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International audience; The properties of powerful (flux >10-19 W m-2 Hz-1) type III bursts observed in July - August 2002 by the radio telescope UTR-2 at frequencies 10 - 30 MHz are analyzed. Most bursts have been registered when the active regions associated to these bursts were located near the central meridian or at 40° - 60° to the East or West from it. All powerful type III bursts drift from high to low frequencies with frequency drift rates 1 - 2.5 MHz s-1. It is important to emphasize that according to our observations the drift rate is linearly increasing with frequency. The duration of the bursts changes mainly from 6 s at frequency 30 MHz up to 12 s at 10 MHz. The instantaneous frequency bandwidth does not depend on the day of observations, i.e. on the disk location of the source active region, and is increasing with frequency. The results of observational properties are discussed in the frame of the standard plasma model of type III bursts radio emission.

International audience; Aims: Decameter radio observations of the solar corona reveal the presence of numerous faint frequency drifting emissions, similar to ``solar S bursts'' which are reported in the literature. We present a statistical analysis of the characteristics of these emissions and propose a mechanism to excite the Langmuir waves thought to be at the origin of these emissions. Methods: The observations were performed between 1998 and 2002 with the Digital Spectro Polarimeter (DSP) receivers operated at the UTR-2 and Nançay decameter radio telescopes in the frequency range 15-30 MHz. Our theoretical explanation is based on Vlasov-Ampère simulations. Results: Based on the frequency drift rate, three populations of structures can be identified. The largest population presents an average negative frequency drift of -0.9 MHz s-1 and a lifetime up to 11 s (median value of 2.72 s). A second population shows a very small frequency drift of -0.1 MHz s-1 and a short lifetime of about 1 s. The third population presents an average positive frequency drift of +0.95 MHz s-1 and a lifetime of up to 3 s. Also, the frequency drift as a function of frequency is consistent with the former results, which present results in higher frequency range. No specific relationship was found between the occurrence of these emissions and the solar cycle or presence of flares. Assuming that these emissions are produced by ``electron clouds'' propagating the solar corona, we deduce electron velocities of about 3-5 times the electron thermal velocity. As previously shown, a localized, time-dependent modulation of the electron distribution function (heating) leads to low velocity electron clouds (consistent with observations), which, in turn, can generate Langmuir waves and electromagnetic signals by nonlinear processes.

International audience; Here we report a radio burst in absorption at 9 30 MHz observed with the UTR-2 telescope. This event occurred on 19 August 2003 about 11:16 11:26 UT, against solar type IV/II emission background. It is the first event where absorption was observed below 30 MHz. The absorption region, comparable with the solar radius size, traveled a long distance into the upper corona from the Sun. We show that the burst minimum corresponds to the almost full absorption of the solar radio emission up to a background level of the quiescent Sun. This supports the interpretation of the phenomenon as an absorption. The result is examined independently with the Nançay Decameter Array measurements and the Wind WAVES instrument records.

Decameter wavelength radio emission is finely structured in solar bursts. For their research it is very important to use a sufficient sensitivity of antenna systems. In this paper we study an influence of the radiotelescope-antenna effective area on the results of decameter solar radio observations. For this purpose we compared the solar bursts received by the array of 720 ground-based dipoles and the single dipole of the radiotelescope UTR-2. It's shown that a larger effective area of the ground-based antenna allows us to measure a weaker solar emission and to distinguish a fine structure of strong solar events. This feature has been also verified by simultaneous ground- and space-based observations in the overlapping frequency range., Comment: 7 pages, 3 figures

International audience; We review radio detection of planetary lightning performed by Voyager, Galileo (including in-situ probe measurements), Cassini, and other spacecraft, and compare the information on the underlying physics derived from these observations---especially the discharge duration, at Jupiter and Saturn---with our knowledge of terrestrial lightning. The controversial evidence at Venus is discussed, as well as the prospects for lightning-like discharges in Martian dust-storms (and studies on terrestrial analogues). In addition, lightning sources provide radio beacons that allow us to probe planetary ionospheres. Ground-based observations of Saturn's lightning have been attempted several times in the past and have been recently successful. They will be the subject of observations by the new generation of giant radio arrays. We review past results and future studies, focussing on the detection challenges and on the interest of ground-based radio monitoring, in conjunction with spacecraft observations or in standalone mode.

Starting from 1960s Type III-like bursts (Type III bursts with high drift rates) in a wide frequency range from 300 to 950MHz have been observed. These new bursts observed at certain frequency being compared to the usual Type III bursts at the same frequency show similar behaviour but feature frequency drift 2-6 times higher than the normal bursts. In this paper we report the first observations of Type III-like bursts in decameter range, carried out during summer campaigns 2002 - 2004 at UTR-2 radio telescope. The circular polarization of the bursts was measured by the radio telescope URAN-2 in 2004. The observed bursts are analyzed and compared with usual Type III bursts in the decameter range. From the analysis of over 1100 Type III-like bursts, their main parameters have been found. Characteristic feature of the observed bursts is similar to Type III-like bursts at other frequencies, i.e. measured drift rates (5-10 MHz/s) of this bursts are few times larger than that for usual Type III bursts, and their durations (1-2 s) are few times smaller than that for usual Type III bursts in this frequency band., Published in Proceedings of the 14th Young Scientists Conference on Astronomy and Space Physics, Kyiv, Ukraine, April 23-28, 2007

