Physics of auroral phenomena : proceedings of the 38th annual seminar, Apatity, 2-6 march, 2015 / [ed. board: A. G. Yahnin, N. V. Semenova]. - Апатиты : Издательство Кольского научного центра РАН, 2015. - 189 с. : ил., табл.

Strange VLF events at auroral latitudes The QP event of 16 December 2011. This event was observed in the morning and lasted about 6 hours. Some details of this event were discussed in the paper [Manninen et al, 2013]. The (1-4 kHz) spectrogram of this event is given in Fig.l (upper panel). It is seen that the signal repetition period gradually decreased and the signal low frequency decreased also from ~ 2.3 kHz to ~1.7 kHz. That could be explained as a result of removing the VLF emission generation region to the higher L-values (to lower values of the electron gyro-frequency). However, the signal upper frequency did not change (bottom panel in Fig. 1). The separated signals present a strange duplet structure. The temporal signal structure is not yet theoretically explained in details. The QP event of 18 December 2011 . This event was observed in the evening and lasted also about 6 hours. The 1-5 kHz spectrogram of this event and a few 10-min intervals from it is given in Fig. 2. Some details of this event were discussed in the paper [Manninen et a l, 2012]. The event presented a sequence of discrete rising signals in the frequency band of -2-4 kHz with a time-changing spectral structure. There was a superposition, at least, o f three different emissions: at higher frequencies (-3-5 kHz) - hiss, replaced by quasi-periodic discrete non-dispersive emissions at the same frequency band; at middle frequencies (-2.0-3.5 kHz) - QP emissions with decreasing repetition periods; and at lower frequencies (-1.8-2.2 kHz) - short discrete rising signals, each having the duration of about 15 s. The appearance and generation of the superposition of these different emissions at different frequency seems to be independent. As in previous event, now there is no theoretical explanation o f such phenomena. TOTAL POWER iLEHTO TDTAl nrtxn 18.12.2011 МИЮКИТО lo ru p o w n 18.12.2011 18.12.2011 KAMMUSlEHTO Figure 2. The (1-5) kHz spectrogram of the QP VLF event on 18/12/2011 and four 10-min successive intervals The series of puzzling short VLF bursts on 22 March 2013. A series of short (-1 min) and strange VLF bursts at -2-4 kHz were observed during few hours on magnetically quiet day with Kp=0-1 after geomagnetic disturbances recorded in the previous day. Two examples of such signals are shown in Fig. 3. The event on left panel looks like a superposition of two short hiss bursts: one burst at the constant frequency at about 3.5 kHz with the decreasing frequency band from -1 kHz in the beginning to -200-300 Hz in the end, and the second one at the slowly increased frequency from 2.5 kHz to 3.5 kHz showing the intensity variations with periodicity o f -3 s and an the increased frequency band from -200 Hz to -1.5 kHz. The event on right panel shows a multi-hop whistler with -3 s periodicity of slowly dispersed signals superposing on a short hiss burst. Both events could argue an existence of a complicated wave interaction in the magnetosphere plasma. SMi р * к S vifi » 1 ITyfc* Figure 3. 1-min spectrograms of short VLF events on 22/03/2013 in the frequency range o f 1-6 kHz: the left plot - 02.57-0.58 UT, the right plot - 03.28-03.29 UT 61