NON-TYPICAL SERIES OF QUASI-PERIODIC VLF EMISSIONS

New types of the series of quasi-periodic (QP) VLF (very low frequency) emissions in frequency range of 1-5 kHz, not associated with geomagnetic pulsations, has been discovered at auroral latitudes (L=5.3) during the Finnish VLF campaign (held in December 2011). Several unusually spectacular events with duration of several hours have been observed in the night-time under conditions of quiet geomagnetic activity (Kp=0-1), while the usual QPs occurrence is the daytime. Contrary to the typical daytime QP emissions, the spectral structure of the reported QP events represented an extended complicated sequence of the combination of repeated discrete mostly rising diffuse VLF signals with the repetition periods ranged from ~several tens of seconds up to ~10 min. Four of such non-typical events are reported in this paper. The fine structure of the separated QP elements may represent a mixture of the different frequency bands signals, which seem to have independent origins. The VLF signals can come from different directions. Most of them have right-hand polarization indicating the location of the ionosphere VLF wave exit points being almost above the ground receiver. However, some events show the left-hand polarization, which can be interpreted as the wave arriving from ionospheric exit points far from the receiver, after propagation in the Earth-ionosphere waveguide. We suppose that these considered QP emissions can be attributed to auto-oscillations of the cyclotron instability of the Earth’s radiation belts [Bespalov, 1982; Bespalov and Trakhtengerts, 1986]. However, there is no complete theory, which could adequately explain the revealed type of QP emissions and their generation mechanism is not yet understood. Introduction The typical quasi-periodic emissions (QP) represented a sequence of repeated noise bursts of ~20-50 s duration, in which each burst may consist of a number of discrete events or periodic emissions [Helliwell, 1965]. The period between bursts is typically measured in tens of seconds. These emissions are understood to be whistler mode waves of magnetospheric origin that have propagated through the ionosphere to the ground. There are two types of QP emissions: associated and non-associated with geomagnetic pulsations. The first type of QP is more widely referred in the literature [e.g., Kitamura, 1974; Sato and Fukunishi, 1981; Bespalov and Kleimenova, 1989; Sazhin and Hayakawa, 1994; Manninen et al., 2012]. However, there was only a few papers analyzed the second type of QP emissions, i.e. non-associated with geomagnetic pulsations [e.g., Smith et al., 1998; Engebretson et al., 2004]. Based on Antarctic data these authors claimed that QP bursts have been observed more frequently during daytime in summer. The series of non-typical quasi-periodic (QP) 1-5 kHz VLF emissions, not described previously in the literature, have been observed during the recent VLF campaign in December 2011 in Northern Finland near obs. Sodankyla Geophysical Observatory (L~5.3). These QP emissions were not associated with geomagnetic pulsations. At least five unusually spectacular events with duration of several hours have been observed in the night-time under conditions of quiet geomagnetic activity (Kp=0-1), and only one event was recorded in the late morning, while the usual QPs occurrence is the daytime. The description of some revealed events is the object of this paper. Observations The Finnish winter VLF campaign applying the very high-sensitivity VLF receiver with the threshold of the order of 10 –6 nT has been carried out in Northern Finland (at the temporary station Kannuslehto: φ=67.74° N, λ=26.27° E; L=5.3), located about 40 km to the North-West from Sodankylä Geophysical Observatory. It was the first time when VLF observations have been carried out in the midwinter solstice under very quiet geomagnetic activity, and for the first time there was revealed a number of events of unknown types of QP emissions. The geomagnetic (Dst and AE indices), solar wind, and IMF data for 15-25 December 2011 are given in Fig. 1. It is seen that during this VLF campaign, the magnetic activity was very low. Proceedings of the 9th Intl Conf. “Problems of Geocosmos” (Oct 8-12, 2012, St. Petersburg, Russia)