Physics of auroral phenomena : proceedings of the 35th Annual seminar, Apatity, 28 Februaru – 02 March, 2012 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2012. - 187 с. : ил., табл.

“Physics ofAuroral Phenomena", Proc. XXXVAnnual Seminar, Apatity, pp. 71 - 74, 2012 ©Kola Science Centre, Russian Academy of Science, 2012 ULF ACTIVITY IN THE AURORAL OVAL AS OBSERVED BY THE MICROSATELLITE ASTRID-2 AND THE GREENLAND CHAIN V. Pilipenko1, T. Neubert2, N. Ivchenko3, G. Marklund3, L. Blomberg3, F. Primdahl 4 ;Space Research Institute, Moscow 2Danish Space Research Institute, Copenhagen 3Alfven Laboratory, KTH, Stockholm 4Danish Technical University, Copenhagen Abstract. Composition of ULF observations along the Greenland magnetometer chain and measurements of electromagnetic fields in the upper ionosphere on-board microsatellite Astrid-2 gives possibility to study the coupling between auroral arc, auroral electrojet and electromagnetic disturbances of different spatio-temporal scales. Spectral analysis of electromagnetic bursts with the MEM technique reveals the spectral power enhancements at frequencies -few Hz, corresponding to spatial scales -few km. The possible mechanism of the frequency-dependent enhancement of electromagnetic noise may arise during the resonant conversion of ULF disturbances into dispersive Alfven waves. Introduction The localized electromagnetic bursts observed by low-orbiting satellites at auroral latitudes could be used as simple indicators of field-aligned currents (FAC) [Gary et al., 1998]. However, the physical nature of these bursts must be clearly identified for a reliable application. So far, most studies were devoted to the analysis of intense events detected by satellites, especially in the cusp and night auroral region [Chaston et al., 1999]. Here we consider typical features of electromagnetic bursts observed during the satellite pass through FAC systems with moderate intensity in the morning/dusk sector. In this study we use the high sensitivity measurements of electric and magnetic fields made with the EMMA instrument onboard the Astrid-2 micro-satellite [Marklund et al., 2001]. Auroral Electrojet and Pc5 waves: image on ground and in space In virtually every pass of Astrid-2 through the auroral oval in dawn and dusk hours, even with weak auroral and magnetic signatures, bursts of wide-band noise are observed in both electric and magnetic components. We have analyzed more than 20 events during February-March 1999, when moderate magnetic activity is observed at the Greenland array. As a typical example, we consider the ascending orbit of Astrid-2 for Feb. 14, 1999 (DOY=045). Its geomagnetic projection on the ionosphere passes over Greenland west coast array around 1115 UT. At Greenland magnetometer array the intensification of westward auroral electrojet (AEJ) is observed, producing maximal H-component disturbance -300 nT around 10 UT at station SKT (geomagnetic latitude 72.0°). The intensification of westward AEJ is accompanied by an enhancement of ULF activity in the Pc5 band (central frequency - 3 mHz) with amplitudes up to few tens of nT. The localization and dynamics of AEJ and Pc5 ULF activity are both displayed in Fig. 1. The distribution of the ionospheric east-west current, reconstructed from ground H and Z magnetic data using the technique [Popov et al., 2001], shows that in the interval 0930-1130 UT the westward AEJ is centered at CGM latitude - 72°. On the same plot in Fig. 1 the spatial-temporal distribution of ULF total power in the band 2-8 mHz is shown by contour lines. As can be seen from this plot, ULF intensity is closely coupled with the AEJ: the ULF spectral power reaches maximum at the same latitude and evolves in time in a similar way as the AEJ. When Astrid-2 crosses at -1114 UT the probable latitude of AEJ an electromagnetic burst is detected by onboard magnetometer and electric sensor. Most evidently the burst can be seen in electric (El) component. Variations of the magnetic field (В 1 component) have been spline-interpolated and then differentiated to derive an estimate of FAC. The resultant FAC and high-pass filtered (cut-off frequency 0.1 Hz) components are shown in Fig. 2. The burst occurrence coincides spatially with the peak of FAC intensity, - 2pA/m2, at ~11:13:50 UT. Even visual inspection of detrended data reveals quasi-periodic variations of electric (especially evident in El, -30 mV/m) and magnetic (B2, -15 nT) components. However, with common spectral analysis it is hard to identify the actual frequencies of a burst, because the data are contaminated with the despun interference in the same frequency band. Using the dynamic spectrogram based on the maximum entropy method (MEM), which provides a better resolution for short impulsive signals as compared with standard FFT, it was possible to differentiate spectral peaks caused by spin-related interference and burst. The MEM spectrogram reveals the presence of quasi-periodic component with / - 3.5 Hz immersed into the electromagnetic burst. The assumption that the response at this frequency corresponds to spatial variations of electromagnetic field gives the relevant scale -2 km. Polar Geophysical Institute 71