Abstract
Abstract. 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 “bullets” 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.
Highlights
The whistler mode very low frequency (VLF) emissions, observed on the ground and onboard satellites, can exhibit a quasi-periodic (QP) occurrence of the separated signal patches in the frequency range of about 0.5–4 kHz, and due to that, they are termed QP emissions (e.g., Carson et al, 1965; Kitamura et al, 1969; Sato et al, 1974; Morrison et al, 1994; Sazhin and Hayakawa, 1994; Engebretson et al, 2004; Nemec et al, 2016)
Even at auroral latitudes, the ground-based VLF measurements are mostly covered by strong atmospherics (e.g., Yamashita, 1978; Yedemsky et al, 1992; Volland, 1995) which hide all natural emissions with weaker amplitudes
Tweek atmospherics are electromagnetic pulses with duration of ∼ 10–100 ms that originated from lightning discharges and propagate over distances of a few thousands of kilometers in the Earth–ionosphere waveguide (e.g., Ohya et al, 2015)
Summary
The whistler mode very low frequency (VLF) emissions, observed on the ground and onboard satellites, can exhibit a quasi-periodic (QP) occurrence of the separated signal patches in the frequency range of about 0.5–4 kHz, and due to that, they are termed QP emissions (e.g., Carson et al, 1965; Kitamura et al, 1969; Sato et al, 1974; Morrison et al, 1994; Sazhin and Hayakawa, 1994; Engebretson et al, 2004; Nemec et al, 2016). To study the waves of the magnetospheric origin, we applied the special method of digitally filtering out the strong impulsive atmospherics with duration less than 30 ms. This processing allowed us to discover completely new types of high-frequency VLF emissions Besides separated discrete emissions, discussed by Manninen et al (2016), we found some previously unknown unusually high-frequency quasi-periodic VLF emissions lasting up to several hours. Such emissions have not been reported earlier. To reject the influence of the strong narrow-band navigation transmitter signals, we analyzed the VLF emissions at the frequencies less than 12 kHz
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