Mohammed Yahia Boudjada

An oblate beaming cone for Io-controlled Jovian decameter emission

We investigate the emission cone of the Io-controlled Jovian decameter radiation from the layout of the sources in the CML-Io phase diagram where the occurrence of the radiation is plotted versus the central meridian longitude (CML) and the phase of the satellite Io. Four zones of enhanced probability are revealed in this diagram and named Io-controlled sources Io-A, Io-B, Io-C and Io-D. We propose an angular representation of the CML-Io phase diagram in a coordinate system linked to the local magnetic field in the radio source and the gradient of the magnetic field. The angular distribution of the sources in such a diagram clearly shows that the radio emission is radiated in a hollow cone which is not axisymmetrical around the magnetic field gradient but flattened in the direction of the magnetic field vector. The use of elliptic coordinates allows us to compute the variable opening angle and the flattening of the cone in both Jovian hemispheres. Finally it comes from our investigation that the flattening of the emission cone allows the theoretical existence of a Jovian active longitude sector much more easily than in the case of an axisymmetrical cone, and the zones of maximum theoretical amplification correspond to the areas of high occurrence probability observed in the CML-Io phase diagram.

Digital techniques for ground-based low-frequency radio astronomy

Far prior to any other wavelength domains, decameter radioastronomy has become a challenging task because of man made interference due to the increasing needs in telecommunications. However most of the astrophysical studies in this frequency band, -- particularly those of non thermal radiations from magnetized objects --, require high gain multi-polarization antennas and the use of high sensitivity, high dynamic range spectroscopy techniques. New generation of wide band spectrum analyzers, well suited to overcome these constraints, are presently developed in Meudon observatory in collaboration with the Space Research Institute (Graz, Austria). They are based on the utilization of newly available, high performance, programmable digital circuits for signal processing, arranged in dedicated, parallel architecture, which can directly compute the power spectrum of the input signal. As an example, we describe here a specialized real-time analyzer for studying polarized decameter emissions from Jupiter and the Sun. With this device, the full spectral analysis (four Stokes parameters) of a 10 MHz bandwidth can be performed at the millisecond time scale, over 1024 channels and within a 70 dB dynamic range. The unprecedented capabilities of this analyzer were evaluated during the last Jovian opposition, by using the Decameter Array in Nancay, France. Some examples are presented, with emphasis on results on the shortest fine structures, which are characteristics of cyclotron maser radiations from planets. Perspectives for extending to shorter wavelengths (namely decimeter or centimeter ones) and for enhancing capabilities by implementing dedicated softwares are discussed. The offered possibility of processing the telescope signal in real time -- in order, for instance, to manage the man made radio interference problem --, indeed appears as a key to maintain acceptable sensitivity in any future, large radio telescope system operating on the ground.

Reply to comment by B. Cecconi on “Spectral features of SKR observed by Cassini/RPWS: Frequency bandwidth, flux density and polarization”

The main purpose of the paper by Galopeau et al.[2007] was to classify the spectral features of the Saturniankilometric radiation (SKR) starting from three physicalobserved parameters: the frequency bandwidth, the fluxdensity, and the pol arization. We show in the presentresponse that an unsupervised application of arbitrary auto-matic criteria during the data processing (such as a signal-to-noise ratio greater than 23 dB) can totally judge a weaknatural emission as a background noise. As a consequence,such a situation may lead to consideration of only the datapresenting a degree of circular polarization close to 100%and neglect a huge part of the data. Galopeau et al. [2007]considered a phenomenological aspect and gave an estima-tion of the Stokes parameters. This approach leads to firstrecognizing spectral components (flux density and band-width) in the frequency range from 3.5 kHz to 1200 kHz,and then deriving the Stokes parameters for each compo-nent. The Cassini/RPWS instrument provides long-lastingcoverage of radio emissions at Saturn with unprecedentedinstrumental capabilities.

Emission cone of radio waves generated by cyclotron maser instability in nonaxisymmetrical inhomogeneous medium

Four zones of enhanced probability appear in the CML-Io phase diagram, where the occurrence of the Jovian radio emissions at decameter wavelength is plotted versus the central meridian longitude (CML) and the orbital phase of Io. These zones are the so-called Io-controlled sources Io-A, Io-B (emitted from Jupiters northern hemisphere), and Io-C, Io-D (emitted from the south). We have plotted the occurrence probability in a polar diagram linked to the local magnetic field, making the assumption that the magnetic field intensity gradient ∇B plays the role of an optical axis for the wave propagation, and introducing an azimuth angle measured relatively to the direction of the magnetic field vector B. The results of our study allow us to conclude that the Io-controlled decameter Jovian radiation is emitted in a hollow cone flattened in a particular direction. The existence of such an emission cone leads us to understand the location of the Io-controlled sources (Io-A, Io-B, Io-C, and Io-D) in the CML-Io phase diagram and to interpret their dependence on the longitude as the manifestation of a Jovian active longitude sector, where the emission mechanism is the most efficient. We study the origin of the flattening of the emission cone in the framework of a radio emission produced by the cyclotron maser instability in an inhomogeneous medium where the local magnetic field B and the gradient of its modulus ∇B are not parallel, i.e., in a geometry without axial symmetry. We consider that the radiation propagates in the source region in the X-mode near its cutoff frequency.

Emission cone of Io-controlled Jovian decameter radiation inferred from occurrence diagram

Four zones of enhanced probability are found in the CML-Io phase diagram, where the occurrence of the Jovian decameter radio emissions is plotted versus the central meridian longitude (CML) and the orbital phase of Io. These zones are the so-called Io-controlled sources Io-A, Io-B (emitted from the northern hemisphere), and Io-C, Io-D (emitted from the south). In a recent work, we have studied the occurrence probability in a polar diagram linked to the local magnetic field, making the assumption that the magnetic field intensity gradient plays the role of an optical axis for the wave propagation. For a given Jovian magnetic field model, the four sources Io-A, Io-B, Io-C and Io-D are plotted as a function of the colatitude angle θ relative to the gradient of the magnetic field (radial coordinate) and an azimuth angle ψ linked to the direction of magnetic field vector. Our previous results revealed that the angle θ is not constant and that the Jovian decameter emission controlled by Io is radiated in a hollow cone which is not axi-symmetrical around the magnetic field gradient but flattened in the direction of the magnetic field vector. The relative directions of the magnetic field and its gradient within the radio source seem to play a crucial role in the angular distribution of the occurrence probability. Thus we analyze the effect of the choice of the magnetic field model (in particular the O6, VIP4, VIT4 and VIPAL models) on this distribution and the consequences for the emission cone. The use of elliptic coordinates in a frame linked to the local magnetic field is very relevant for such a study.

Observations of electromagnetic emissions inside the Earth’s plasmasphere from the Interball-1 satellite

The spectrum analyzer AKR-X onboard the Interball-1 satellite at the beginning (August–October 1995) and at the end (August–October 2000) of satellite operation in perigees of its orbital motion recorded and analyzed electromagnetic emissions of the inner regions of the Earth’s plasmasphere in the frequency band 100–1500 kHz at distances of 1.1–1.8 RE. The observations have shown that the electromagnetic modes (the Z and LO modes escaping the magnetosphere) which are formed at the altitudes 600–4000 km are associated with the subauroral nonthermal continuum and with the recently discovered kilometric continuum. There are noticeable differences in the spectral character of these emissions during the minimum (1996) and maximum (2000) solar activity, when, as a rule, the LO mode escaping the plasmaphere and the continua are not present.