Lunar occultation of the diffuse radio sky: LOFAR measurements between 35 and 80 MHz

TL;DR

This paper introduces a novel interferometric technique to measure large-scale diffuse radio emission using lunar occultation at 35-80 MHz, providing insights into the Galactic background and RFI impacts for Cosmic Dawn experiments.

Contribution

The study demonstrates for the first time how lunar occultation can be used to measure the diffuse radio sky and characterize RFI at low frequencies.

Findings

Lunar occultation appears as a negative flux source at 35-80 MHz.

Reflected RFI is concentrated at the lunar center and measurable independently.

Moon-based Cosmic Dawn experiments require >80 dB Earth-isolation to reduce RFI below 1 mK.

Abstract

We present radio observations of the Moon between $35$ and $80$ MHz to demonstrate a novel technique of interferometrically measuring large-scale diffuse emission extending far beyond the primary beam (global signal) for the first time. In particular, we show that (i) the Moon appears as a negative-flux source at frequencies $35<\nu<80$ MHz since it is `colder' than the diffuse Galactic background it occults, (ii) using the (negative) flux of the lunar disc, we can reconstruct the spectrum of the diffuse Galactic emission with the lunar thermal emission as a reference, and (iii) that reflected RFI (radio-frequency interference) is concentrated at the center of the lunar disc due to specular nature of reflection, and can be independently measured. Our RFI measurements show that (i) Moon-based Cosmic Dawn experiments must design for an Earth-isolation of better than $80$ dB to achieve an RFI temperature $<1$ mK, (ii) Moon-reflected RFI contributes to a dipole temperature less than $20$ mK for Earth-based Cosmic Dawn experiments, (iii) man-made satellite-reflected RFI temperature exceeds $20$ mK if the aggregate cross section of visible satellites exceeds $80$ m$^2$ at $800$ km height, or $5$ m$^2$ at $400$ km height. Currently, our diffuse background spectrum is limited by sidelobe confusion on short baselines (10-15% level). Further refinement of our technique may yield constraints on the redshifted global $21$-cm signal from Cosmic Dawn ($40>z>12$) and the Epoch of Reionization ($12>z>5$).