The Frequency Agile Solar Radiotelescope (FASR) will observe the Sun over a wide range of radio frequencies and make high spatial resolution images at many frequencies nearly simultaneously. FASR will need to be able to observe both the very bright, usually compact emission from solar flares as well as much fainter fluctuations in the solar chromosphere across a broad range of spatial scales (from 1 arcsec to 1 degree) at high time resolution, and these constraints impose severe requirements on telescope design. We discuss the problem of imaging the Sun at radio wavelengths and present simulations of imaging the thermal free-free emission from the Sun's atmosphere using models based on EUV data.
The Low Frequency Array (LOFAR) will be a radio astronomy interferometric array operating in the approximate frequency range 10-240 MHz. It will have a large collecting area achieved using active dipole techniques, and will have maximum baselines of up to 500 km to attain excellent spatial resolution at long wavelengths. The Sun will always be in LOFAR's beam during daylight hours, and particularly during periods of high solar activity the Sun will be a prominent (and highly variable) feature of the low-frequency sky. A diverse range of low-frequency emissions is generated by the Sun that carry information about processes taking place in the Sun's atmosphere. Study of these emissions with LOFAR will make possible major advances in our understanding of particle acceleration and shocks in the solar atmosphere, and of coronal mass ejections and their impact on the Earth. In this paper we summarize LOFAR's capabilities and discuss
the solar science that LOFAR will address.
For many years, ground-based radio observations of the Sun have proceeded into two directions: (1) high resolution imaging at a few discrete wavelengths; (2) spectroscopy with limited or no spatial resolution at centimeter, decimeter, and meter wavelengths. Full exploitation of the radio spectrum to measure coronal magnetic fields in both quiescent active regions and flares, to probe the thermal structure of the solar atmosphere, and to study energy release and particle energization in transient events, requires a solar-dedicated, frequency-agile solar radiotelescope, capable of high-time, - spatial, and -spectral resolution imaging spectroscopy. In this paper we summarize the science program and instrument requirements for such a telescope, and present a strawman interferometric array composed of many (greater than 40), small (2 m) antenna elements, each equipped with a frequency- agile receiver operating over the range 1 - 26.5 GHz.