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.

Temporal change o fthe VLFhiss polarization: Case study o fApril 12, 2011 12 APRIL 2011 12.04.2011 0445-0502 ОТ X . NRK( 67.7s) _ KEVl 66.2'') _ IV A{ 64.8°) . SOD( 63.8' ) . OUJ( 60.9°) _ MEK( 59.Г) .. HAN( 58.6°) . NUR( 56.6°) 00 06 12 18 24 UT Fig. 3. The variations of the AE index of magnetic activity. Fig.4. The night and morning sides magnetograms. The hiss emission very suddenly very suddenly disappeared at 06.05 UT with a substorm onset at the night side (Fig.4). 8-50 mHz 8 10 12 14 16 18 20 mHz Fig.6. The spectra of the geomagnetic pulsations. 3. Discussion Based on the morphological properties of the considered VLF hiss burst, it can be attributed to the typical plasmaspheric hiss. In many papers, for example, [Inan and Bell, 1977, Thorne et al., 1979], when considering the conditions of propagation of the whistler mode VLF emissions in the magnetosphere, the importance of the plasmapause in the guidance of whistler waves is noted. The ground data also showed that the strongest day­ time hiss emissions were observed near the plasmapause location [e.g., Smirnova et al., 1976]. Thus, the plasmapause can be a main wave canal guiding the plasmaspheric hiss from the magnetosphere to the ionosphere. Then, the VLF waves propagate though the ionosphere and due to travelling in the Earth-ionosphere, the waves can arrive to the ground receiver, located far away from the wave exit point. The sense of the polarization vector rotation provides the information how far is the VLF wave receiver location from ionosphere exit point. We suppose that in the beginning of our event, the VLF receiver (at KAN station) was located far away (left-hand polarized waves) from the wave ionospheric exit which is roughly coincides with the plasmapause projection to the ionosphere. Probably, in that time the plasmapause was located well equatorward from KAN. This suggestion is well reasonable. The strong solar wind dynamic pressure (~14 nPa), observed on the previous day (11 April), significantly compressed the magnetosphere, and at that time the plasmapause could be located at L-value much lower than L=5.3 (KAN station). To -05.30 UT on 12 April, the solar wind dynamic pressure dropped up to ~1.5 nPa, and the plasmapause started to move to higher L-values. At this time we 09 UT 112-5*) 10в.7*> 110 - 2 *) 10».7*) 110 . 1 ^ 107.7*) ioe.5*) A±Sf) 14.5") 329.0*) 2»3.3*) BRW 250.1*) 0520-0533 UT X .. NRK( 67.7' ) . к т - ш ' . IVA( 64.8°) . SOD( 63.8°) _ OUJ( 60.9°) _ MEK( 59.1°) . HAN( 58.6°) . NUR( 56.6°) S3.8*. 107.7*) ( 60.9*, ЮБ.5’) HAN ( 58 6\ Ite.O*) NUR С 5Б.е*. ЮЗ.О0 04.30 05.00 05.30 06.00 UT Fig.5. The filtered geomagnetic pulsations at the IMAGE meridian stations and the hiss spectrogram 69