image: Panel (a) shows the trajectory of MSS1-A crossing the SAA on a geographic map, the color indicates the integral flux of 40-750 keV electrons, accompanied by L-shell contour lines. The star marks the start position of the trajectory. Panel (b) shows the spectrogram of the electron omni differential flux, the wisp can be seen as an energy-dispersed spike. The real-time L-shell is shown with the white dashed line along the ordinate on the right. Panel (c) presents the electron omni differential flux of several energy bands with lines of different colors. Panel (d-f) are in the same format, capturing observations from MSS1-B approximately 8 minutes later.
Credit: ©Science China Press
This research work was conducted by Professor Zong and his team at Peking University and State Key Laboratory of Lunar and Planetary Science, Macau University of Science and Technology. They developed the Medium-energy Electron Spectrometer onboard the Macao Science Satellite – 1, which enabled a comprehensive measurement of the full pitch angle distribution at Low-Earth-Orbit. The Macao Science Satellite - 1 (MSS) comprises two satellites orbiting the Earth at an inclination of 41°. MSS1-A follows a circular orbit at an altitude of 450 km, while MSS1-B's orbit is elliptical, ranging between 450 and 500 km in altitude.
With energetic electron observation from MSS-1, they report the initial observation of a 'wisp' precipitation with peak intensity detected inside the South Atlantic Anomaly. The South Atlantic Anomaly is a region within near-Earth space where the geomagnetic field is significantly weaker and the flux of energetic particles is higher compared to other areas. This anomaly poses a space weather hazard to Low-Earth-Orbit satellites.
Evidence has been found that Very Low Frequency (VLF) waves emitted by the powerful ground VLF radio transmitter in Australia, known as NWC, can scatter the pitch angle of energetic electrons in the inner radiation belt. This scattering directs electrons into the drift loss cone, forming a 'wisp,' which is characterized by its peak intensity outside the South Atlantic Anomaly region (SAA), and a 'rift' with minimal intensity within the SAA.
With MSS-1 data, however, the researchers found unusual peak ‘wisp’ instead of minimum inside SAA related to NWC. The researchers attribute the 'wisp' within the anomaly to a specific pitch angle range just outside the drift loss cone, a measurement that was unattainable by previous satellites. Moreover, the researchers distinguished between the trapped and precipitating electron populations. The analysis, directly derived from the ratio of these populations, revealed that approximately 2-5% of trapped electrons will be lost in this specific wisp due to the influence of the ground VLF transmitter. The results not only complement existing evidence of energetic electron pitch angle scattering facilitated by the ground VLF transmitter but also provide a quantitative estimation of its impact.
See the article:
Sun, Y., Liu, Y., Zong, Q. et al. Radiation belt electron wisp inside South Atlantic Anomaly due to terrestrial VLF transmitter observed by MSS-1. Science China Earth Sciences, 68(2): 538–548 (2025), https://doi.org/10.1007/s11430-024-1465-x
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Science China Earth Sciences