Using large radio telescopes at decametre wavelengths

Details Using large radio telescopes at decametre wavelengths Using large radio telescopes at decametre wavelengths

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja....13264l&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #13264

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With the aim of evaluating the actual possibilities of doing, from the ground, sensitive radio astronomy at decametre wavelengths (particularly below 40 MHz), 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 Decameter 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 nanoseconds, 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, 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 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 decrease the effects of artificial and natural radio interference. We give several examples of successful signal detection in case of much weaker radio sources than Solar System ones, down to the ˜1 Jy intensity level. In summary, we conclude that searching for sensitivity improvement at decametre wavelength is justified, and is now technically feasible, in particular by building giant, phased antenna arrays of much larger collecting area (as in the LOFAR project). One must also take into account some specifics of this wavelength range - but somewhat unusual in "classical" radio astronomy -, i.e. a very high level and density of radio interference (telecommunications) and the variable ionosphere. Some applications to Solar System radio astronomy are briefly outlined.

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