Decametric Emission Research Articles

Striated spectral activity in Jovian and Saturnian radio emission

Examination of high time resolution frequency‐time spectrograms of radio emission measured near the Voyager 1 and 2 encounters with Jupiter reveals occasional striation patterns within the normally diffuse hectometric radiation. The patterns are characterized by distinctive banded structures of enhanced intensity meandering in frequency over time scales of minutes to tens of minutes. This banded form of striated spectral activity (SSA) has an occurrence probability of the order of 5% during the three weeks before and after Jupiter encounters. Plots of single 6‐s frequency sweeps often exhibit a slow rise in intensity followed by a sharp drop‐off in each band as frequency decreases. Banded SSA is often preceded or followed by chaotic SSA in which banding of the emission becomes discontinuous or unrecognizable, although the intensity modulation is still evident. Although SSA normally occurs in the frequency range of roughly 0.2–1.0 MHz, similar but longer‐lasting patterns have been found occasionally in decametric emission above 10 MHz. Analogous modulation has also been observed in the Saturnian radio emission, suggesting that SSA may be a common feature intrinsic to the radio emission at both planets. Observed properties suggest a number of constraints on the possible source mechanism which make theoretical interpretations difficult.

Decametric Emission by Pulsars

A brief review of the main features of radio emission by pulsars in the decametre range is presented. Data on intensity, spectra, main pulse form, structure and properties of the interpulses are also presented.

Ray tracing of Jovian decametric radiation from southern and northern hemisphere sources: Comparison with Voyager observations

Because of a lack of readily usable information pertaining to the polarization of the Voyager 1 and 2 high‐frequency band data, a technique has been developed that aids the identification of Io‐dependent decametric radiation originating from the southern hemisphere of Jupiter. This technique compares the results of model ray tracing calculations with the Planetary Radio Astronomy (PRA) observations. A large portion of the Voyager 1 and 2 PRA observations are sorted into bins (±3° wide) centered on a specific Io central meridian longitude. When the data are plotted (as a frequency‐longitude spectrogram) in this coordinate system, Io‐dependent features can be identified and compared with ray tracing calculations performed in a model Jovian magnetosphere where it is assumed that the decametric emissions are generated in the RX mode from low‐altitude source regions along the instantaneous Io flux tube. Two different magnetic field models are used, and the results are contrasted. In this study, we compare the observations for constant sub‐Io longitudes of 260° and 300° with the corresponding model ray tracings. The results permit the identification of decametric spectral features from source locations in both the northern and southern hemispheres: (1) The emission traditionally designated “Io‐B” originates at the Io flux tube footprint in the northern hemisphere when the sub‐Io system III longitude λIII is equal to 260°. (2) The component traditionally designated “Io‐C” is a combination of emissions emanating from the Io flux tube footprints in both northern and southern hemispheres when Io is located at longitudes 260° and 300°. (3) The traditional “non‐Io‐A” emission is, in fact, Io related at both Io configurations studied. When Io is located at λIII = 260°, this emission originates in the southern hemisphere flux tube footprint. When Io is at λIII = 300°, this component (“non‐Io‐A”) originates from the flux tube footprint in the northern hemisphere.

A theory of Jovian shadow bursts

The shadow events in the dynamic spectra of Jovian decametric emission are explained as the result of interaction between electron bunches responsible for S and L emissions. The relevant dispersion relation is derived for the fast extraordinary mode in the cold magnetospheric plasma in the presence of S and L electron bunches. The growth rate of the synchrotron maser instability is studied in the presence and absence of S-electrons. It is shown that the synchrotron maser instability responsible for L-emission can be temporarily quenched by the invasion of S-electrons, thereby stopping the L-emission. The theory accounts for various observed features of the shadow events.

Beaming of Jupiter's decametric emission

Dynamic spectra of a Jovian non-Io-A storm recorded simultaneously by the Voyager 1 spacecraft and by the Kiiminki radio spectrograph are compared. It seems that the emission beam of the storm co-rotates with the planet and has a sloped leading edge, in accordance with the result of Maeda and Carr (1984).

IMF control and convection in the Jovian magnetosphere.

