[en] From a detailed analysis of the intensity distribution of Saturn electrostatic discharges (SED) as a function of time during the Voyager 1 encounter with Saturn, the total beaming pattern of the SED source, which is found to be isotropic, is determined. This result allows for the derivation of the diurnal variations of the peak electron density over the dayside equatorial ionosphere of Saturn, and thus explains the puzzling features observed during the Voyager 1 encounter with Saturn - the longer extent of the SED visibility afer closest approach and the existence of double-humped SED episodes before the encounter - by ionospheric absorption of the SED radio emission. 18 references

[en] A theoretical model is developed here in order to determine an envelope for the average spectrum of the Saturnian kilometric radiation (SKR). The microscopic generation mechanism is supposed to be the so-called synchrotron (or cyclotron) maser instability. As in recent works on the terrestrial kilometric radiation, the effect of the magnetic field inhomogeneity on the generation process must be taken into account. Then, assuming that the emission is nonlinearly saturated by trapping, the calculation allows the authors to put an upper limit on the SKR spectral intensity very simply: the maximum level of the wave electric field within the source region is indeed linked to a few macroscopic plasma parameters, which can be derived from the observations (the structure of the magnetospheric magnetic field, the cold plasma density, and the characteristic energy of the hot emitting electrons). They have used a dipolar magnetic field model, while the plasma distribution results from the superposition of two components, an ionospheric population and a plasma disc, whose scale heights have been roughly determined from Voyager measurements. The energetic electrons responsible for the emission are supposed to precipitate along the high-latitude magnetic field lines where SKR emission is known to take place. The width of the source region can be self-consistently estimated from the model. A very good agreement is obtained between the theoretical spectrum and the observational radio data. The calculated spectral intensities exceed the most intense observed intensities by up to 1 order of magnitude, suggesting that the SKR emission is only marginally saturated by nonlinear processes

[en] The paper deals with the long-term fluctuations of the jovian emission at hectometre and kilometre wavelengths. Observations from the Planetary Radio Astronomy (PRA) experiment aboard the two Voyager space-craft are used. The emissions are shown to be strongly affected by the magnetic sector structure at Jupiter. The position of the sources of emission in the jovian magnetosphere is discussed. (author)

[en] We review the basic principles and recent or planned applications of passive, radio frequency electric antennas for in situ measurements of dusty plasmas in space. Electric antennas as passive wave detectors are reliable and versatile tools for such measurements, with the technique of Quasi Thermal Noise Spectroscopy and its generalization to dusty plasmas. The technique has been applied in the interplanetary medium, cometary plasma and dust tails, plasma environments of the Earth, Venus, Jupiter (including the Io plasma torus), Saturn (including the plasma torus and the E ring), with antennas of various shape aboard a number of spacecraft, including, most recently, Cassini. The technique is in the course of use on STEREO (NASA) in interplanetary magnetic clouds, is selected on MMO/Bepi-Colombo (JAXA-ESA) for in situ plasma measurements at Mercury, and planned on the Solar Orbiter (ESA) and other spacecraft projects. The diagnostic is based on the spectral analysis of the electric potential induced by the plasma particles as they pass by the antennas, and/or impact them or the spacecraft. The technique has a great advantage over usual electron detectors: its cross section for detection is much larger than the surface of the detector itself, ensuring a great sensitivity and a quasi-immunity to spacecraft perturbations. The spectral density induced by the passage of plasmas particles - with their dressing popularly known as Debye shielding, Langmuir waves, Bernstein waves and other members of the plasma menagerie - is easily calculated under stable conditions from the theory of the plasma quasi-thermal fluctuations. Around the plasma frequency, the spectrum is dominated by the QTN while at lower frequencies the spectrum is dominated by the shot noise produced by particles impacts on the antenna (in dipole mode) or on the spacecraft (in monopole mode). In this last case, the shot noise can be strongly enhanced by the impacts (and subsequent volatilization/ionization/recollection) of dust grains on the spacecraft, and a refined spectroscopy may provide both plasma and dust measurements.

[en] The habitability of an exoplanet depends on many factors. One such factor is the impact of stellar eruptive events on nearby exoplanets. Currently this is poorly constrained due to heavy reliance on solar scaling relationships and a lack of experimental evidence. Potential impacts of coronal mass ejections (CMEs), which are the large eruption of magnetic field and plasma from a star, are space weather and atmospheric stripping. A method for observing CMEs as they travel though the stellar atmosphere is the type II radio burst, and the new Low Frequency Array (LOFAR) provides a means of detection. We report on 15 hr of observation of YZ Canis Minoris (YZ CMi), a nearby M dwarf flare star, taken in LOFAR’s beam-formed observation mode for the purposes of measuring transient frequency-dependent low-frequency radio emission. The observations utilized the Low Band Antenna (10–90 MHz) or High Band Antenna (110–190 MHz) for five three-hour observation periods. In this data set, there were no confirmed type II events in this frequency range. We explore the range of parameter space for type II bursts constrained by our observations. Assuming the rate of shocks is a lower limit to the rate at which CMEs occur, no detections in a total of 15 hr of observation places a limit of ${\mathrm{\nu <!--\; ??\; -->}}_{\mathrm{t}\mathrm{y}\mathrm{p}\mathrm{e}\mathrm{I}\mathrm{I}}<<!--\; <\; -->0.0667$ shocks/hr ≤ ν _CME for YZ CMi due to the stochastic nature of the events and the limits of observational sensitivity. We propose a methodology to interpret jointly observed flares and CMEs which will provide greater constraints to CMEs and test the applicability of solar scaling relations.

[en] Magnetospheric processes seen in gas giants such as aurorae and circularly polarized cyclotron maser radio emission have been detected from some brown dwarfs. However, previous radio observations targeted known brown dwarfs discovered via their infrared emission. Here we report the discovery of BDR J1750+3809, a circularly polarized radio source detected around 144 MHz with the Low-Frequency Array (LOFAR) telescope. Follow-up near-infrared photometry and spectroscopy show that BDR J1750+3809 is a cold methane dwarf of spectral type T6.5 ± 1 at a distance of ${65}_{-<!--\; ???\; -->8}^{+9}\mathrm{p}\mathrm{c}$. The quasi-quiescent radio spectral luminosity of BDR J1750+3809 is ≈5 × 10¹⁵ erg s⁻¹ Hz⁻¹, which is over two orders of magnitude larger than that of the known population of comparable spectral type. This could be due to a preferential geometric alignment or an electrodynamic interaction with a close companion. In addition, as the emission is expected to occur close to the electron gyrofrequency, the magnetic field strength at the emitter site in BDR J1750+3809 is B ≳ 25 G, which is comparable to planetary-scale magnetic fields. Our discovery suggests that low-frequency radio surveys can be employed to discover substellar objects that are too cold to be detected in infrared surveys.