Results of the last observations of solar sporadic radio emission at the UTR-2 radio telescope (Kharkov, Ukraine) at the frequencies 10 - 30 MHz are presented. The use of new backend facilities, the DSP and 60-channel spectrometer, allows us to obtain data with time resolution up to 2 ms and frequency resolution of 12 kHz in the continuous frequency band 12 MHz. Usual Type III bursts, Type IIIb bursts, U- and J-bursts in the decameter range are discussed. Special attention is paid to detection and analysis of Type II bursts and their properties, newly discovered fine time structures of Type III bursts, Type III-like bursts, s-bursts, new observational features of drift pair bursts, and ‘absorption’ bursts.

Radio astronomy in the decameter to centimeter wavelength range is facing new challenges because of man made interferences due to increasing needs in telecommunications. At the Radioastronomy department of Paris Meudon Observatory, we have been working since four years on high dynamic range digital receivers based on Digital Signal Processors (DSP). The first achievement is a digital spectro- polarimeter devoted to spectroscopy of astrophysical radiation in decameter range, now in operation at the Nancay Decameter array. The block diagram of the receiver includes a high dynamic range analogue section followed by a 12 bits analogue to digital converter. The digital part makes use of high power, programmable digital circuits for signal processing, arranged in a dedicated parallel architecture, able to compute in real time the power spectrum and the correlation of the input signals. This receiver was also used, as spectrometer backend, at Nancay decimetric radiotelescope and has performed very well in the presence of very strong interferences. We are presently working on a new digital receiver with broader bandwidth. The objective is 2 × 25 MHz band with at least 60 dB dynamic range. This new receiver will use additional computation power in order to recognise and avoid man made interferences which corrupt the radio astronomical signal. At the Nancay Radioastronomy Observatory, we have started to develop a new digital configurable receiver with 8 times 25 MHz band and ten thousand channels. For low frequency radioastronomy, direct spectrum computation technique is really powerful and offers new capabilities for real time interferences excision. Fig. 1 shows pulsar observations in the presence of interference made with the DSP receiver on the UTR-2 radiotelescope. Fig. 2 shows the effect of satellite interfernce on OH observations made with the Nancay telescope. Fig. 3 shows the block diagram of the DSP system and demonstrates how offline excision of interference in the frequency time-domain enables recovery of the signal. The final spectrum had 960 minutes integration on and off source and took 8045 minutes of procession on a 450 MHz Pentium II.

AD Leonis is a very active, single dMe flare star. The similarities between this type of star and the Sun has led to study their radio radiation, which originates from their corona. The high brightness temperatures and other characteristics of most dMe radio bursts can be attributed to a non-thermal, coherent mechanism: plasma radiation or a cyclotron maser instability (CMI) are both plausible explanations. Even for the strongest burst of AD Leo which reached 940 mJy at 21 cm, it was not possible to discriminate between these two mechanisms (Bastian et al. 1990).

Here we present an intense burst from AD Leo, exhibiting strong spikes for which the CMI seems to be the only reasonable explanation. In Sect. 2 we describe the observations, and in Sect. 3 we give an interpretation for this event.

AE Aquarii is a magnetic cataclysmic variable containing a white dwarf and a K3-K7 star which lies slightly above the main sequence. The white dwarf is the most rapidly rotating known (Prot ≃ 33.08 s, Patterson 1979), and it is the most strongly asynchronous with its revolution (Porb = 9.88 hr). The white dwarf accretes matter from the K star, which approximately fills the Roche lobe. AE Aqr exhibits flares in the soft X-rays, the ultra-violet, and almost continuously in the visible and the radio regimes. Rapid optical and TeV γ-ray bursts have also been discovered, which are modulated with the period of the white dwarf and at half of this period (de Jager & Meintjes 1993). This modulation, also found in X-rays, is interpreted as the accretion of matter onto the white dwarf’s magnetic poles. The strength of the white dwarf’s magnetic field is not well-determined, it is estimated to be ∼ 6.104 - 105 G (Lamb & Patterson 1983, Cropper 1986) at the white dwarf’s surface. Eracleous et al. (1994) recently suggested that the magnetic dipole axis lies close to the equatorial plane (∼ 20°). De Jager et al. (1994) discovered a rapid spin down of the white dwarf leading to a spin down power which exceeds the accretion power. They suggest that a significant fraction of the spin down power may be converted to the acceleration of particles, which may explain the radio and the γ-ray emissions. Both the characteristics of the optical flares and the existence of TeV γ-rays suggest a relation with the non-thermal radio flares.