Concept of an experimental ultralong-wavelength radio array

The Ultralong-Wavelength (ULW) regime of longer than 10 m (corresponding to frequencies below 30 MHz) remains as the last virtually unexplored window in radio astronomy and is presently attracting considerable attention as an area of potentially rewarding studies. However, the opaqueness of the Earth's ionosphere makes the ULW celestial radio emission very difficult to detect with ground-based instrumentation. The impact of the ionosphere on ULW radio emission depends on the solar cycle activities and varies with time. In addition, the ULW spectrum region is densely populated by intensive artificial radio frequency interference. An obvious solution of these problems is to place an ULW radio telescope in space. However, this solution is expensive and poses nonnegligible technological challenges. An alternative approach is triggered by recent studies showing that the period of post 2020 will be most suitable for exploratory ground-based ULW radio observations due to the expected "calm" state of the ionosphere; the ionospheric cutoff frequency could be well below 10 MHz, even in the day time. In anticipation of this upcoming opportunity, we propose and present in this paper a concept of an experimental ULW radio array, with the intention of setting it up in Inner Mongolia, China. This ULW facility will use the infrastructure of the currently operational Mingantu spectral radio heliograph. The proposed ULW array covers the frequency range from 1 to 72 MHz. This experimental array will be used for exploratory studies of celestial radio emission in the ULW range of the spectrum.