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Abstract

Broadband very low frequency (VLF) radio waves measured in northern Finland, is used to study a new type of VLF emissions displaying several strip-like structure observed in the 5–39 kHz frequency range. We call these emissions banded structure (BS). Analyzing campaigns from 2006 to 2022, we found that BS emission are observed in the local evening, from 16 to 01 MLT, during quiet geomagnetically periods, and usually last about 1 hour. The strip elements vary in time and frequency over the event duration and the frequency separation between adjacent bands of BS lies in the range 300–800 Hz. The emissions can be observed either in the high VLF frequency ranges or spanning from low to high frequency ranges. We also examined magnetometers data, from sensors located roughly at the same longitude as the VLF receiver. The results show that about 65% of the events stop been observed at about the beginning of the growth phase of substorms. We discuss whether their origin might be due to plasma instabilities in the magnetosphere, as in the case of auroral hiss.