Physics of auroral phenomena : proceedings of the 34th Annual seminar, Apatity, 01 - 04 March, 2011 / [ed.: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 231 с. : ил.

J. Manninen et al. As in the initial phase of the storm (Fig. 2), the auroral hiss in the recovery phase of the storm (Fig. 3) was characterized by the pure right-hand polarization, and the chorus demonstrated the presence of both kinds of the polarizations, i.e. circular polarizations. We may conclude that the ionosphere exit points of the chorus signals were spread in a relatively large area. In the recovery phase, the night-time chorus was accompanied as the classical dawn chorus by the Pi 1C type geomagnetic pulsation with the circular polarization, usually associated with the electron precipitation [e.g., Troitskaya and Kleimenova, 1972]. During this considered event (1 Mar 2008) we had the VLF recordings on the board of the low-altitude (700 km) Demeter satellite crossing the late morning sector of the magnetosphere (-10 MLT). Demeter data from two orbits: the down part in the Northern hemisphere (at ~21.00 UT) and the up part in the Southern hemisphere (at ~21.30 UT) at the latitude range of L ~ 5-7, are shown in Fig.4. The chorus emissions (rising tones at f<3 kHz) were observed at the large latitude area in both hemispheres. Such large latitudinal chorus distribution in the ionosphere was previously reported by Jiricek et al. (1981), as a result of the non-ducting signal propagation above the ionosphere. DENIETER 01 .03.2008 -1 0 MLT ~21 UT N -2 1 .3 0 UT s Fig. 4. VLF data from Demeter satellite: upper panel - the part of falling orbit in the Northern hemisphere at ~21 UT; bottom panel - the part of up going orbit in the Southern hemisphere at -21.30 UT. It is important to note, that during the both considered chorus bursts (27 Feb and 1 Mar), the chorus emissions were also observed simultaneously at the Yakutsk meridian [Mullayarov, private communication], located at -05-06 MLT, which is between the Scandinavian meridian (22-23 MLT) and the Demeter satellite position (-10 MLT). Thus, we may suppose that the discussed chorus was generated in a very large longitude region, at least, from midnight to the late morning. Both night-time chorus events under consideration demonstrated the signature of a typical morning chorus like it has been recently reported by Manninen et al. (2010). Both above-discussed chorus were accompanied by the riometer absorption enhancement at Scandinavian meridian (Fig. 5) indicating the energetic particle precipitation. Simultaneous increasing of the riometer absorption was observed at the TIX [Samsonov, private communication], located in this time in the local morning at -04-05 MLT. The strong riometer absorption was also observed in this time by the 30 MHz HAARP VHF riometer at Gakona, located in the auroral latitude near the meridian of the Demeter position (~10 MLT). Thus, there were energetic particle precipitations at all sites, where the chorus was observed. It suggests that the large longitude area of the simultaneous chorus occurrence is the result of the chorus generation instead of behaviour of signal propagation. 27.02.2008 R UT Fig. 5a. The riometer data at two Scandinavian stations during chorus, observed in the initial phase ofthe storm. 01.03.2008 R UT Fig. 5b. The same as in Fig. 5a for recovery phase of storm. In the previous observations, Titova et al. (1998) found that the strongly localized variations of the trapped and precipitated energetic electron flux and associated the whistler wave intensification have been observed in the evening sector near the plasmapause during the recovery phase of magnetic storms due to the reconstruction of the cold plasma density. Similar magnetosphere situation could be occurred during our considered chorus events 84