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

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 approximate to 10 degrees 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. Thunderstorm lightnings generate electromagnetic emission involving extremely low frequency (ELF) waves which can propagate several times around the Earth in the spherical resonance cavity formed between the Earth surface and the ionosphere and forming the Schumann resonances at 8 Hz, 14 Hz, etc. As a result, a single ELF measurement station can monitor thunderstorm activity over whole Earth by separating different thunderstorm regions with registered waves' azimuths. Such measurements were usually done by analyzing strong individual impulses. Here we propose a novel method involving ELF magnetic signals in a full available amplitude range. We show how to filter the 50 Hz electric grid network signal perturbations to allow usage of low amplitude impulses for azimuth determination. The method is applied to 3 kHz ELA11 measurements from our Hylaty station in Poland and allows for continues monitoring thunderstorm activities in main centers on Earth. A strong point-like ELF signal from the Hunga Tonga volcano eruption in January 2022 allowed us to measure deviation of the signal azimuth when propagating in the polar region, close to the solar terminator, and demonstrate how the method allows for separate analysis of the much-dispersed long propagating Tonga signals from nearby thunderstorm signals at the same azimuth. A new method for deriving wave arrival azimuths with parametric temporal filtering of electromagnetic waves in the ELF band is introduced A multitude of thunderstorms on Earth varying during the day at different azimuths are resolved The Hunga Tonga volcano eruption signals are diffracted by similar to 10 degrees when propagating in the Earth-ionosphere cavity over the polar regions