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.

Night-time VLF chorus in the magnetic storm on 27 Feb - 03 Mar 2008 Later some authors [e.g., Spasojevic et al., 2003; Yahnin et al., 2006] showed that after geomagnetic disturbances, related with a southward turning of the IMF, a plasmapause density structure could be highly irregular within the plumes even in the night side. We suppose that such cold plasma irregularities in night side of the plasmapause could be occurred in our case of 27 Feb, supporting the Pci wave generation in the night side of the magnetosphere. In the initial phase of the magnetic storm (27 Feb), the fresh injected particles (both electrons and protons) were accelerated by the magnetosphere compression associated with strong increase in solar wind density (see Fig. 1) and could be a source of an electron-cyclotron (chorus) and ion- cyclotron (Pci pulsations) instabilities.We speculate that the large longitude area of the simultaneous chorus generation could be a result of the presence of the strong values of the solar wind velocity providing the magnetosphere compression and particle acceleration. 3. Summary The analysis of the VLF emissions, measured on the ground at auroral latitudes (L=5.3) during the moderate magnetic storm, demonstrated the occurrence of VLF chorus lasting about one-two hours near the local magnetic midnight. The first chorus burst was observed in the storm initial phase (27 Feb 2008), and the second one - in the recovery phase of the storm (1 Mar 2008). In both events, the chorus was preceded by the short bursts of the auroral hiss associating with the substorm onset, the riometer absorption enhancement, and the Pi2 and PilB geomagnetic pulsations. The auroral VLF hiss was characterized by right-hand polarization and could be interpreted as the ionosphere VLF exit point location was not very far from the ground receiver. But the chorus demonstrated the presence of both type of polarization with mostly right- hand one, represented waves simultaneously coming from different distances. The night chorus, observed in the storm initial phase after very strong magnetic substorm (AE index >1200 nT), was accompanied by the Pci geomagnetic pulsations with very complicated spectral structure, and the night chorus in the recovery phase of the storm was accompanied by Pi 1C geomagnetic pulsations. In both events the simultaneous chorus emissions were observed on the ground in the morning side as well. 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