资源环境科技发展态势分析平台

Very low frequency (VLF) emissions of natural origin were identified for the first time by analyzing 1-hr ground-based magnetic field spectrograms in the 0.2- to 39-kHz frequency range. Data were used from the Kannuslehto radio receiver (L-shell 5.5), recorded during different campaigns between 2006 and 2019. The spectrograms exhibit banded structures, which consist of several strip elements that vary in time and frequency over the event duration. Statistical analysis of 95 events shows that they are observed in the frequency range that extends from 2 to 37 kHz, and mainly appearing above 16 kHz. The events span from 4 to 110 min and occur in the evening sector (17-01 magnetic local time), mostly during quiet geomagnetic conditions. Furthermore, they are primarily left-handed polarized and are associated with bursts of lightning-related radio emissions such as sferics and tweeks.

Plain Language Summary Advances in modern technology have created tangible scientific and societal benefits, allowing us to make new discoveries, perform better observations, and monitor important parameters of our Earth's condition. In this study, new digital processing tools are used to uncover a natural, namely, non-man-made, very low frequency (VLF) radio wave emission never observed before. These emissions consist of several strips as appear on our detector screens, with structures having frequency drifts that vary during the event duration. Moreover, the range of frequencies in which these emissions are observed, 16-39 kHz, is higher than the typical one used for VLF observations. The measurements were made in Northern Finland during different campaigns, between 2006 and 2019, using 10-m size square-loop radio antennas installed on the ground. The duration of the structured emissions ranges from about four to a hundred minutes. These emissions occur during evening and early night hours, and when the Earth's magnetic conditions are calm. Additionally, we learned that the field perpendicular to the wave propagation rotates clockwise when looking at the incoming wave. A remarkable characteristic of these emissions is that they are coincidental in time with the occurrence of bursts of lightning VLF emissions.