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.

Trans-polar propagation ofPil wave burst as observed by an Antarctic array during the Themis 2007 /03 /23 substorm the distant region, r » r 0, the electromagnetic field of the fast wave trapped in the ionospheric waveguide is dominant. In this region the direct signal falls off —2 rapidly as oc r , but the waveguide field weakly depends on radial distance В oc r~xn e x p ( - r / A ) , where attenuation length Л is very large, especially for nighttime conditions. The ducted wave has a lower cutoff frequency. The fundamental cutoff frequency is a ~VA12 D , where VA is the characteristic Alfven velocity inside the waveguide, and D is its width. The excitation factor of a waveguide mode decreases rapidly with increasing frequency, therefore the ground signal is expected to have a larger amplitude near cut­ off frequencies. Attenuation of the ducted wave (mostly caused by the ionospheric Joule loss) is an increasing function of frequency, larger for out-of-geomagnetic meridian plane propagation, and is minimized at со*. Observations of Pci propagation at mid-latitudes have shown that the damping rate is ~10dB/100km as a maximum in the injection region and ~2.5dB/100km in the region beyond 500 km [Hayashi et al., 1981]. Spatial attenuation is larger in the daytime than in the nighttime. Thus, the band-limited enhancement of Pil bursts can arise owing to the combination of two factors: cutoff at lower frequency, and weaker excitation and more severe attenuation of higher frequencies. The observed tendency of diminishing frequency of the high-frequency enhancement agrees with the modeling predictions of the damping increase for higher frequency. On the other hand, the band-limited enhancement may be caused by the resonant response/transmission of the auroral ionosphere [ Lysak, 1988; Pilipenko et a l, 2002]. The IAR excitation may occur only in the region of the Pil source. Because the IAR fundamental frequency COA ~ VA sin I / 2D (/ is the declination) is commonly less than the ionospheric waveguide cutoff frequency, (0A < CO. , the waveguide mode cannot excite the IAR upon its propagation. We suppose that at auroral latitudes the enhancements at -0.42 Hz (MCQ) and -0.35 Hz (POK) are caused by IAR effects. The difference in the highlighted frequencies is quite natural, because the conjugate ionospheres are not identical. This study demonstrates that Antarctica has a unique dense array of search-coil magnetometers corresponding to all magnetospheric domains: the sub-auroral and auroral regions, cusp, and polar cap, which may be used by the space community as an effective tool for substorm-related research. 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