH) waves observed in the magnetosphere with the Plasma Wave Instrument (PWI) onboard the GEOTAIL spacecraft. The dynamic spectra show various types of ECH waves particularly near the dayside magnetopause. Superimposed on weak ECH waves, intense bursts with multiple harmonic bands are observed intermittently and sporadically in time, which is called “Totem Pole” emission (TP emission). The present paper focuses on the relation between the ECH waves and the corresponding electron velocity distribution functions obtained by the GEOTAIL LEP. The electron velocity distributions consist of multiple components including hot electrons at energy of a few keV and warm electrones of about hundred eV. To correct the total density estimated from the velocity distribution, we added a cold Maxwellian below 70 eV. As studied in linear theory, the GEOTAIL LEP observation also shows that the density ratio between the hot and cold electrons determines unstable harmonic modes. Linear dispersion analysis with the observed velocity distributions modified with the cold component indicates that ECH waves are marginally stable for each case. With increased loss cone factors of the hot electrons, however, multiple harmonic modes become unstable.

Plasma waves in slow-mode shocks observed by geotail spacecraft

The present paper provides plasma wave characteristics observed around slow-mode shocks in the geomagnetic tail region using the Plasma Wave Instrument (PWI) and Low Energy Particle experiment (LEP) onboard Geotail spacecraft. We also apply the direction finding method to the Electrostatic Solitary Waves (ESW) observed in the upstream region. By the direction finding analyses, we found that there exist counterstreaming ESW in the foreshock. Further, we show the observed electron velocity distribution is consistent with the counterstreaming ESW. We propose the observation model that conterstreaming ESW are emitted in the foreshock region of the slow-mode shock and in the distant neutral line.

Direction‐finding measurements of heliospheric 2–3 kHz radio emissions

Using data from the Voyager 1 plasma wave instrument, a series of direction‐finding measurements is presented for the intense 1992–93 heliospheric 2‐ to 3‐kHz radio emission event, and several weaker events extending into 1994. Direction‐finding measurements can only be obtained during roll maneuvers, which are performed about once every three months. Two parameters can be determined from the roll‐induced intensity modulation, the azimuthal direction of arrival (measured around the roll axis), and the modulation index (the peak‐to‐peak amplitude divided by the peak amplitude). Measurements were made at two frequencies, 1.78 and 3.11 kHz. No roll modulation was observed at 1.78 kHz, which is consistent with an isotropic source at this frequency. In most cases an easily measurable roll modulation was detectable at 3.11 kHz. Although the azimuth angles have considerable scatter, the directions of arrival at 3.11 kHz can be organized into three groups, each of which appears to be associated with a separate upward drifting feature in the radio emission spectrum. The first group, which is associated with the main 1992–93 event, is consistent with a source located near the nose of the heliosphere. The remaining two groups, which occur after the main 1992–93 event, have azimuth angles well away from the nose of the heliosphere. The modulation indexes vary over a large range, from 0.06 to 0.61, with no obvious trend. Although the variations in the directions of arrival and modulation indicies appear to reflect changes in the position and angular size of the source, it is also possible that they could be caused by refraction or scattering due to density structures in the solar wind.

Second harmonic hectometric radio emission at Jupiter

Galileo has been in orbit around Jupiter since December 1995. The plasma wave instrument on board the spacecraft has occasionally detected a rotationally modulated attenuation band in the hectometric (HOM) emission that most likely is due to scattering of the radiation from density fluctuations along the Io L‐shell, as reported earlier [cf. Gurnett et al., 1998]. The occurrence of the attenuation band is likely to be dependent on Io activity and the presence of density scattering centers along the Io L‐shell as well as the location of the source region. Some of the attenuation bands show clear indications of second harmonic emission. Without polarization measurements, it is difficult to place constraints on the local generation conditions based on the cyclotron maser instability, but the results imply that second harmonic emission could be present in the decametric (DAM) radiation as well. A survey of the data has revealed about 30 examples of second harmonic HOM.

Cusp energetic ions: A bow shock source

Recent interpretations of cusp energetic ions observed by the POLAR spacecraft have suggested a new energization process in the cusp [Chen et al., 1997; 1998]. Simultaneous enhancement of H+, He+2, and O>+2 fluxes indicates that they are of solar wind origin. In the present study, we examine H+ and He+2 energy spectra from 20 eV to several 100 keV measured by the Hydra, Toroidal Imaging Mass‐Angle Spectrograph (TIMAS), and Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE) on POLAR. The combined spectrum for each species is shown to be continuous with a thermal distribution below 10 keV/e and an energetic component above 20 keV/e. Energetic ions with comparable fluxes and a similar spectral shape are commonly observed downstream from the Earth's quasi‐parallel (Q∥) bow shock. In addition to the similarity in the ion spectra, electric and magnetic field noise and turbulence detected in the cusp by the Plasma Wave Instrument (PWI) and Magnetic Field Experiment (MFE) onboard POLAR are similar to the previously reported observations at the bow shock. The waves appear to be coincidental to the cusp energetic ions rather than causal. We suggest that these ions are not accelerated locally in the cusp. Rather, they are accelerated at the Q∥ bow shock and enter the cusp along open magnetic field lines connecting both regions.

Source of auroral myriametric radiation observed with Geotail

The Geotail plasma wave instrument has found the auroral myriametric radiation (AMR). Its frequency range is about 1–30 kHz similar to the trapped continuum radiation, and its occurrence shows a good correlation with auroral kilometric radiation (AKR). This suggests the source location of AMR is similar to that of AKR. Ray tracing is performed for L‐O mode waves radiated just above the local plasma frequency in the auroral plasma cavity. The waves are guided in the cavity, propagate far from the Earth, arid are observed as AMR. In the source region these guided waves are observed by the DE 1 satellite and reported as the trapped L‐O mode waves within the cavity. A limitation on initial conditions of wave normal angles for the ray tracing derived from a comparison with these observations suggests that its generation mechanism is similar to that of the nonthermal continuum. It is, however, generated in the auroral plasma cavity and it is not mode‐converted from the upper hybrid mode waves. The ray tracing results are consistent with AMR observations by Geotail.