New high-frequency (7–12 kHz) quasi-periodic VLF emissions observed on the ground at L ∼ 5.5

• Published in: Annales Geophysicae
• Main Authors: J. Manninen, N. Kleimenova, T. Turunen, L. Gromova
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2018
• Subjects: Science; Q; Physics; QC1-999; Geophysics. Cosmic physics; QC801-809; Northern Finland
• Online Access: https://doi.org/10.5194/angeo-36-915-2018; https://doaj.org/article/558c577faaae4e92938e617df28c1550

We reveal previously unknown quasi-periodic (QP) very low frequency (VLF) emissions at the unusually high-frequency band of ∼ 7–12 kHz by applying the digital filtering of strong atmospherics to the ground-based VLF data recorded at Kannuslehto station (KAN). It is located in northern Finland at L ∼ 5.5. The frequencies of QP emissions are much higher than the equatorial electron gyrofrequency at L ∼ 5.5. Thus, these emissions must have been generated at much lower L shells than KAN. Two high-frequency QP emission events have been studied in detail. The emissions were right-hand polarized waves indicating an overhead location of the exit area of waves in the ionosphere. In one event, the spectral–temporal forms of the emissions looked like a series of giant <q>bullets</q> due to the very abrupt cessation. Unfortunately, we could not explain such a strange dynamic spectral shape of the waves. In the second event, the modulation period was about 3 min under the absence of simultaneous geomagnetic pulsations. The studied emissions lasted about 4 h and were observed under the very quiet geomagnetic activity. The adequate mechanisms of the generation and propagation of the revealed high-frequency QP emissions have not yet been established. We speculate that studied QP emissions can be attributed to the auto-oscillations of the cyclotron instability in the magnetospheric plasma maser.