A roadmap towards a space-based radio telescope for ultra-low frequency radio astronomy

Solar radio observations provide a unique diagnostic of the outer solar atmosphere. However, the inhomogeneous turbulent corona strongly affects the propagation of the emitted radio waves, so decoupling the intrinsic properties of the emitting source from the effects of radio-wave propagation has long been a major challenge in solar physics. Here we report quantitative spatial and frequency characterization of solar radio burst fine structures observed with the LOw Frequency Array (LOFAR), an instrument with high time resolution that also permits imaging at scales much shorter than those corresponding to radio-wave propagation in the corona. The observations demonstrate that radio-wave propagation effects, and not the properties of the intrinsic emission source, dominate the observed spatial characteristics of radio burst images. These results permit more accurate estimates of source brightness temperatures, and open opportunities for quantitative study of the mechanisms that create the turbulent coronal medium through which the emitted radiation propagates.

/pdf/imaging-spectroscopy-of-solar-radio-burst-fine-structures-2697ao5p96.pdf

Imaging Spectroscopy of Solar Radio Burst Fine Structures

The Basics of Interferometry

Lightning was detected by Voyager 2 at Uranus and Neptune, and weaker electrical processes also occur throughout planetary atmospheres from galactic cosmic ray (GCR) ionisation. Lightning is an indicator of convection, whereas electrical processes away from storms modulate cloud formation and chemistry, particularly if there is little insolation to drive other mechanisms. The ice giants appear to be unique in the Solar System in that they are distant enough from the Sun for GCR-related mechanisms to be significant for clouds and climate, yet also convective enough for lightning to occur. This paper reviews observations (both from Voyager 2 and Earth), data analysis and modelling, and considers options for future missions. Radio, energetic particle and magnetic instruments are recommended for future orbiters, and Huygens-like atmospheric electricity sensors for in situ observations. Uranian lightning is also expected to be detectable from terrestrial radio telescopes.

/pdf/atmospheric-electricity-at-the-ice-giants-vdrbdadp7u.pdf

Atmospheric Electricity at the Ice Giants.

The Nancay Decameter Array (NDA) routinely observes low frequency (10-100 MHz) radio emissions of Jupiter and the Sun since 4 decades. The NDA observations, acquired with a variety of receivers with increasing performances, were the basis for numerous studies of jovian and solar radio emissions and now form a unique long-term database spanning >3 solar cycles and jovian revolutions. In addition, the NDA historically brought a fruitful support to space-based radio observatories of the heliosphere, to multi-wavelength analyses of solar activity and contributes to the development of space weather services. After having summarized the NDA characteristics, this article presents latest instrumental and database developments, some recent scientific results and perspectives for the next decade.

/pdf/1977-2017-40-years-of-decametric-observations-of-jupiter-and-29wp5nh11k.pdf

1977-2017: 40 years of decametric observations of Jupiter and the Sun with the Nancay Decameter Array

The current status of the large decameter radio telescope UTR-2 (Ukrainian T-shaped Radio telescope) together with its VLBI system called URAN is described in detail. By modernization of these instruments through implementation of novel versatile analog and digital devices as well as new observation techniques, the observational capabilities of UTR-2 have been substantially enhanced. The total effective area of UTR-2 and URAN arrays reaches 200 000 m2, with 24 MHz observational bandwidth (within the 8–32 MHz frequency range), spectral and temporal resolutions down to 4 kHz and 0.5 msec in dynamic spectrum mode or virtually unlimited in waveform mode. Depending on the spectral and temporal resolutions and confusion effects, the sensitivity of UTR-2 varies from a few Jy to a few mJy, and the angular resolution ranges from ~ 30 arcminutes (with a single antenna array) to a few arcseconds (in VLBI mode). In the framework of national and international research projects conducted in recent years, many new results on Solar system objects, the Galaxy and Metagalaxy have been obtained. In order to extend the observation frequency range to 8–80 MHz and enlarge the dimensions of the UTR-2 array, a new instrument – GURT (Giant Ukrainian Radio Telescope) – is now under construction. The radio telescope systems described herein can be used in synergy with other existing low-frequency arrays such as LOFAR, LWA, NenuFAR, as well as provide ground-based support for space-based instruments.

The modern radio astronomy network in Ukraine: UTR-2, URAN and GURT

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The modern radio astronomy network in Ukraine: UTR-2, URAN and GURT