Abstract
Abstract. A new type of the series of quasi-periodic (QP) very low frequency (VLF) emissions in frequency range of 1–5 kHz, and not associated with geomagnetic pulsations, has been discovered at auroral latitudes (L = 5.3) during the Finnish VLF campaign (held in December 2011). At least five unusually spectacular events, each with a duration of several hours, have been observed during the night under conditions of quiet geomagnetic activity (Kp = 0–1), although QPs usually occur during the daytime. Contrary to the QP emissions typically occurring during the day, the spectral structure of these QP events represented an extended, complicated sequence of repeated discrete rising VLF signals. Their duration was about 2–3 min each, with the repetition periods ranging from ~1 min to ~10 min. Two such nighttime non-typical events are reported in this paper. The fine structure of the separated QP elements may represent a mixture of the different frequency band signals, which seem to have independent origins. It was found that the periodic signals with lower frequency appear to trigger the strong dispersive upper frequency signals. The temporal dynamics of the spectral structure of the QPs studied were significantly controlled by some disturbances in the solar wind and interplanetary magnetic field (IMF). This finding is very important for future theoretical investigations because the generation mechanism of this new type of QP emissions is not yet understood.
Highlights
Quasi-periodic (QP) very low frequency (VLF) emissions were first reported by Gallet (1959), Pope and Campbell (1960), and Lokken et al (1961)
Based on Antarctic observations, these authors concluded that QP2 are most common in austral summer under conditions of quiet geomagnetic activity, their diurnal maximum of occurrence is near noon, and the period of separated VLF bursts repetition ranged from ∼20 s to ∼100 s
A new type of ensemble of different quasi-periodic (QP) VLF emissions have been recorded in Northern Finland at
Summary
Quasi-periodic (QP) very low frequency (VLF) emissions were first reported by Gallet (1959), Pope and Campbell (1960), and Lokken et al (1961). The period between bursts is typically measured in tens of seconds. Based on Antarctic observations, these authors concluded that QP2 are most common in austral summer under conditions of quiet geomagnetic activity, their diurnal maximum of occurrence is near noon, and the period of separated VLF bursts repetition ranged from ∼20 s to ∼100 s. J. Manninen et al.: New type of ensemble of quasi-periodic, long-lasting VLF emissions. Very often these VLF emissions were accompanied by periodic emissions with periods of a few seconds. The repetition period of the separate signals was large, and ranged from about one to about ten minutes. These events, not previously described in the literature, are the object of this paper
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