Physics of auroral phenomena : proceedings of the 35th Annual seminar, Apatity, 28 Februaru – 02 March, 2012 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2012. - 187 с. : ил., табл.

“Physics o fAuroral Phenomena”, Proc. XXXVAnnual Seminar, Apatity, pp. 67 - 70, 2012 © Kola Science Centre, Russian Academy of Science, 2012 TEMPORAL CHANGE OF THE VLF HISS POLARIZATION: CASE STUDY OF APRIL 12, 2011 J. Manninen1, N.G. Kleimenova2, O.V. Kozyreva 2 1Sodankyla Geophysical Observatory, Finland 2Institute o f the Earth Physics RAS, Moscow, Russia Abstract. Two hours lasting 1.5-4.0 kHz VLF hiss burst was observed on Apr. 12, 2011 at 04-06 UT in northern Scandinavia at the temporal station Kannuslehto (L=5.3) in the end of the initial phase of the small magnetic storm. In the previous day (11 Apr. at -16 UT), the very high solar wind dynamic pressure (up to 14 nPa) was observed and that shifted the plasmapause to the lower L-values. The VLF burst can be attributed to a typical plasmaspheric hiss. The temporal dynamic of the wave polarization properties was found. At the beginning, the VLF hiss was characterized by the left-hand polarization which can be interpreted as the waves long travelling in the Earth- ionosphere guide. The most part of the waves arrived from the north-south direction, probably from lower latitudes. About one hour later, the wave polarization gradually changed and turned to the strong right-hand polarization, which can indicate that the ionosphere wave exit point was nearly overhead. To this time, the solar wind dynamic pressure fell down up to 1 nPa and the magnetosphere (and the plasmapause) began to extend to its ordinary state. Thus, now the plasmapause can be mapped near Kannuslehto station (L=5.3). The VLF waves arrived from the north-south direction as well, but the intensity of northern and southern signals became comparable. We suppose that such wave polarization change can be interpreted as the temporal plasmapause location dynamics. The VLF hiss was modulated by simultaneously observed geomagnetic pulsations of the Pc4 range which demonstrated their resonance nature. The Pc4 spectral maxima shifted with time from higher frequency to lower, that can be a result of a latitude shift of the resonant field lines. Thus, the temporal dynamics of the VLF hiss polarization as well as the change of the resonant Pc4 pulsations demonstrate the temporal dynamic of the plasmapause location. Polar Geophysical Institute 1. Introduction VLF hiss is a broadband, noise-like whistler mode emission in the frequency range between about 100 Hz and several kHz. On the ground, the hiss emissions are usually divided into two types: ( 1 ) auroral hiss which is related to aurora and look like several minutes impulsive burst of emissions and ( 2 ) plasmaspheric hiss represented a burst of noise-like VLF emissions lasting up to several hours. Satellite observations at the plasmasphere showed that these noise VLF emissions are almost always presented [e.g., Thome et al., 1973]. It is widely accepted that the plasmaspheric hiss get their energy from gyroresonant interaction with the electrons of the inner radiation belt near the plane of the magnetic equator [Kennel and Petschek, 1966]. Plasmaspheric hiss can persist during relatively quiet conditions, but the emission intensifies during magnetic storms or substorms [e.g., Kleimenova et al., 1968; Thome et al., 1974; Larkina and Likhter, 1982; Merendith et al., 2004]. The recent Finnish VLF observation campaign has been carried out in April 2011 at the temporal station Kannuslehto (KAN, geograph, ф =67.74° N,^. = 26.27° E, 64.2°; 107.9° CGM, L=5.3), located near Sodankyla observatory. The description of the instrument is given by Manninen (2005). One of the most spectacular VLF event, observed during this campaign, was the burst of the VLF hiss emissions on April 12, 2011 at -04-06 UT, marked by the change of the sense of rotation of the wave polarization with time. The aim of the present work was to analyse the properties of this hiss event. 2. Observation results and discussion The small magnetic storm (Dst — 40 nT) started on April 11, 2011. In the storm initial phase, the solar wind density (Np) was very strong and the solar wind dynamic pressure (P) reached 14 nPa under IMF Bz>0. The storm main phase started at -06 UT on April 12, 2011 when the IMF Bz turned to the negative values. The time history of this storm is given in Fig. 1. 1. At 04-06 UT on 12 April, the strong VLF hiss burst was observed at KAN station. The frequency band was -2.3-4.2 kHz in the beginning and -1.3-3.3 kHz in the end of the event. The temporal dynamics of the hiss spectrogram and the wave polarization is presented in Fig.2, where the upper panel demonstrates the temporal variations of the total power of the VLF hiss, the middle panel - the power of the pure right-hand (R) polarized waves, and the bottom one - the power of the pure left- hand (L) polarized waves. It is seen (Fig. 2) that in the event beginning, the polarization of VLF hiss was mostly left-handed, which can be interpreted as the ionosphere wave exit point was located far away from this station and the waves came due to long distance propagation in the Earth- ionosphere wave-guide [Yearby and Smith, 1994]. 67