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.

V.A. Pilipenko et al. Discussion and conclusion The comparison of two successive auroral intensifications show that Pil power drops rapidly away from a source. Indeed, during the first auroral activation at ~1110 UT which occurs -15° eastward from MCQ, a sharp Pil power enhancement is evident, but it is not very strong at MCQ. However, during subsequent activation at -1120 UT, when MCQ happens to be just beneath the auroral bulge, the intensification of emission is much stronger. Most probably, the source of the Pil burst is located in the epicenter o f the auroral activation. Fig. 5. Dynamic spectra of search coil magnetometer data from conjugate stations MCQ (D component) and РОК (H component) for the period 1050-1140 UT, and fluxgate magnetometer (Bx - component) from Themis-C. Power is coded using log-scale according to the color bars at the right. 8 8 0.5 || 1 ч Л . .. ... i« m ' ■ ; ^----------— # i" j I; ■■■ ii im i ' T t И |- 1 ; ж <»*., П., 0 1 [ l.l..- ---- ^ --- j --- Н -— ----- H i CSY(-80.7°) ;>■; w .: ...... II 0.3 • : : i , , , ; ,. г ■ Р0Ц-80.10) ; ; l ....... .... T ’ T " r ! 1 B " 0.2 * »** **»<* у ■у-* 10.40 10.50 11 11.10 11.20 11.30 11.40 11.50 UT Fig. 4. Dynamic spectra o f the nightside polar stations MCQ (D), TNB (D), MCM (H), DMC (D), P5 (D), CSY (D), and PI (D) for the period 1040-1150 UT. The spectra of Antarctic station in the time interval 1115-1125 UT (Fig. 6) demonstrate a tendency for the main frequency of the Pil enhancement to decrease with increase of distance from a source. This dependence may indicate an increase of damping with frequency. spectral content of the burst is not entirely irregular: frequencies />0.35 Hz are highlighted. The enhancement of noise in the band around 0.3-0.4 Hz can be seen at all stations. Thus, Pil burst can propagate effectively (better than Pi2 pulsations) to very high latitudes, throughout the polar cap, and even on other side of the geomagnetic pole. The observed Pil bursts at distant stations from both activations are of comparable amplitudes. This indicates that away from the source the Pil attenuation becomes much weaker, enabling a signal to be detected at very large distances, up to a few thousands of km. 2007 082 Log10(PSD) M CO (-79.9°1 0.2 0.3 0.4 0.5 _________________________M C M f—79.9°1 0.4 0.5 Р М « -8 .8 .У ) 0.4 0.5 ___ С S V f—80.7*4 Fig. 6. Spectra (log-scale) o f the nightside polar stations MCQ (D), TNB (D), MCM (H), DMC (D), P5 (D), CSY (D), and PI (D) for period 1115-1125 UT. The Pil bursts observed at distant stations during both activations are o f comparable amplitudes. This indicates that away from a source the Pil attenuation becomes much weaker, enabling a signal to be detected at very large distances, up to a few thousands of km. It seems that Pil bursts can propagate poleward to large distances, even across the polar cap. At the same time, we could not identify any Pi2 wave pattern at the polar cap stations during the same substorm. Thus, in contrast to Pi2, the propagation of Pil bursts turns out to be more effective. We suppose that this difference is due to different transmission mechanisms: a quasi-static response for Pi2, and an ionospheric wave propagation forPil. These propagation features are expected from basic analytical and numerical models of Pci MHD wave excitation and propagation in the ionosphere [Greifinger, 1972; Fujita and Tamao, 1988; Fujita, 1988]. Qualitatively different situations are expected to arise depending on the lateral distance from the center of the incoming wave: (a) For lateral distances r<r0 that are small in comparison to the lateral size r 0 of the incoming disturbance (probably, -100 km), one may expect that the ground magnetic structure is dominated by the incident disturbance (e.g. Alfven wave); (b) In