2025ApJ...994..226A

The ultralong wavelength sky (ν ≲ 30 MHz) is still largely unexplored, as the electromagnetic wave is heavily absorbed and distorted by the ionosphere on Earth. The farside of the Moon, either in lunar orbit or on lunar surface, is the ideal site for observations in this band, and the upcoming Moon-based interferometers will obtain multifrequency high-resolution sky maps. Making use of the lunar occultation of the sky and the anisotropy of antenna primary beam response, we propose a novel method to reconstruct the ultralong wavelength spectral shape in multiple directions in the sky using only one antenna on lunar orbit. We apply the method to one antenna on one of the nine daughter satellites of the proposed Discovering the Sky at Longest wavelength project. Using simulated observation data between 1 and 30 MHz from one dipole antenna, we find that the spectra for different regions on the sky can be reconstructed very well, and the free-free absorption feature in each region can be derived from the reconstructed spectra. This work demonstrates the feasibility of reconstructing the unbiased anisotropic spectra using very limited instrumentation on a lunar orbit, with mature technologies already in place. It extends the application of such kind of satellite in revealing the distribution of free electrons in the Galactic interstellar medium from the distribution of absorption features in the ultralong wavelength sky.