New Method for Determining Azimuths of ELF Signals Associated With the Global Thunderstorm Activity and the Hunga Tonga Volcano Eruption

Abstract

A new method is proposed for deriving extremely low frequency (ELF) wave arrival azimuths using the wide range of signal amplitudes, contrary to previously applied high amplitude impulses only. The method is applied to observations from our new magnetic sensor in the Hylaty station with an 18 bit dynamic range and a 3 kHz sampling frequency. We analyzed a day of 15 January 2022, to test the procedure against the ability to extract ELF signals generated during the Hunga Tonga volcano eruption. With complementary filtering of power line 50 Hz signatures, precise azimuth information can be extracted for waves from a multitude of thunderstorms on Earth varying during the day at different azimuths. A phenomenon of successive regular variation—decay or activation—of thunderstorms activity with varying azimuth is observed, possibly due to passing over the solar (day/night) terminator, and signatures of azimuth direction change during this passage can be noted. We also show that the erupting Hunga Tonga volcano associated impulses dispersed due to a long propagation path are clearly revealed in the azimuth distribution with analysis using parameters fitted to measure slowly varying signals, but not for fast varying impulses. We show that the Hunga Tonga related signals arrive from the azimuth ≈10° smaller than the geographic great circle path. The discrepancy is believed to be due to propagation through the polar region and in the vicinity of the solar terminator.

Periodic Oscillation of VLF Transmitter Signals Measured in Low and Middle Latitude Regions

“…Measurements of VLF transmitter signals can be inverted to verify the theoretical ionospheric models (e.g., Bekker et al, 2022;Chowdhury et al, 2021) and remotely sense the electron density of D-region ionosphere (e.g., Gołkowski et al, 2018Gołkowski et al, , 2021Inan et al, 2010;Marshall et al, 2008;Xu et al, 2021), thereby investigating various space weather events, for example, solar flares (e.g., Han & Cummer, 2010;McRae & Thomson, 2004) and eclipses (e.g., Chakraborty et al, 2016;Cheng et al, 2023;Xu et al, 2019), and energetic particle precipitation from the radiation belts (e.g., Inan et al, 2007;Sauvaud et al, 2008). As for atmospheric processes, VLF measurements have been traditionally utilized to monitor lightning discharge (e.g., Bozóki et al, 2023;Qie et al, 2013) and thunderstorm activity (e.g., Kubisz et al, 2024;Qie et al, 2022). In addition, various high-energy radiation and transient luminous events in the Earth's atmosphere are associated with unique VLF signatures (Xu, Qie, et al, 2023;Yang et al, 2020;Zhang et al, 2020).…”