Jovian Kilometric Radiation Research Articles

Interpretation of the zebra pattern in the Jovian kilometric radiation

AbstractThe origin of a fine structure as quasi‐harmonic parallel drifting stripes of enhanced brightness (zebra pattern) in the dynamic spectrum of Jovian kilometric radiation is discussed. A possible interpretation of the observed structure based on the effect of double plasma resonance (DPR) in the Jupiter magnetosphere is analyzed. It is shown that the observed features of the zebra pattern cannot be attributed to the DPR effect at electron cyclotron harmonics. The proposed scheme consists of excitation of ion cyclotron waves at the low hybrid frequency in the ion DPR regions and succeeding coalescence of these waves with a longitudinal wave at the upper hybrid frequency. The source parameters necessary for matching the expected and observed properties of the Jupiter zebra pattern are discussed.

Theory of Jovian Kilometric Radiation

AbstractThis report presents a theoretical model of extraordinary mode radiation in presence of low frequency whistler mode. We show that the generation mechanism of extraordinary mode emission in the presence of whistler mode is based on the plasma maser theory. Application of theoretical results to Jovian Kilometric radiation is examined.

Remote sensing of planetary plasma by means of natural radio emissions

Voyagers 1 and 2 discovered radio emissions at Jupiter, Saturn and Uranus which were similar to those already studied in the Earth's magnetosphere. Terrestrial myriametric radiation, TMR, emanates from electrostatic upper hybrid, ESUH, waves, the conversion being believed to occur as described by the linear mode conversion window, LMCW, theory. A technique, based on the theory, has been devised which allows TMR, observed by satellites, to be used to remote-sense the source regions so as to obtain the location and plasma density of the source plasma. The technique can also be applied to study the sources of the analogous radiations observed at the other planets: Jovian kilometric radiation and narrowband electromagnetic emissions at Saturn and Uranus. The results of using this technique with the four planetary radiations are presented. They provide further evidence for the validity of the LMCW theory and for the importance of plasma density gradients in the conversion process.

Analysis of Jovian kilometric radiation simultaneously observed by voyagers 1 and 2

Broadband Jovian radio emission in the kilometric wavelength range (bKOM) observed by the Voyagers 1 and 2 (V1 and V2) planetary radioastronomy, PRA, experiments have been analysed for the period January to December 1979 which includes the Jupiter encounters. The pre-encounter observations were made on the nightside, the difference in local time between the two spacecraft being about one hour. It is therefore possible to study local time, and source - observer distance effects. bKOM beaming exhibits large temporal variations. The beaming is believed to arise from the manner in which the source electrostatic upper-hybrid waves are converted to electromagnetic radiation in the plasma density gradients at the Io torus. In this case the beaming depends on the characteristic frequencies of the source plasma, and temporal variations in the beaming are expected to reflect fluctuations in the Io torus. Remote-sensed density profiles have been computed from V1 and V2 simultaneous bKOM observations and compared with in situ measurements by V1 when it passed through the torus. Good agreement is found between the density gradients obtained, and the in situ measurements. The differences between V1 and V2 profiles may possibly be explained by the difference in local time of the two spacecraft. The computed profiles from dayside and nightside observations are nearly coincident. A large temporal variability of the Io torus density, or of the latitudinal position of the source, is apparent.

The emission of narrow‐band Jovian kilometric radiation

A model based on the nonlinear coupling of electrostatic plasma waves is proposed to explain the emission of the narrow‐band Jovian kilometric radiation (nKOM) observed by the Voyager spacecraft, it is shown that upper‐hybrid branch electrostatic waves propagating through the inhomogeneities in the outer periphery of the Io plasma torus can attain the proper geometry for localized upconversion interactions leading to pump depletion. Plasma waves propagating into a weak density gradient (increasing ωp/Ωe) and reflected at the critical layer (ωL = ωp), interact with the incident waves leading to the electromagnetic emission at 2ωUH, which is beamed at large angles with respect to the background magnetic field. In general, both L‐O and R‐X mode waves can be generated. The observed power and net polarization (L‐O) are consistent with pump depletion of electrostatic waves at a level of ∼10 mV/m. A possible excitation mechanism for the electrostatic waves is also discussed.

Io plasma torus and the source of jovian narrow-band kilometric radiation

Narrow-band jovian kilometric radiation (nKOM) is distinguishable from broad-band jovian kilometric radiation (bKOM), both discovered by the Voyager spacecraft1–3, by its generally lower intensity, its more restricted range of centre frequency, its narrower bandwidth, and its temporally less sporadic nature4. When observed at northern and southern magnetic latitudes nKOM polarization was predominantly left-handed and right-handed respectively, opposite to bKOM4,5. At nearly the same longitude on consecutive jovian rotations nKOM sometimes lagged the magnetic field rotation4 by 3–5%; no such lag was detected for bKOM6,7. It was proposed that bKOM is produced within ˜2° of the magnetic equator in the outer lo plasma torus by conversion of electrostatic upper hybrid (ESUH) waves to ordinary (O) mode electromagnetic radiation via a radio window8,9. Here we show that nKOM seems to be produced by the same conversion process, also operating in the lo torus, but at latitudes ≥8°. The difference in latitude for nKOM and bKOM sources stems from the condition required for mode conversion that the plasma density gradient should be normal to the magnetic field vector. The difference in these locations allows an explanation of most of the similarities and differences between the two radiations.