The dependence of the Jovian non-Io-related decametric emission on solar wind parameters demonstrates that the energy of the solar wind is imparted to the Jovian magnetosphere. This paper considers if, and then how, the steady-state reconnection model can be used for the understanding of this energy transfer process. Referring to the observationally obtained values of key parameters as much as possible, we investigate if the steady state model as commonly applied for the earth's magnetosphere can be adopted as a reasonable approximation for the Jovian magnetosphere as well. We find that rapid rotation of the planet does not present a serious obstacle to a steady progress of reconnection since the motion of the field lines is governed by the magnetosheath plasma and the slipping occurs at the ionospheric ends. Next we discuss the convection generated by reconnection for northward and southward polarities of the interplanetary magnetic field, and address the field line motion in the tail and the current system generated in the ionosphere. Influence of the centrifugal force on the motion of field lines in the inner region of the magnetosphere is also discussed.

Bursts of type N in Jupiter's decametric radio spectra

Dynamic spectra of Jupiter's decametric emission often display narrow-band features, referred to as events of type N (Carr et al., 1983). The average bandwidth of these emissions is in the vicinity of 200 kHz, their durations are typically in the decasecond range, and their f-t slopes are small and random. Although the N-bursts can be described as narrow-band L-bursts, it seems that they are realted to S-bursts in their area of occurrence in the Io-B region, the durations of the emission envelopes, and their bandwidths. Possible implications are discussed.

IN SEARCH OF URANIAN DECAMETRIC EMISSION

ABSTRACT An experiment to detect decametric radiation from Uranus is described. It is argued that in light of recent discoveries of a magnetic field on Uranus, it may be possible to detect low-frequency radio emission similar to the decametric emission from Jupiter. The experiment consisted of 106 hours of monitoring using the 26.3 MHz array of the University of Florida Dixie County Radio Observatory. No radiation from Uranus was detected. An upper flux density limit of 400 Jy was obtained.

Jupiter's and Saturn's fine-scale magnetic fields

Jupiter's field is strongly dipolar but with relatively large high order moments compared to the Earth's. In situ magnetic field data allow us to interpret most of the Earth-based microwave observations of Jupiter, with the exception of Branson's hot spot. Decametric emissions have a complex rotational pattern which has been stable since 1950; their agreement with the spacecraft magnetic fields is much less satisfactory than that of the microwaves. We conclude that the extrapolation of magnetic fields from the spacecraft to the surface of Jupiter is in error by 40% in the Southern Hemisphere. Saturn's radio emissions show complexities similar to Jupiter's. They are strongly asymmetric about the rotational axis, although Saturn's Field is nearly axisymmetric. Their strong asymmetry suggests strong longitudinal variations in the magnetic field a few thousand kilometers from the cloud tops, in conflict with the field measured aboard Pioneer 11. The magnetic fields within a few thousand kilometers of either Jupiter's or Saturn's cloud tops are probably unknown. It is discouraging that more is not known about the fields after a total of 7 encounters. Perhaps the Galileo probe can test usefully models of the Jupiter field, even if its measurements refer to just one trajectory through the clouds. An arguable case can be made that the giant planets exhibit complexity of magnetic structure similar to the Sun.

Jovian decametric arcs: An estimate of the required wave normal angles from three‐dimensional ray tracing

A three‐dimensional ray tracing code which incorporates the O‐4 magnetic field model (Acuna and Ness, 1976) and a realistic plasma model has been used to model high‐curvature decametric arcs observed by the Voyager planetary radio astronomy instrument. Two examples of intense, isolated, vertex‐late, high‐curvature arcs were singled out for study. The source point wave normal angle was incremented at each of a full range of frequencies until the model rays identically matched the observed arcs of the planetary radio astronomy data. Several different Doppler shifts were assumed at the source point. By this procedure an accurate relationship between the wave normal angle Ψ and the frequency was obtained, with the variation being 70° < Ψ < 80°, and Ψmax = 80° for the arcs considered. As the assumed Doppler shift at the source point increased, the value of Ψmax was found to decrease, but the general shape of the Ψ(f) curve was unaffected. By comparing the ray tracing results with the independent results of Staelin (1981) regarding emission from beaming electrons, it was found that larger Doppler shifts (f/fg > 1.1) at the source point produce Ψmax > 70°, in agreement with the ray tracing results for the two arcs considered, only for υ∥ > 0.7 and υ∥/c > 0.32 (where υ∥ is the electron velocity parallel to B). Actual values of υ∥/υ are unknown, but independent observations indicate that υ∥/c ≈ 0.1. Since the low‐curvature arcs are believed to result from larger wave normal angles, our results indicate an upper limit to the Doppler shift of Jovian decametric emissions of f/fg < 1.1.

