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 magnetic storm. Two night-time VLF chorus events were measured during this storm, particularly during the initial and recovery phases of the storm. It is well known that chorus emissions are typical for the local morning. However, in the initial phase of the storm (27 Feb 2008), the chorus burst with the increasing frequencies from ~1.3 to ~2.8 kHz was observed near the local magnetic midnight (~22-23 MLT). The strong substorm with AE-index ~ 1350 nT (Fig. 1), observed after SC, was preceded the chorus occurrence. The substorm onset at the Scandinavian meridian was accompanied by the breakup of the visible auroras, the burst of the auroral hiss, the Pi2 and Pi IB geomagnetic pulsations, and the riometer absorption enhancement. About 40 min later, the chorus emissions started at KAN and lasted about one hour. Total power 27.02.2008 kann u sleh to 18 19 20 21 Fig. 2. The dynamic spectra o f chorus and its polarization, the bottom panel - the dynamic spectrum of the PilB and Pci pulsations for 27 Feb 2008 event. The dynamic spectrum of chorus is given in Fig. 2 (upper panels). In the first part of the chorus burst (-19.20-20.00 UT), the frequency band of the strongest signals was recorded at -0.4-1.4 kHz, and in the second part (-20.00-20.30 UT), the frequency increased from -1.4 kHz to -2.5 kHz. Two middle panels in Fig. 2 demonstrate the pure right-hand and pure left-hand polarized signal power. The auroral hiss at -18.40 UT was characterized by the pure right-hand polarization. According to Yearby and Smith (1994) it could be interpreted as the ionosphere VLF exit point location not very far from the ground receiver. The chorus emissions demonstrated the presence of both polarizations, but mostly the right-hand one. It means that chorus emissions came from different distances, and there was superposition of many ionosphere exit points of signal, located at rather large area, not only overhead. The time of chorus frequency increasing (-20 UT) coincided with the onset of the Pci type of geomagnetic pulsations (bottom panel in Fig. 2) with the left-hand polarization and the complicated dynamic spectrum, which could be attributed to the so-called “goose” Pci structure [Feigin et al., 2009]. At that time, the onset of the Pc5 geomagnetic pulsations at 2-7 mHz (not shown here) was observed in the morning side of the Earth (-11 MLT) by geostationary GOES-11 satellite and at the conjugated ground station (YKC). The 2-7 mHz geomagnetic pulsations were observed also on the ground the near midnight (SOD). The second chorus burst was observed near the midnight in the recovery phase of the storm (1 Mar 2008). As in the initial phase of the storm, before the chorus there were recorded the auroral hiss associated with the substorm onset, the riometer absorption enhancement, and the Pi2 and PilB pulsation bursts. In this case the chorus frequencies were lower than -1.2 kHz. The chorus dynamic spectra and the right-hand and left-hand polarized signal power are shown in Fig. 3. T o t a l P o w e r 0 1 M A R 2 0 0 8 K o n n u s l o h t o 1 9 2 0 2 1 2 2 Pure rivht baud polarized pow er 1 9 2 0 2 1 2 2 Pure left hand polarized pow er 1 9 2 0 2 1 2 2 1 9 2 0 2 1 2 2 Fig. 3. The same as in Fig. 2 for 0 1 March 2008 event. 83