Io plasma torus and the source of jovian kilometric radiation (bKOM)

One of the many discoveries by Voyagers 1 and 2 during their flyby of Jupiter was the detection of radio emission at kilometric wavelengths1,2 in the frequency range 20–500 kHz. Broadband jovian kilometric radiation bKOM was observed to be beamed away from the zenomagnetic equatorial plane, the beaming angle increasing with increasing frequency1–3. It was suggested that bKOM is created by the linear conversion of electrostatic upper hybrid waves to electromagnetic waves in the density gradients at the lo torus4. The beaming is inherent to the conversion mechanism and depends on the characteristic frequencies of the source plasma. Here I present results, based on this theory, of remote sensing of the lo torus using bKOM observed by Voyager 2 when inbound towards Jupiter. The density profiles obtained are in remarkable agreement with measurements of the torus densities obtained in situ by Voyager 1. This seems to provide very strong evidence that bKOM is produced at the lo torus by the linear mode-conversion mechanism and indicates that the technique used for remote sensing could be a powerful method of determining the torus characteristics when no in situ measurements are available.

Narrow‐band Jovian kilometric radiation: Occurrence, polarization, and rotation period

A statistical analysis of the narrowband Jovian kilometric radiation, or nKOM, observed by the planetary radio astronomy experiment (Voyager 1 and Voyager 2) from January to December 1979, is reported. Occurrence, polarization, and rotation period are studied. The sense of circular polarization changes with magnetic latitude of the observer. Left‐hand polarized emission is associated with the northern hemisphere, which suggests a radiation mechanism in the ordinary mode. The emission is beamed in magnetic latitude with an obscuration at the equator, and the measured axial ratio increases with the magnetic latitude of the observer. The nKOM source regions do not rotate with a single period and coherent phase. Long‐term variations in the rotation are shown, the dominant regimes being system III and 2.5–3% corotation lags. During sequences of corotation with system III, the nKOM events are observed mainly in the ranges 40° and 300° central meridian longitude.

Broadband Jovian kilometric radiation: New results on polarization and beaming

Using the Voyager 1 and 2 (V1 and V2) planetary radio astronomy experiment, we have analyzed the data in the kilometric wavelength range from January to December 1979, the period which includes the Jupiter encounters for V1 and V2. Statistical studies of the broadband kilometric radiation (bKOM) reveal new results on the polarization and beaming of the emission. The northern and southern components are right‐hand and left‐hand, respectively polarized, without reversal at encounter. The central meridian longitude occurrence of these two components is dependent on the zenographic latitude of the observer. The shadowing by the Io plasma torus is confirmed, the boundaries of the emission beam have been deduced, and it is shown that the latitudinal beaming of the emission is dependent on local time, from dayside to nightside observations. Our results are discussed in terms of a source radiating in the extraordinary mode. It is suggested that the source regions are at the inner boundary of the Io torus all around the planet and that the emission is beamed into a hollow conical sheet whose axis coincides with the magnetic dipole of the planet. Radiation mechanisms are briefly discussed; the maser synchrotron instability and the non linear conversion of electrostatic waves could contribute to bKOM, each one in a limited frequency range.

Radio wave emission from the IO torus

A theory is proposed to explain radio wave emissions observed by the Voyager spacecraft to have their source in Jupiter's inner magnetosphere. The emissions, termed Jovian kilometric radiation (JKR), are proposed to come from the Io plasma torus - a region of enhanced plasma density circling the planet at the orbit of Io. It is suggested that intense upper-hybrid waves, known to exist at the zenomagnetic equator, couple to the L-O magnetoionic mode at the plasma frequency. In order to produce the observed wave polarisation it is necessary to invoke a distended plasma torus, and ray tracing in a realistic Jovian magnetosphere shows that many characteristics of JKR can be explained by this theory.

Latitudinal beaming of planetary radio emissions

The Voyager missions have revealed that jovian kilometric radiation (emanating from Jupiter's magnetosphere) is beamed away from the zenomagnetic equator. Results from GEOS 1 show that terrestrial non-thermal continuum or myriametric radiation is similarly beamed away from the geomagnetic equator. A mode-coupling mechanism is proposed such that measurements of the direction of propagation of the escaping O-mode radiations may allow the construction of the first high-resolution maps of the shapes of the equatorial plasmapause and Io torus.

