We review both observational and theoretical aspects of the generation of auroral radio emissions at the outer planets, trying to organize the former in a coherent frame set by the latter. Important results have been obtained in the past few years on these radio emissions at the five magnetized planets, from the observations of Ulysses at Jupiter and of Wind and other Global Geospace Science spacecraft in Earth orbit, from the reanalysis of Voyager data about Saturn, Uranus, and Neptune, from ground-based high frequency-time resolution and full polarization measurements, and from pioneering multispectral observations of the Jovian and Saturnian aurorae (radio/UV/IR). In parallel, considerable progress has been made in their generation theory (Cyclotron-Maser operating in small-scale, laminar, hot-plasma-dominated radio source structures), mostly on the basis of in situ observations of terrestrial radio sources. Particle acceleration and precipitation is also better documented, thanks to in situ measurements in the Earth auroral zones and to multispectral studies of Jupiter and Saturn. Finally, the modeling of the planetary magnetic field and magnetospheric plasma at these two planets has also been considerably improved. To organize the wealth of observational results within a coherent theoretical frame, we emphasize unresolved questions (e.g., planetary radio bursts generation) and contradictions and propose ways to answer them. Our ability, already significant, to perform remote sensing of magnetoplasmas at the giant planets and, hopefully, at other distant radio sources (solar, stellar) in the near future, depends on the good understanding of the physical processes underlying the generation of auroral electromagnetic emissions. The question of the existence of exoplanetary radio emissions and the possibility to detect and study them is briefly discussed.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JE01323

Auroral radio emissions at the outer planets: Observations and theories

Jovian S-bursts are intense impulsive decameter radio spikes drifting in frequency in tens of milliseconds over several hundreds of kHz up to a few MHz. Their generation scenario has been much debated for 30 years. The automated analysis of an extensive set of digital radio observations at very high temporal and spectral resolutions is presented here. It strongly suggests that S-bursts are the cyclotron-maser emission of electron populations with ∼5 keV energy, accelerated near Io and then in quasi-adiabatic motion along magnetic field lines connecting the Io torus to Jupiter's auroral regions. This scenario is consistent with Voyager observations of Alfven waves near Io's wake and with recent infrared observations of the Io flux tube footprint. The total energy, velocity and pitch angle of the radiating electrons, and the extent of the bursts radiosources are deduced from the radio measurements, which appear as a promising remote sensing tool for the Jovian magnetosphere, and possibly that of other “radio” planets.

A scenario for Jovian S-bursts

[1] Jupiter's radio emissions are dominated in intensity by decametric radio emissions due to the Io-Jupiter interaction. Previous analyses suggest that these emissions are cyclotron-maser emissions in the flux tubes connecting Io or Io's wake to Jupiter. Electrons responsible for the emission are thought to be accelerated from Io to Jupiter. We present simulations of this hot electron population under the assumption of acceleration by Alfven waves in the Io flux tube. Outside of limited acceleration regions where a parallel electric field associated with Alfven waves exists, the electrons are supposed to have an adiabatic motion along the magnetic field lines. Near Jupiter a loss cone appears in the magnetically mirrored electron population, which is able to amplify extraordinary (X) mode radio waves. The X-mode growth rate is computed, which allows us to build theoretical dynamic spectra of the resulting Jovian radio emissions, whose characteristics match those observed for Jovian S bursts.

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Jovian S burst generation by Alfvén waves

We have developed a method for displaying the spectral structure of Jupiter's decametric radio S bursts on timescales down to a few microseconds, 2 orders of magnitude finer than has been achieved elsewhere. By employing an extremely sensitive antenna (640 dipoles, at 26 MHz) and selecting only relatively weak S bursts that possess the simplest possible spectral shape, we identify frequently occurring structural features that we associate with localized emission centers. On timescales having better than about 30 μs resolution we find that the S burst baseband oscillation (and therefore the RF oscillation) is modulated to form distinct pulses, which we refer to as subpulses. Still finer time resolution reveals that within individual subpulses the baseband (and RF) oscillation often displays segments in which the usually drifting phase term abruptly becomes essentially constant and, after remaining so for 10 to 100 μs, abruptly resumes its random drift. It is these abruptly starting and ending segments of phase coherence that we attribute to isolated powerful centers of cyclotron maser wave amplification, which happen for brief intervals to be the only ones that are active. We believe that the more usual phase-incoherent condition (i.e., one in which the instantaneous frequency drifts randomly within the emission band) is due to the fact that the resultant radiation is the sum of two or more components from neighboring wave amplification centers emitting at slightly different frequencies, with independently varying intensities. Possible models for the production of subpulses and phase coherent intervals are discussed.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999JA900342

Microstructure of Jovian decametric S bursts

Aims. The wide-band dynamic spectra of Jovian decameter emission obtained over the last decade with high-frequency and high time resolution equipment on the largest decameter band antenna array, the Ukrainian T-shape Radio telescope (UTR-2), are presented. Methods. We analyzed the data obtained with the Digital SpectroPolarimiter (DSP) and WaveForm Reciever (WFR) installed at UTR-2. The combination of the large antenna and high performance equipment gives the best sensitivity and widest band of analysis, dynamic range, time and frequency resolutions. The wavelet transform method and the Fourier technique was used for further data processing. Results. The main characteristics of already known and newly detected modulation events were investigated and specified. The new receiving-recording facilities, methodology and program of observations are described in detail.

/pdf/modulation-structures-in-the-dynamic-spectra-of-jovian-radio-d4a2z7az0w.pdf

Modulation structures in the dynamic spectra of Jovian radio emission obtained with high time-frequency resolution

Like the comparable AKR radio emissions from earth's magnetosphere, the well-known decametric radio S-bursts from Jupiter, observed in France and Australia at frequencies from 10 to 26 MHz, have been found to exhibit equally spaced discrete spectral components which can be attributed to the adjacent longitudinal oscillation modes of natural radio lasers. Implying sizes of only a few kilometers for the individual radio lasers producing the S-bursts, the frequency spacing of these modes was roughly constant with frequency and about 30 to 50 kHz. Their corresponding temporal spacings, however, varied inversely proportional to the observing frequency, suggesting that the radio lasers producing the S-bursts were expanding uniformly at a rate of about 4 km/s. Presumably caused by the projected motion of Io with respect to the planet, this expansion of the S-burst radio lasers would account for the downward frequency drifts of the S-bursts without the energetic electron bunches which have heretofore always been assumed necessary to account for such behavior.

G. R. A. Ellis

Papers

Natural Radio Lasing at Jupiter