Measuring the Earth's Synchrotron Emission from Radiation Belts with a Lunar Near Side Radio Array

TL;DR

This paper proposes using a lunar near side radio array to image Earth's radiation belt synchrotron emission, enabling near real-time monitoring of belt responses to solar activity through detailed modeling and synthetic aperture imaging.

Contribution

It introduces a novel method combining physical modeling, lunar surface data, and synthetic aperture imaging to detect Earth's radiation belts from the Moon.

Findings

Detection possible every 12-24 hours with a 16,384 element array

Electron density affects measurement speed, faster at lunar night

Simulation shows feasibility of lunar-based radiation belt imaging

Abstract

The high kinetic energy electrons that populate the Earth's radiation belts emit synchrotron emissions because of their interaction with the planetary magnetic field. A lunar near side array would be uniquely positioned to image this emission and provide a near real time measure of how the Earth's radiation belts are responding to the current solar input. The Salammbo code is a physical model of the dynamics of the three-dimensional phase-space electron densities in the radiation belts, allowing the prediction of 1 keV to 100 MeV electron distributions trapped in the belts. This information is put into a synchrotron emission simulator which provides the brightness distribution of the emission up to 1 MHz from a given observation point. Using Digital Elevation Models from Lunar Reconnaissance Orbiter (LRO) Lunar Orbiter Laser Altimeter (LOLA) data, we select a set of locations near the Lunar sub-Earth point with minimum elevation variation over various sized patches where we simulate radio receivers to create a synthetic aperture. We consider all realistic noise sources in the low frequency regime. We then use a custom CASA code to image and process the data from our defined array, using SPICE to align the lunar coordinates with the Earth. We find that for a moderate lunar surface electron density of 250/cm^3, the radiation belts may be detected every 12-24 hours with a 16384 element array over a 10 km diameter circle. Changing electron density can make measurements 10x faster at lunar night, and 10x slower at lunar noon.