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.

"Physics ofAuroral Phenomena", Proc. XXXIVAnnual Seminar, Apatity, pp. 82 - 85 2011 © Kola Science Centre, Russian Academy of Science, 2011 Polar Geophysical Institute N IGHT -T IM E V L F CHORUS IN THE M AGN E T IC STORM ON 27 FE BRU AR Y - 03 MARCH 2008 J. Manninen1, N.G. Kleimenova2, O.V. Kozyreva2, T. Raita1, T. Turunen1, M. Parrot3 1Sodankyla Geophysical Observatory, Finland 2Institute o f the Earth Physics RAS, Moscow, Russia 3CNRS, Orleans, France Abstract. The Finnish VLF campaign was carried out in 25 Feb - 04 Mar 2008 at L=5.3 (KAN) during a moderate magnetic storm, driven by the solar wind high-speed stream with Vsw~ 800 km/s. Several non-typical VLF events were measured during this storm. Two events of chorus emissions were recorded near local midnight: one - in the storm initial phase and the second - in the storm recovery phase. In both events, typical night-time short bursts of auroral hiss were observed, associating with the substorm onset, the auroral break-up, the riometer absorption enhancement, and the Pi2 and PilB geomagnetic pulsations. The chorus emissions were observed near the local magnetic midnight and lasted about 1-2 hours. In the initial phase of the storm, the chorus at frequency of 1.5-2.5 kHz was accompanied by the Pci geomagnetic pulsations with the complicated dynamic spectrum (so called “goose” structure). The night-time chorus in the recovery phase of the storm was accompanied by the Pi 1C type geomagnetic pulsations like a typical dawn chorus. The auroral hiss demonstrated the pure right-hand polarization, interpreted as the ionosphere VLF exit point location not very far from the ground receiver. But the chorus demonstrated the presence of both type of polarization with mostly the right-hand one. It means that chorus emissions represented waves coming from different distances. In the second event there were the VLF data from the low-altitude (~ 700 km) Demeter satellite. It observed the presence of the chorus at ~10 MLT sector in the same time as the night-time chorus at KAN. It is suggested that during this event, chorus was generated at very large longitudinal region, at least from the late morning till midnight. I. Introduction One of the most spectacular kinds of the magnetospheric plasma waves is the whistler mode chorus. Chorus activity is typical for the early morning horn's, and therefore these emissions were called “dawn chorus”. The properties and nature of chorus were discussed in number, of experimental and theoretical papers. The chorus generation can be attributed to the so-called backward wave oscillator (BWO) regime of the electron cyclotron instability near the plasmapause [Trakhtengerts, 1995; Trakhtengerts and Rycroft, 2000]. These VLF emissions can be a driver of an energetic electron precipitation, typically accompanied the chorus occurrence [e.g., Manninen et al., 1996, Bortnik and Thome, 2007; Golkowski and Inan, 2008] due to the electron-cyclotron resonance and pitch-angle diffusion [Pasmanik and Trakhtengerts, 1999]. It was found [Tsurutani and Smith, 1974; Anderson and Maeda, 1977] that the onset of the chorus bursts coincided with an injection of substorm electrons with energy of ~10- 100 keV. The Finnish VLF observation campaign has been carried out during the period of 25 February to 04 March 2008 (during a moderate magnetic storm) at the temporal station Kannuslehto (KAN, geograph. <p =67.74° NД = 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). The aim of the present work was to study the chorus observed during the initial and recovery phases of this magnetic storm in the night-time, which is non-typical 82 for chorus occurrence. 2. Observation results and discussion The considered magnetic storm was one of the 27-days sequences of recurrent storms (CIR-storms) caused by the solar wind high-speed streams. The Dst index and the variations of IMF and solar wind parameters are presented in Fig. 1. The relatively small negative Dst value (-46 nT) and high solar wind velocity (up to 800 km/s) are typical for CIR magnetic storms. wind, Bz IMF, and AE-index variations during considered