Radiation belt electron wisp inside South Atlantic Anomaly due to terrestrial VLF transmitter observed by MSS-1

Abstract

The South Atlantic Anomaly represents a region within near-Earth space characterized by a significantly weaker geomagnetic field and a higher flux of energetic particles compared to other areas. It is a space weather hazards to Low-Earth- Orbit satellites. There has been evidence that the Very Low Frequency (VLF) waves from the powerful ground VLF radio transmitter in Australia, known as NWC, have the capacity to scatter energetic electrons’ pitch angle in the inner radiation belt. The scattering directs electrons into the drift loss cone, forms a “wisp”, characterized by its peak intensity outside the South Atlantic Anomaly (SAA), and a “rift” exhibiting minimal intensity within SAA. Our findings mark the initial observation of a “wisp” precipitation, an unusual occurrence with peak intensity detected inside the SAA, observed via the Macao Science Satellite-1. Enabled by the Medium-energy Electron Spectrometer onboard Macao Science Satellite-1, we were able to comprehensively measure the full pitch angle distribution at Low-Earth-Orbit. This allowed us to attribute the “wisp” within the anomaly to a specific pitch angle range just outside the drift loss cone, a measurement unattainable by previous satellites. This “wisp” occurrence aligns with previous model predictions, despite being overlooked. Moreover, we distinguished between the trapped and precipitating electron populations. Directly derived from the ratio of these populations, our analysis revealed that approximately 2%‒5% of trapped electrons will be lost in this specific wisp due to the influence of the ground VLF transmitter. Our results not only complement existing evidence of energetic electron pitch angle scattering facilitated by the ground VLF transmitter but also offer a quantitative estimation of its impact.