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.

TRAN S -PO LAR PROPAGATION OF P il W A V E BU R ST A S O B SER VED B Y AN AN TARC T IC A R R A Y DURING TH E TH EM IS 2007/03/23 SUBSTORM V.A. Pilipenko1’2, O.M. Chugunova1, M J . Engebretson2, and M. Lessard3 (I> Institute o f the physics o f the Earth, Moscow (2j Augsburg College, Minneapolis <3>University o fNew Hampshire, Durham “Physics ofAuroral Phenomena", Proc. XXXTVAnnual Seminar, Apatity, pp. 86 - 89 2011 Geophysical © Kola Science Centre, Russian Academy of Science, 2011 Institute Abstract. We have analyzed bursty Pil emissions that occurred during an interval of substorm activity on March 23, 2007. Magnetometer observations from the multi-spacecraft Themis mission and from the search- coil magnetometers in Antarctica are augmented by UVI images from the Polar satellite. Dynamic spectra from high-latitude stations reveal that Pil bursts can propagate poleward, even across the polar cap. Surprisingly, Pil propagation at high latitudes is more efficient than that of Pi2 pulsations. We infer that the propagation mechanism for Pil is the partial wave energy trapping in the ionospheric waveguide. The band-limited spectral structure of Pil burst can arise owing to the combination of a cutoff at lower frequency, and a weaker excitation and stronger attenuation at higher frequencies. Introduction Accurate timing and locating of substorm onsets continues to be a matter of considerable importance as the space physics community tries to evaluate competing onset mechanisms [ Liou et al., 1999]. Although UV satellite imagers provide ionospheric projection of electron precipitation related to the onset, the satellite limited imaging cadence underscore the need for complementary ground-based monitoring techniques. Pi 2 observations can be made over a large latitudinal range, but the long-period nature of these signals provides only approximate timing (~few min) [Liou et al., 2000]. Observations of Pil bursts (so called PiB emission), because of their higher frequency, hold the promise of providing better temporal resolution (~few sec) [Bossinger and Yahnin, 1987]. ULF waves in the Pci/Pil band are expected to be produced through field-aligned injection of localized Alfven waves into the ionosphere. MHD waves in the frequency range around 1 Hz can propagate in the horizontal direction, being trapped in the upper ionosphere. Thus, the spatial structure of Pcl/Pil magnetic signals is determined by both mode conversion from incident Alfven waves into horizontally propagating fast magnetosonic waves and trapping of fast waves in the ionospheric F-layer [Greifinger and Greifmger, 1968]. Therefore, the spatial and frequency dependences of the magnetic signals observed on the ground are expected to be different in regions near the injection center than in regions with distance much larger than the scale of the incident wave. A non-monotonic Alfven velocity profile in the upper ionosphere results not only in the occurrence of the fast mode waveguide, but also the ionospheric Alfven resonator (IAR). The IAR is bounded from below by the highly-conductive E-layer, and from above by the steep vertical gradient o f KA(z). In the auroral region, additional effective reflection of Alfven waves may occur from the bottom boundary of the auroral acceleration region (AAR) [Pilipenko et al., 2002]. The propagation effects o f structured Pci waves from various sources [e.g., Pilipenko et al., 2005] have been studied by many researchers, and those studies confirmed most of the theoretical predictions. However, we are not aware of any observational study that has unambiguously shown that Pil propagation along the Earth’s surface is due to the ionospheric waveguide. PilB as well as Pi2 are closely associated to the substorm onset, so P ilB are sometimes described as a high frequency extension of Pi2. Pil bursts are not sudden enhancements of broad-band power, but have a fine structure: the frequencies around 0.2-0.3 Hz are often highlighted [Kangas et al., 1978]. This fine spectral feature of P ilB was suggested to be related to the IAR occurrence [Lysak, 1988], or the oscillatory nature of the anomalous conductivity regime in the region of field-aligned currents [Pilipenko et al., 1999]. The temporal evolution of P ilB is sometimes observed to be composed of two stages [Amoldy et al., 1998]. First, a weak increase o f emission intensity occurs in a wide spatial range practically simultaneously at widely separated stations, caused by the propagation in the ionospheric waveguide. In the second stage, the main increase of intensity occurs at individual stations, which moves relatively slowly from one station to another. This stage is caused by the approaching of the auroral intensification region. There is still no generally accepted view of the mechanism of the primary P il source. There were suggestions that P il is either an ionospheric phenomenon, caused by fluctuations o f the electron precipitation [Arthur and McPherron, 1980], or a magnetospheric phenomenon, caused by bursty plasma flow in the magnetotail [Lessard et al., 2006]. The substorm on March 23, 2007 has been studied by the Themis community [Keiling et al., 2008]. In this paper we will augment their analysis by examining the 86