A small-size (less than 1 min of ARC) decametric radio source in the Perseus cluster

Results of observations of decametric readio emission from the Perseus cluster with the aid of URAN-1 interferometer are presented. The 42.2 km baseline of the interferometer is oriented along the parallel. The object is shown to contain a radio source with dimensions which do not exceed 60″ and whose flux is not less than 95 Jy over the frequency range from 16.7 to 25 MHz. Several source models are considered and a comparison with higher-frequency measurements is carried out. It seems possible that the source is a component of 3C84A which was reportedly observed earlier by the interferometry technique at 408 and 1415 MHz.

Alfvén wave propagation in the Io plasma torus

Gurnett and Goertz (1981) proposed that the large number of discrete arcs observed in the Jovian decametric radio emission is caused by multiple reflections of Alfvén waves excited by Io. In this paper the plasma measurements that were made by the Voyager 1 plasma science experiment have been combined with a model of Jupiter's magnetic field to calculate the time an Alfvén wave takes to travel between Io and Jupiter's ionosphere and the period of subsequent bounces between the northern and southern hemispheres. The result is a wave pattern extending around Jupiter as the multiply reflected Alfvén waves are carried away from Io by the corotating magnetospheric plasma. Although the whole pattern continually changes over the 13 hours Io takes to move through 360° of Jovigraphic longitude, a general longitudinal structure is exhibited independent of the position of Io, due to the geometry of the magentic field and the distribution of plasma in the Io torus. If the Alfvén waves stimulate the decametric radio emission, then the wave pattern predicts specific properties of the decametric emission which can be compared with radio observations.

Interaction of S- and L-bursts in Jupiter's decametric radio spectra

Dynamic spectra of Jupiter's decametric emission are observed with a high-resolution radio spectrograph. Certain events observed in region Io-B display interaction effects between the S- and L-emissions. The effect appears as a gap in the L-emission after a passage of an S-burst. In a typical case the L-emission has a relatively narrow bandwidth, the S-burst appears as a sloping line crossing it, and the duration of the emission gap is a substantial fraction of a second. The gap has sharp, sloped edges; its leading edge has anf-t slope which is the same as that of the S-burst while the slope of the trailing edge is lower. A kind of ‘tilted V-pattern’ is thus formed. It is suggested that some more complicated spectral patterns may also be produced this way. It seems that an emission model consisting of S-bursts from bunches of gyrofrequency-emitting electrons ascending a flux tube and interacting with L-emitting bunches of electrons having more stationary guiding centers is inadequate to explain the complex S-L interactions.

Fast-drift shadow events in Jupiter's decametric radio spectra

High-resolution dynamic spectra of Jovian S-bursts frequently reveal sloping gaps crossing bands of L-burst emission with drift rates comparable to those of S-bursts. These “fast-drift shadow” (FDS) events are often sharply bounded on one edge by an S-burst, and sometimes on both edges by a pair of S-bursts emanating from a common vertex. It is suggested that the investigation of such S- and L-burst interactions may provide new insights of considerable importance in the search for the Jovian decametric emission mechanism.

Structure of the source of jovian decametric emission and interplanetary scintillation

Since its discovery in 1955, Jupiter decametric emission has been extensively studied with ground-based instruments1, and more recently by planetary radioastronomy experiment on board the Voyager spacecraft2,3. However, we are still far from understanding the origin of the emission, mainly because we have no direct information about the position and structure of the source, due to the low spatial resolution of instruments at those wavelengths. Previous theories have assumed that the source emission takes place along magnetic field lines, close to the local gyrofrequency. We propose here a method to test this hypothesis by studying interplanetary scintillations which modulate the emission when received on Earth, and quantify the predicted effects. Preliminary results indicate that the emission is probably spatially distributed and could occur along field lines, and that Io controlled A and B sources are on opposite sides of Jupiter.

Synoptic observations of Jupiter's radio emissions: Average statistical properties observed by Voyager