International audience; We analyze the records of Jupiter's decameter radio emissions obtained during an Io-A S-burst storm on 15 March 2005. The observations were performed at the world's largest decameter array, UTR-2, which is equipped with a digital receiver capable of catching waveforms of duration ~3 s with temporal resolution defined by the sampling rate of ~66 MHz. A Hilbert transform based algorithm has been applied to study narrow-band spectral patterns demonstrating quasi-linear drift over time-frequency plane. The instantaneous amplitude and phase information has been extracted from the recorded waveforms with the purpose of analyzing microsecond-scale coherent events in the S-burst emission. A statistical model of narrow band random process is proposed for describing such features in the observed waveforms as coherent segments, phase jumps, nonlinear frequency drift, etc. It is shown that the study of coherence properties in terms of instantaneous phase is equivalent to Fourier analysis of a narrowband signal. This implies that no particular mechanism (such as superimposed modulation or oscillation) is required for generating the observed coherent phase structures of S-burst emission: those, as well as the pulse-like envelope structures, emerge naturally at the output of a narrow band filter applied to a random noise. It is further suggested that probability distribution function of instantaneous amplitude gives an important insight into the underlying physical mechanism of S-burst generation. In particular, it is demonstrated that models based on the concept of ``generator,'' i.e., a nonlinear system with feedback, are less suitable for reproducing the observational characteristics of S-bursts at microsecond time scale resolution. On the other hand, the concept of ``amplifier,'' i.e., a linear system (without feedback) that enhances the fluctuations within a narrow band, fits the observational data well. This conclusion is consistent with S-burst generation mechanism via cyclotron-maser instability, which is indeed a resonant wave amplification process.

International audience; We present the results of radio telescope UTR-2 observations of solar Type II radio bursts in the 10-30 MHz frequency range. These events possess a fine structure consisting of fast drift sub-bursts similar to Type III bursts. The frequency drift rate of the Type II bursts at decameter wavelengths is smaller than 0.1 MHz s-1. One of these bursts with herringbone structure has a wave-like backbone that almost does not drift. The features of the observed bursts are discussed.

International audience; The Low Frequency Array (LOFAR) will come on line with unprecedented radio sensitivity and resolution between 10 and 240 MHz. Such a system will provide a factor of 10-30 improvement in sensitivity in the pursuit of the weak radio emission from extrasolar planets. To date, previous examinations of extrasolar planetary systems with the most advanced radio telescopes have yielded a negative result. However, the improvement in sensitivity by LOFAR over current systems will increase the likelihood of extrasolar planet detection in the radio. We apply radiometric models derived previously from the study of planets in our solar system to the known extrasolar planets, and demonstrate that approximately 3-5 of them should emit in the proper frequency range and with enough power to possibly become detectable at Earth with LOFAR.

International audience; We develop a statistical approach aimed at the detection of weak sporadic pulses on the noise background. The results are applied to modeling the observational time series where pulsed radio emissions have to be recognized against the sky background fluctuations. The proposed methodology demonstrates the efficiency of using the statistics of peak values (integrated tail of probability distribution function of the intensity) for the purpose of signal detection. It is established that the highest sensitivity is reached with this method at low values of the filling factor (duty cycle) of the pulsed signals. If in addition the pulses have sufficiently high intensity, the discussed approach performs better than simple integration over the observational time. Then we discuss the possibility of detecting radio pulses from exoplanetary magnetospheres, especially from known "hot Jupiters" found by radial velocity measurements in the visible and we report our results from extensive observations of several candidate exoplanets with the world largest decameter telescope UTR-2. Although no detection of pulsed emission from exoplanets has been found to date, the analysis demonstrates the feasibility of detection with more stable receivers and longer observational time.

International audience; With the aim of evaluating the actual possibilities of doing, from the ground, sensitive radio astronomy at decametre wavelengths (particularly below ˜50MHz), an extensive program of radio observations was carried out, in 1999-2002, by using digital spectral and waveform analysers (DSP) of new generation, connected to several of the largest, decametre radio telescopes in the world (i.e., the UTR-2 and URANs arrays in Ukraine, and the Nançay Decametre Array in France). We report and briefly discuss some new findings, dealing with decametre radiation from Jupiter and the Solar Corona: namely the discovery of new kinds of hyper fine structures in spectrograms of the active Sun, and a new characterisation of Jupiter's "millisecond" radiation, whose waveform samples, with time resolution down to 40 ns, and correlated measurements, by using far distant antennas (3000 km), have been obtained. In addition, scattering effects, caused by the terrestrial ionosphere and the interplanetary medium, could be disentangled through high time resolution and wide-band analyses of solar, planetary and strong galactic radio sources. Consequences for decametre wavelength imaging at high spatial resolution (VLBI) are outlined. Furthermore, in spite of the very unfavourable electromagnetic environment in this frequency range, a substantial increase in the quality of the observations was shown to be provided by using new generation spectrometers, based on sophisticated digital techniques. Indeed, the available, high dynamic range of such devices greatly decreases the effects of artificial and natural radio interference. We give several examples of successful signal detection in the case of much weaker radio sources than Solar System ones, down to the ˜1Jy intensity level. In summary, we conclude that searching for sensitivity improvement at the decametre wavelength is scientifically quite justified, and is now technically feasible, in particular by building giant, phased antenna arrays of much larger collecting area (as in the LOFAR project). In this task, one must be careful of some specifics of this wavelength range - somewhat unusual in "classical" radio astronomy - i.e., very high level and density of radio interference (telecommunications) and the variable terrestrial ionosphere.