The occurrence rate, polarization character, and intensity of broadband Jovian kilometric radiation

We describe the major observational features of one new component of Jupiter's radio emission spectrum, the broadband kilometer wavelength radiation, or bKOM. This study, using the Voyager Planetary Radio Astronomy (PRA) experiments, reveals that the overall occurrence morphology, dynamic spectra, and polarization character of bKOM are strong functions of the latitude and/or local time geometry of the observations. The postencounter data show a decline in the mean occurrence rates and power level of bKOM and, in particular, a depletion in the occurrence rate at those same longitudes where the detection rate is a maximum before encounter. Additionally, the polarization sense undergoes a permanent reversal in sign after encounter, whereas the time‐averaged wave axial ratio and degree of polarization remain relatively unchanged. Finally, no evidence of any control by Io is found. The strong dependence of the morphology on local time suggests a source whose beam is nearly fixed relative to the Jupiter‐sun line.

Narrow‐band Jovian kilometric radiation: A new radio component

We describe a new component of Jupiter's radio spectrum. This component emits in a very narrow bandwidth (≤ 40 kHz) near 100 kHz. Its waveform is a very smooth and gradual rise and subsequent fall in intensity over typically two hours. The emission is polarized with left‐hand polarization associated with the Jovian northern magnetic hemisphere and righthand with the south. The most interesting feature of the emission is its deviation from a strict System III rotation period repetition rate. The emission source of this narrow‐band component clearly rotates slower by 3‐5% than all other forms of Jovian radio emission. Propagation considerations coupled with this observed lack of corotation point to a source region near the magnetic equatorial plane at the outer ‘edge’ (8‐9 RJ) of the Io plasma torus.

Ray tracing of Jovian kilometric radiation

Results of computer ray tracing of Jovian kilometric radiation from 56.2 kHz to 1 MHz in a model Jovian magnetosphere with an Io torus are presented. Ray tracing calculations indicate that the Io torus presents a propagation barrier to the radiation and that the Jovian kilometric radiation must be generated in the L‐O mode from a source near Jupiter on field lines passing through the Io torus. One effect of the Io torus is to refract the rays away from the magnetic equator forming a shadow zone at radial distances beyond the torus. In general, at radial distances greater than 10 Jovian radii, as the wave frequency increases (> 200 kHz) so does the magnetic latitude of the shadow zone. These and other features of the ray tracing calculations are in good qualitative agreement with the observations from the plasma wave receiver and planetary radio astronomy experiment on board both Voyagers 1 and 2.

Spatial and temporal studies of Jovian kilometric radiation

Plasma wave measurements taken during the Voyager 1 and 2 Jupiter encounters are used as a basis for synoptic studies of Jovian kilometric radiation. The studies reveal the existence of a shadow zone near the magnetic equator within which kilometric radiation is seldom or weakly observed. This shadow is presumably cast by the Io plasma torus whose density is high enough to refract electromagnetic waves generated either near the torus or closer to Jupiter. At 56.2 kHz this shadow zone has a half width of about 10°. In addition to a latitudinal shadow zone, there occasionally appear to be long‐lived longitudinal variations of kilometric radiation intensities. That is, individual features may reappear on successive rotations at nearly constant System III longitudes. This may imply longitudinally asymmetric structures in the kilometric source region or intervening magnetosphere such as the Io torus which corotate and have lifetimes of up to several days. Some of the features appear to drift to higher longitudes with increasing time suggesting deviations from corotation ranging up to a few percent. Gross changes in the character of Jovian kilometric radiation on time scales of a few days imply dramatic changes in the inner Jovian magnetosphere on the same time scale and may be indicative of magnetospheric substorms or changes in plasma injection rates from Io.

Low frequency radio emissions from Jupiter: Jovian kilometric radiation

A new component of the Jovian radio spectrum has been observed by the plasma wave instruments on Voyagers 1 and 2 at frequencies ranging from about 10 to 56 kHz or higher. This Jovian kilometric radiation is characterized by storms of emissions lasting typically 45 minutes at 56.2 kHzhowever some events persist for as long as four hours. The storms usually exhibit impulsive bursts with time scales of a few seconds to several minutesalthough some events show smoothly varying intensities as a function of time. High resolution frequency‐time spectrograms reveal a continuum‐like background with more intensenarrowband features superimposed. The narrowbandor discretefeatures tend to decrease in frequency with increasing timefalling ∼ 1 kHz in 5 to 60 seconds. The maximum power emitted assuming an isotropic radiator near Jupiter and a bandwidth for the most intense bursts of ∼ 10 kHz is about 109 watts. The Jovian kilometric radiation is most likely observed within ±45° of 200° System III longitudeλIII although there is a secondary maximum near λIII = 25°