Physics of auroral phenomena : proceedings of the 38th annual seminar, Apatity, 2-6 march, 2015 / [ed. board: A. G. Yahnin, N. V. Semenova]. - Апатиты : Издательство Кольского научного центра РАН, 2015. - 189 с. : ил., табл.

STRANGE VLF EVENTS AT AURORAL LATITUDES J. Manninen1, N.G. Kleimenova2, T. Turunen 1 'Sodankyla Geophysical Observatory, Sodankyla, Finland 2Schmidt Institute o f the Physics o f the Earth RAS, Moscow, Russia e-mail: jyrki.manninen@sgo.fi, kleimen@ifz.ru Abstract. Few puzzling VLF events were found during several Finnish VLF winter campaigns at temporal station KAN located at ~40 km from Sodankyla Geophysical Observatory. Some expressive VLF emissions are presented here: (i) a few hours lasting quasi-periodic emissions in the frequency range o f 1-5 kHz with the multi-scale time periodicity of the structure as a mixture of the different frequency band signals, which seem to have independent origins; (ii) a series of short separated bursts of the VLF emissions in the frequency range of 2-3 kHz with a complicated do not repeated spectral structure; (iii) the unusual daytime peculiar discrete right-hand polarized VLF signals in the high frequency range of ~6-10 kHz which were discovered after the special filtration of impulsive signals like atmospherics; (iv) the morning unstructured VLF left-hand polarized hiss at ~ 4 -ll kHz and some others VLF emissions. The most part of the discussed VLF events were observed under very quiet geomagnetic conditions and only in the w inter season. T he m ain part of quasi-periodic VLF emissions may be generally attributed to the auto-oscillations of the Earth radiation belts cyclotron instability as it was developed by V.Yu. Trakhtengerts and PA. Bespalov. However, an exact theoretical interpretation of the unusual signal details remains unknown. Introduction In the Earth’s magnetosphere where different types of the natural whistler mode emissions occur at frequencies from a few hundreds of Hz to several kHz, well known as chorus and hiss. Most of them are generated in the disturbed magnetosphere as discussed in chapters 9 and 12 of Trakhtengerts and Rycroft monograph (2008). However, there is another type of VLF emission representing a spectacular sequence of repeated spectral elements or short noise bursts (lasting from several to tens of seconds). These events can be attributed to the quasi-period (QP) type of the VLF emissions [e.g., Sazhin and Hayakawa, 1994]. During last years, several VLF campaigns have been carried out at Kannuslehto (KAN), in Northern Finland (ф=67.74° N, X=26.27° E, L~5.5). Several puzzling QP emissions were found there and some results of their analysis have been published in Manninen et al. (2012, 2013, 2014a and 2014b). Most of these long-lasting QP emissions were not described in the literature early. The considered events were mostly observed in the winter time and under quiet solar wind and Interplanetary Magnetic Field (IMF) conditions. That provides the idea that such QP emissions could be interpreted as the self-oscillations of the magnetospheric maser [ Bespalov, 1982; Bespalov and Trakhtengerts, 1986]. The aim of this paper is to show several recently observed events of puzzling VLF emissions which need the future development of a theory. Observations of some strange VLF events In the first part of this paper, we show three examples of long lasting (up to several hours) events o f QP emissions with a strange spectral structure. These events were not associated with geomagnetic pulsations. Further we present two expressive high-frequency VLF events, which analysing is just in beginning. “P hysics o f Auroral P henom ena", Proc. XXXVIII A n n u a l Sem inar, Apatity, pp. 60-63, 2 0 1 5 © Kola Science Centre, Russian Academy of Science, 2015 Polar Geophysical Institute |The iow frequency limit decreasedw'tthtirrg. ~| Figure 1. The 1-4 kHz spectrogram of QP VLF emissions on 16/12/2011 (upper panel) and its separate signals (bottom panel). 60