# Concept of an Experimental Ultra Long-wavelength Radio Array

## TL;DR

This paper proposes an experimental ultra-long wavelength radio array in Inner Mongolia, designed to explore celestial radio emissions below 72 MHz, leveraging upcoming low-ionosphere activity periods and existing infrastructure.

## Contribution

It introduces a novel ground-based ULW radio array concept utilizing existing infrastructure, aiming to explore the largely uncharted ULW spectrum in radio astronomy.

## Key findings

- Design of a 1-72 MHz ULW radio array in Inner Mongolia
- Potential to study celestial emissions in a largely unexplored spectrum
- Utilization of upcoming low-ionosphere activity periods for observations

## Abstract

The Ultra-Long 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 (RFI). An obvious solution of these problems is to place an ULW radio telescope in space. However, this solution is expensive and poses non-negligible 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 Miangtu Spectra Radio Heliograph (MUSER). 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.