Low Frequency Observations of the Moon with the Murchison Widefield Array

TL;DR

This study explores the use of the Moon as a calibration source for low frequency radio observations related to the Epoch of Reionization, revealing challenges due to Earth's reflected radio emissions.

Contribution

It provides initial observational data of the Moon at low frequencies with the Murchison Widefield Array, highlighting Earth's reflected radio emissions and their impact on EoR experiments.

Findings

The Moon's spectrum is affected by Earth's radio reflections, especially in the FM band.

Reflected Earth's radio emission can be estimated and has implications for EoR detection.

Earth's radio power in the FM band can be quantified from lunar observations.

Abstract

A new generation of low frequency radio telescopes is seeking to observe the redshifted 21 cm signal from the Epoch of Reionization (EoR), requiring innovative methods of calibration and imaging to overcome the difficulties of widefield low frequency radio interferometry. Precise calibration will be required to separate the small expected EoR signal from the strong foreground emission at the frequencies of interest between 80 and 300 MHz. The Moon may be useful as a calibration source for detection of the EoR signature, as it should have a smooth and predictable thermal spectrum across the frequency band of interest. Initial observations of the Moon with the Murchison Widefield Array 32 tile prototype show that the Moon does exhibit a similar trend to that expected for a cool thermally emitting body in the observed frequency range, but that the spectrum is corrupted by reflected radio emission from Earth. In particular, there is an abrupt increase in the observed flux density of the Moon within the internationally recognised Frequency Modulated (FM) radio band. The observations have implications for future low frequency surveys and EoR detection experiments that will need to take this reflected emission from the Moon into account. The results also allow us to estimate the equivalent isotropic power emitted by the Earth in the FM band and to determine how bright the Earth might appear at metre wavelengths to an observer beyond our own solar system.