Observations of Jupiter's low‐frequency radio emissions collected over one‐month intervals before and after each Voyager encounter have been analyzed to provide a synoptic view of the average statistical properties of the emissions. Compilations of occurrence probability, average power flux density, and average sense of circular polarization are presented as a function of central meridian longitude, phase of Io, and frequency. The results are compared with ground‐based observations. The necessary geometrical conditions and preferred polarization sense for Io‐related decametric emission observed by Voyager from above both the dayside and nightside hemispheres are found to be essentially the same as those observed in earth‐based studies. On the other hand, there is a clear local time dependence in the Io‐independent decametric emission. The emission is prevalent at longitudes >200° when observed from over the dayside hemisphere but is dominant at longitudes <200° when observe from over the postmidnight sector. Decametric emission, which comprises the dynamic spectral lesser arcs near 10 MHz, displays a distinct, bimodal polarization pattern that is predominantly in the left‐hand sense at longitudes below 150° and in the right‐hand sense at longitudes above 150°. The central meridian longitude distributions of occurrence probability and average flux density at hectometric wavelengths appear to depend significantly on both the observer's latitude and local time. Io appears to have an influence on average flux density of the emission down to below 2 MHz. The average power flux density sectrum of Jupiter's emission has a broad peak near 9 MHz. Integration of the average spectrum over all frequencies and all longitudes gives a total radiated power for an equivalent isotropic source of 4 × 1011 W.

Models for Jupiter's decametric arcs

Several papers of the present issue attempt to explain the remarkable arc‐shaped structures that dominate Jupiter's decametric emissions as recorded by Voyagers 1 and 2. Since these papers approach this subject from very different points of view, the introduction is designed to place these perspectives in context. The present paper understands these arcs in terms of a magnetic fine structure undetected by Pioneer 11 on account of its rather large closest approach distance (50 or 60 times the smallest scales in the structures involved). The nested sequences of arcs manifest the occurrence of widespread fine structures, similar in size and shape to the white ovals so ubiquitous over Jupiter's visible surface. An arc concave toward increasing time (vertex early) occurs at the east limb passage of such a structure, and the arc convex (vertex late) occurs at the west limb passage. This is consistent with the early source producing vertex early arcs and the late (and main) source producing vertex late arcs. Since arcs seem naturally to classify themselves into two large families—greater arcs (reaching frequencies ≥20 MHz) and lesser arcs (below 20 MHz)—polarized invariably right hand, or right hand or left hand symmetrically in longitude, respectively, we identify the right‐hand greater arcs with north polar cap sources, the lesser right‐hand (left hand) arcs with north (south) temperate sources, and call for the absence of fine magnetic structure in Jupiter’s southern polar cap.

Radio Jupiter after Voyager: An overview of the planetary radio astronomy observations

We present an overview of Jupiter's low‐frequency radio emission morphology as observed by the planetary radio astronomy (PRA) instrument onboard the Voyager spacecraft. The PRA measurement capabilities and limitations are summarized following over two years of experience with the instrument. As a direct consequence of the PRA spacecraft observations, unprecedented in terms of their sensitivity and frequency coverage, at least three previously‐unrecognized emission components have been discovered: broadband and narrow‐band kilometric emission and the lesser‐arc decametric emission. Their properties are reviewed here. In addition, the fundamental structure of the decameter wavelength and hectometer wavelength emission, which is now believed to be almost exclusively in the form of complex but repeating arc structures in the frequencytime domain, is described here for the first time. Dramatic changes in the emission morphology of some components as a function of the sun‐Jupiter‐spacecraft angle (local time) are described. Finally, the PRA in situ measurements of the Io plasma torus hot‐to‐cold electron density and temperature ratios are summarized.

Standing Alfvén wave current system at Io: Voyager 1 observations

The enigmatic control of the occurrence frequency of Jupiter's decametric emissions by the satellite Io has been explained theoretically on the basis of its strong electrodynamic interaction with the corotating Jovian magnetosphere leading to field‐aligned currents connecting Io with the Jovian ionosphere. Direct measurements of the perturbation magnetic fields due to this current system were obtained by the Goddard Space Flight Center (GSFC) magnetic field experiment on Voyager 1 on March 5, 1979, when it passed within 20,500 km south of Io. An interpretation in the framework of Alfvén waves radiated by Io leads to current estimates of 2.8 × 106 A. A mass density of 7400–13,600 proton mass units per cm³ is derived, which compares very favorably with independent observations of the torus composition characterized by 7–9 proton mass units per electron for a local electron density of 1050–1500 cm−3. The power dissipated in the current system may be important for heating the Io heavy ion torus, inner magnetosphere, Jovian ionosphere, and possibly the ionosphere or even the interior of Io.

A new high-grain, broadband, steerable array to study Jovian decametric emission

A large array of antennae has been built at the Radioastronomy Observatory, Nancay, France, to study solar and planetary decametric emissions. This array has a high gain (25 db) in a broad range of frequencies and is steerable through a large part of the sky. We present the main characteristics of this array, and the receives which are used to show the importance of the equipment for Jovian studies. We summarize the results already obtained and describe some topics which are presently being studied.