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

The high kinetic energy electrons that populate the radiation belts emit synchrotron radiation from their interactions with the planetary magnetic field, with the brightness of the spectrum revealing the electron distribution across different energy levels. A lunar near side array would be uniquely positioned to image this emission and provide a near real-time global measure of how the Earth's radiation belts are responding to the current solar input. The Salammbô code is used to model the dynamics of the three-dimensional phase-space electron densities in the radiation belts, allowing the realistic prediction of 1 keV to 100 MeV electron distributions trapped in the belts. These distributions are then passed to a simulation that creates the corresponding synchrotron brightness spectrum. We run simulations on a quiet time from the 11th of October 2016 and a stormy time from the 1st of November 2016. These yield brightness maps from 0.1-1 MHz for a Lunar observer with an overall spectral flux density of 1 - 3.75 Janskys.

Using Digital Elevation Models from Lunar Reconnaissance Orbiter's Lunar Orbiter Laser Altimeter (LOLA) data, we select a set of locations near the Lunar sub-Earth point with minimum elevation variation where we simulate radio receivers to create a synthetic aperture, varying both array size and configuration. We decide on a science bandwidth of 0.5-1.0 MHz to avoid most of the transient auroral interference. By using various post processing techniques and order of magnitude arguments, we reduce the noise problem to amplifier noise and electron quasithermal noise, which varies with Solar Zenith Angle (SZA).

We use SPICE to align the LOLA frame to celestial sky coordinates to track the array's relative position with the Sun and Earth. We then use a custom CASA code to process and image the data from our defined array. We find that for a moderate lunar surface electron density of 250/cm³, the radiation belts may be detected at least once a day with a 16384 element array over a 10 km diameter circle. Lunar surface electron densities in the 1000s/cm³ at low SZAs mean there will be too much quasithermal noise to observe the radiation belts in a reasonable time frame. At higher SZAs near or on the night side, such an array could make a snapshot of Earth's radiation belts at least 10 times a day.