Physics of auroral phenomena : proceedings of the 39th annual seminar, Apatity, 29 February-4 March, 2016 / [ed. board: N. V. Semenova, A. G. Yahnin]. - Апатиты : Издательство Кольского научного центра РАН, 2016. - 167 с. : ил., табл.

SC-associated VLF-emissions at two closely spaced auroral stations HUT RBSP A RBSPS Figure 3. Plasmapause loca tion by RBSP satellites. The results of detailed analysis of the VLF emission at KAN and LOZ during 10 minutes around SC are shown in Fig. 5. It is seen that the frequency of the first VLF burst increased very quickly from about 0.5 kHz to ~3.0 kHz, and then two frequency bands have appeared: the lower band at f < 2 kHz which was right-hand polarized at both stations, and the higher one at f -2-4 kHz with the different sign of the polarization rotation at KAN and LOZ. In Fig.5, one can see that at LOZ, the 2-4 kHz waves were strongly left-hand polarized. However, at KAN, during the first minute, these waves demon­ strated the both (left- and right-hand) polarization, and later, after 11.16 UT, the wave became strongly right-hand polarized demonstrating their spatial prox­ imity to the wave exit point from the ionosphere. So, we can conclude that at 11.15-11.19 UT, KAN was located closer to the 2-4 kHz wave exit point than LOZ. 2. Observation results and discussion It was found that the short VLF burst at -2-5 kHz, caused by the SC impulse at 11.14 UT, started 1 min later at both stations and lasted about 5 min. In a few minutes, this burst was replaced by the long lasting VLF hiss, which intensified after -11.32 UT reaching the maximum intensity at the frequencies lower -4 kHz (Fig. 4). The main structure of the VLF emissions at KAN and LOZ was similar de­ spite of the different instrumentation with the different sensitivity. In Fig. 4 a good similarity of the VLF spectral structure at LOZ and KAN is seen, both in 1 hour (a) and 10 min (b) time scales. Note, the computer programs of spectral analysis of VLF signals at these stations although based on the same physical principle, but had a small difference, which sometimes leads to a small difference in color presentation. LOZ Totalnewer T*»« *rurawn Wn* аттЛи KAN LOZ i " i v « * j 3 * ' ' V* » Right T * T 'T 'T —Л— f ~ - r - T ’-'V ШШ Azimuth i . ...... . ,< : >- j I* '’“'men Гот*, min, l i ;10 - 11:20 UT Figure 5. The VLF spectrograms (total power, left-hand and right-hand polarized wave power) and the azimuth of VLF wave arriving at KAN and LOZ in the 10-min interval (11.10-11.20 UT). The azimuth of VLF wave arrival was calculated for both stations. However, it should be noted, at KAN, the wave arrival direction can be calculated with the uncertainty of 180 degrees because only two horizontal components of the magnetic field are recorded. But at LOZ also the vertical electric component was measured, which allow to calculate the azimuth of the wave arrival. The calculation of the azimuth of VLF wave arrival showed (Fig. 5) that at KAN, the 2-4 kHz waves travelled along the North-South direction (red color). But at LOZ, the waves arrived from the West (green color), i.e., from the KAN location. That confirms our assumption that KAN was located close to the wave ionospheric exit point. It is important to mention, that at LOZ, the waves at higher frequency (f >~4 kHz) came from the South (red color) where the plasmapause was located (see Fig. 3). The strong hiss emissions at frequency up to 6 kHz were evolved several minutes after SC (Fig. 4a). The detailed analysis of this interval according LOZ data is presented in Fig. 6. It is clearly seen that in the beginning of the event, polarization of VLF emissions was left-handed, which can be interpreted as the ionospheric exit point of the wave was located far away from this station, and the waves arrived long distance propagating in the Earth- ionosphere wave-guide [e.g., Yearby and Smith, 1994]. KAN Figure 4. The VLF spectrograms (total power) at LOZ and KAN, (a) - at 11-12 UT (60 min full scale), ( b ) - at 11.10 -11.20 UT (10 min full scale). 37