Physics of auroral phenomena : proceedings of the 39th annual seminar, Apatity, 29 February-4 March, 2016 / [ed. board: N. V. Semenova, A. G. Yahnin]. - Апатиты : Издательство Кольского научного центра РАН, 2016. - 167 с. : ил., табл.

Ionosphere response to the intense ULF waves as observed by GPS/ГЕС and EISCAT instruments 4. Discussion Long-period pulsations are the most powerful wave process in the near-Earth environment. The radar observations showed that Pc5 waves can noticeably modulate the ionospheric plasma: the electric field E, plasma convection velocity V, E-layer electron density Ne and the ionosphere conductance I . and electron Te and ion T, temperatures in both F- and £4ayers (see references in [Pilipenko et al., 2014a]). Recent observations by [Pilipenko el al., 2014a; Watson et al, 2015] have demonstrated that Pc5 waves are capable to modulate TEC as well. One may expect that all the Pc5 wave-induced fractional variations of plasma and magnetic field should be of the same magnitude, like in any linear wave. However, GPS observations have revealed that the depth of periodic TEC modulation is sometimes even somewhat larger (e.g., in the event of Oct. 31, 2003 ANT / N r ~ 2.5%) than the geomagnetic field modulation (A B/Bq - 1%). In principle, ULF modulation of energetic electron precipitation, inducing an additional periodic ionization of the lower ionosphere, can cause periodic TEC variations with much higher depth than geomagnetic field variations [Watson et al., 2007]. However, during the event under consideration no periodic electron precipitation occurred as evidenced by simultaneous riometer observations. The mechanism of the field-aligned plasma transport by Alfven waves can produce relative amplitudes of TEC variations larger than that of geomagnetic pulsations. Consideration of possible mechanisms of TEC modulation by magnetospheric Alfven waves has shown that in principle the plasma heating, vertical plasma drift, steep gradient, and field-aligned electron transport can provide a noticeable input into the observed TEC variations transport [Cran-McGreehin et al., 2007]. The field-aligned current transported by an Alfven wave, incident onto the ionosphere from the magnetosphere, provides an additional periodic plasma flow in/out the ionosphere. As a result, the plasma density in the bottom ionosphere periodically increases/decreases. A feature of the field-aligned plasma transport mechanism is that it contributes mainly into the bottom layers of the ionosphere, in accordance with the combined GPS/EISCAT/magnetometer observations. Surely, any conclusive judgments can be staled only after more detailed studies with the use of other ionospheric instruments that will provide more detailed information about ionospheric plasma parameters. 5. Conclusion Long-period Pc5 pulsations being the most powerful wave process in the terrestrial environment can significantly modulate the local densities of the magnetospheric and ionospheric plasma. Even radiopath-integrated TEC has turned out to be sensitive enough to response to intense Pc5 waves. So far, the effect of TEC modulation by ULF waves is a challenge for the MHD wave theory, because responsible mechanisms of such modulation have not been firmly established yet. Analysis of the altitude profile of the electron density fluctuations derived from EISCAT data during the global Pc5 wave event has shown that main contribution into the periodic TEC variations is provided by lower ionosphere, up to -150 km, that is the E-layer and lower F-layer. This observational fact favors the field- aligned plasma transfer induced by Alfven wave as a dominant modulation mechanism. A cknow ledg em en ts. This study was supported by the grant RFBR 16-35-60049 mol_a_dk (BV), RFBR grants 15- 45-05108 (VP) and 15-05-01814 (EF). Dual-frequency 30-s rate GPS measurements are freely available as daily data files in RINEX format from the IGS. We are indebted to the staff of EISCAT for supplying the data. We thank the institutes who maintain the IMAGE magnetometer array. The riometer data originated from the IRIS, operated by the Lancaster University (UK) in collaboration with the Sodankyla Geophysical Observatory. References Pilipenko V., V. Belakhovsky, D. Murr, E. Fedorov, M. Engebretson. Modulation of total electron content by ULF Pc5 waves, J. Geophys. Res., 119,4358-4369. 2014a. Watson С., P.T. Jayachandran, H.J. Singer, R.J. Redmon, D. Danskin. Large-amplitude GPS TEC variations associated with Pc5-6 magnetic field variations observed on the ground and at geosynchronous orbit, J. Geophys. Res., 120, doi:10.1002/2015JA021517. 2015. Pilipenko V., V. Belakhovsky, A. Kozlovsky, E. Fedorov, K. Kauristie. ULF wave modulation of the ionospheric parameters: Radar and magnetometer observations, J. Atmosph. Solar-Terr. Physics, 108, 68-76. 2014b. Kleimenova N.G., O.V. Kozyreva. Spatial-temporal dynamics of Pi3 and Pc5 geomagnetic pulsations during the extreme magnetic storms in October 2003, Geomagnetism and Aeronomy (English translation), 45, 71-79. 2005. Cran-McGreehin A.P., A.N. Wright, A.W. Hood. Ionospheric depletion in downward currents, J. Geophys. Res., 112, A 10309. 2007. 43