Direct Evidence Reveals Transmitter Signal Propagation in the Magnetosphere

• Published in: Geophysical research letters 2021-08, Vol.48 (15), p.n/a
• Main Authors: Gu, Wenyao, Chen, Lunjin, Xia, Zhiyang, Horne, Richard B.
• Format: Article
• Language: English
• Subjects: ducted propagation; magnetosphere; nonducted propagation; VLF transmitters
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL093987

Signals from very‐low‐frequency transmitters on the ground are known to induce energetic electron precipitation from the Earth's radiation belts. The effectiveness of this mechanism depends on the propagation characteristics of those signals in the magnetosphere, and in particular whether the signals are ducted or nonducted along channels of enhanced plasma density, analogous to optical fibers. Here we perform a statistical analysis of in‐situ waveform data collected by the Van Allen Probes satellites that shows that nonducted propagation dominates over ducted propagation in both the occurrence and intensity of the waves. Ray tracing confirms that the latitudinal distribution of wavevectors corresponds to nonducted as opposed to ducted propagation. Our results show the dominant mode of propagation needed to quantify transmitter‐induced precipitation and improve the forecast of electron radiation belt dynamics for the safe operation of satellites. Plain Language Summary Very Low Frequency (VLF) signals emitted from ground‐based transmitters can appear in the Earth's magnetosphere, propagating either in ducted mode along magnetic lines or in nonducted mode. The different propagation modes affect how the signals interact with energetic electrons of the radiation belts and precipitate them into the ionosphere. In this study, we present a statistical study using the observations of Alpha transmitter signals by Van Allen Probes satellites and show that nonducted propagation mode dominates over ducted propagation mode in both signal occurrence and wave intensity. Our result is also supported by ray‐tracing simulations. Our study not only resolves the relative contribution of different propagation modes, but also provides critical parameters for quantifying precipitation by VLF transmitter signals and therefore improving forecast of radiation belt electron dynamics. Key Points First statistical wave normal analysis on very low frequency (VLF) transmitter signals in the magnetosphere is presented Dominance of nonducted signals is revealed in the magnetosphere The power proportions of ducted and nonducted signals are given as a function of L and Kp index