All-Sky Imaging with Vector Sensor Interferometry

Abstract

Radio astronomy observations at frequencies below 10~MHz could provide valuable science, such as measuring the cosmic dark age signal in the redshifted 21~cm hydrogen absorption line, detecting exoplanetary auroral emissions which lead to inferences about magnetic fields and atmospheres, and characterizing the effects of solar wind and coronal mass ejections on the magnetospheres of solar system planets. Despite their value, few resolved measurements in the sub-10~MHz band have been made. At frequencies below 10~MHz, the Earth's ionosphere reflects, attenuates, and distorts radio waves, making radio astronomy in this band possible only from space. However, a space-borne array would need thousands of electrically-small antennas to reach the sensitivity required for detecting faint astronomical signals, and it would need to be positioned far from the Earth to reduce the impact of Earth-based radio interference. Using more efficient antennas would minimize the number needed, and using antennas that are more resilient to interference would reduce the required distance from Earth. To this end, we consider constructing a low frequency array out of vector sensor antennas. These advanced antennas consist of three orthogonal dipole and three orthogonal loop antennas with a common phase centre. Their benefits include direction-finding and polarimetric capabilities, but they have not been considered for this application previously. We show that vector sensors can provide four times more Fisher information during interferometry than tripoles, simpler antennas that are commonly considered for space applications. We also present an all-sky imaging simulation to demonstrate a spherical harmonic imaging technique.