Mapping High Energy Particles Using Augmented Star Trackers On-Board Swarm

Abstract

The ESA Swarm mission, launched on 22 November 2013, consists of three spacecraft each equipped with a Micro Advanced Stellar Compass ($\mu$ASC) from the Technical University of Denmark (DTU). Each $\mu$ASC features three Camera Head Units (CHUs) orientated orthogonally to optimize system accuracy performance and avoid simultaneous blinding. The image sensors inside the CHUs are sensitive to ionizing radiation. When an energetic particle impacts the image sensor, electrons are liberated along the particle’s ionizing track and appear on the source image as white dots dubbed ‘energetic particle detection’ (EPD) events.

For star tracker applications EPDs are normally supressed to support nominal attitude operation. However, in early 2018 software was uploaded to the $\mu$ASCs on-board Swarm, which on top of using the EPD measurements to improve the image for star tracking, is reporting the EPD count to the telemetry to ground. This added functionality enables detection and monitoring of high energy particles. By taking advantage of the sample rates (1-2 Hz), the orientation of the camera heads and simultaneous measurements from all three spacecraft spatial derivatives of the EDP aligned to electric and magnetic fields can be determined. Furthermore, since the Swarm spacecraft are in circular, near-polar orbits at an altitude of 450-510 km the spacecraft continuously monitor and map high energy particles at the South Atlantic Anomaly (SAA) of relevance for future mission planning as well as provide detailed time-radiation relations from charge injections processes from e.g. Coronal Mass Ejections (CMEs). In this work we present processes and analysis of four years of high energy radiation data obtained from the Micro Advanced Stellar Compass ($\mu$ASC) on board ESA’s Swarm mission, from February 2018 to February 2022.