Physics of auroral phenomena : proceedings of the 36th Annual seminar, Apatity, 26 February – 01 March, 2013 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2013. - 215 с. : ил., табл.

V. Pilipenko et al. AN A P _ Иф (5) N - i a + /3 со According to (5), for co~2*10'2 s'1, T~103К, ДТ-250 K, Afi~10'4 s'1, the expected effect is to be AN/N~0.5%. Discussion ULF pulsations are the most powerful wave process in the terrestrial environment and they can significantly modulate the magnetospheric and ionospheric plasma. The radar community has demonstrated that Pc5 waves can modulate the ionospheric electric field and plasma convection velocity; field-aligned current; E-layer electron density and the ionosphere conductance; and electron Te and ion T, temperatures in F- and E-layers. Our observations demonstrate that these waves are capable to modulate TEC as well. Moreover, our observations, as well as other reports, reveal that the relative disturbance o f TEC is even larger than that o f magnetic field. The simultaneous Pc3 waves and TEC periodic fluctuations presented in [Skone et al., 2008] had peak-to-peak amplitudes ANT ~ 0.02 TECU, and ДВ -2 nT, so assuming that typical NT ~ 40 TECU, the relative TEC fluctuations ANT/NT~ 0.05%, whereas the relative magnetic fluctuations ДВ/В ~ 0.004% only. Davies and Hartman [1976] reported the percentage change in TEC ~0.03% and ~0.006% with associated magnetic variations in the range 30—50 s. Okuzawa and Davies [1981] reported that variations in TEC over the 10—50 s band ranged from 0.005% up to 0.1%. Assuming that typical Pc4 wave amplitude is ДВ-10 nT, for background B~3*104 nT, these pulsations had the relative amplitude in the ionosphere -0.03%. Thus, the reported TEC fluctuations are to be caused by magnetic pulsations with much weaker relative amplitudes. In the analyzed Pc5 event of Oct. 31, 2003 Nr - 40 TECU, ANT-1 TECU, hence ANT / NT~ 2.5%. At the same time, the ground magnetic pulsations ДВ - 400 nT, so ЛВ/В - 1%. Thus, during this event, the relative amplitude of TEC variations is about the same or even -2-3 times larger than that of magnetic pulsations. Comparison o f possible mechanisms of TEC modulation by Alfven waves indicate that the mechanisms o f periodic plasma heating and vertical plasma drift might be the most promising. However, any conclusive judgments can be stated only after more detailed studies with the use of other ionospheric instruments and more advance theoretical modeling. Conclusion GPS TEC technique has turned out to be unexpectedly sensitive to ULF waves. Combined usage of magnetometers and GPS/TEC is a very promising way to reveal the physical mechanism o f disturbances. Both earlier observations and the current study have revealed that relative TEC disturbance is about or even larger than that o f the magnetic field. The most promising mechanisms are the periodic ion heating and vertical plasma drift. However, the effect of TEC modulation by Pc5 waves is a challenge for the MHD wave theory, because the responsible mechanism of such modulation has not been firmly established yet. A cknow ledgem ents. This study was supported by NSF grant ATM-0827903 (DM, ME), RFBR grants 12-05- 98522 (EF), 13-05-90436 (VP), 12-05-00273 (VB), and Program № 4 0f the RAS Presidium. References Cran-McGreehin A.P., et al., Ionospheric depletion in downward currents, J. Geophys. Res., 112, A10309, 2007. Davies K., Hartman G.K., Short-period fluctuations in TEC, J. Geophys. Res., 81, 3431, 1976. Lathuillere C., et al., Ionospheric ion heating by ULF Pc 5 magnetic pulsations, J. Geophys. Res., 91, 1619, 1986. Okuzawa Т., Davies K., Pulsations in the total columnar electron content, J. Geophys. Res., 86, 1355 1981. Pilipenko V.A. Fedorov E.N., Modulation of TEC by geomagnetic pulsations, Geomagn. Aeron., 34, 516 1995. Pilipenko V., et al., Determination of the wave mode contribution into the ULF pulsations from combined radar and magnetometer data: Method of apparent impedance, J. Atmosph. Solar-Terr. Phys., 77, 85-95 2012 Poole A.W.V., Sutcliffe P.R., Mechanisms for observed TEC pulsations, J. Atmos. Terr. Phys., 49, 231 1987 Skone C., Mitigating geomagnetic noise in airborne magnetic surveys using GPS, in- ION GNSS 2008 Savannah USA, 2008. Shagimuratov I.I., et al., Observations of GPS TEC fluctuations in Antarctic and Arctic ionosphere during 28 October 2003 storm, "Physics of Auroral Phenomena", Proc. 31 Annual Seminar, Apatity, 167 2008 ? Waters C.L., S.P. Cox, ULF wave effects on high frequency signal propagation through the’ ionosphere Ann Geophys., 27, 2779, 2009. ’ 80