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

We reveal previously unknown quasi-periodic (QP) VLF emissions at the unusual high-frequency band of ~ 7–11 kHz by applying the digital filtering of strong sferics 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 bullets with the very abrupt cessation. Unfortunately, we could not explain such strange shape of the waves. In the second event, the modulation period was about 3 min under the absence of the simultaneous geomagnetic pulsations. The studied emissions lasted about 4 hours 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.