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.

Modulation o f TEC/GPS by ULF Pc5 waves AN N B0 Periodic particle precipitation. In our event no noticeable riometer variations were observed. Though riometer data can detect the precipitation of energetic electrons with E>10 keV, therefore the possibility of periodic low- energy electron precipitation remains unclear. Plasma compression by fast mode. Upon the interaction with the ionosphere the wave properties are expected to be modified considerably, and the problem of TEC modulation needs a special consideration of Alfven wave interaction with the ionosphere. TEC modulation may be related to the plasma/magnetic compression in a wave. An evanescent fast compressional mode can be excited by incident Alfven wave in the upper ionosphere owing to the ionospheric Hall conductance. The mechanism of TEC modulation by an evanescent compressional mode arising during the interaction of an Alfven wave with the anisotropic E-layer was considered by Pilipenko and Fedorov [1995]. According to their estimates, for a large-scale wave with transverse scale larger than the height h of the E- layer (kh< 1) the modulation depth is as follows Л AT T)<S) kh (2) Therefore, relative TEC variations could be comparable with relative magnetic variations only for small-scale poloidal Alfven waves with kh~ 1. However, such small-scale Alfvenic structures are to be screened by the ionosphere from ground magnetometers. Lateral plasma gradient. Another possible TEC modulation mechanism may comprise an advection of lateral gradient of the ionospheric plasma [ Waters and Cox, 2009]. Shagimuratov et al. [2008] found strong TEC fluctuations with periodicity -10 min and associated them with polar patches - large-scale (-100-1000 km) regions of enhanced (by a factor of 2 and more) F-region plasma density travelling through the polar caps due to ionospheric convection. The periodic modulation of polar patch plasma by ULF wave electric field may produce noticeable TEC modulation. However, the feasibility of this effect is hard to evaluate because of the lack of information on lateral F- layer structure. Field-aligned plasma transport. A possible mechanism may be related to field-aligned plasma transport by Alfven waves [Cran-McGreehin et al., 2007]. The field-aligned current transported by an Alfven wave provides an additional in/out periodic plasma flow into the E-layer. An Alfven wave with magnetic disturbance ~400 nT should carry a considerable field-aligned current. For example, Pc5 pulsations carrying ~2(JA/m2 current will cause fluctuations in the E-layer with Ne=10n m"3and Ah=20 km of ANe~6.1010m'3. Thus, the field-aligned current during an intense Pc5 event is sufficient to cause a significant pumping of electron density in the E-layer. However, the relative contribution of the E-layer to the TEC is much smaller than the contribution from the F-layer. Periodic shift of plasma vertical profile. The finite East-West electric component o f the incident Alfven wave Ey causes a vertical plasma drift Vz = Ey cos I / B0- Although the vertical gradient may be severe, there will be no temporal TEC changes provided N= 0 at the end points of integration. However, owing to the strong dependence of ionization and recombination rates on altitude, the vertical shift causes a plasma modification due to the changes of ionization-recombination balance. In the F-layer the recombination rate in (1) is L=PN. The maximum of the vertical profile is formed at the altitude where Q - p N . Let us assume that the recombination coefficient depends exponentially on z , /3 = /?exp[-(z-zm)/H ]. An order of magnitude estimate [Poole and Sutcliffe, 1987] gives Ш_= 2 ^ (3) N co2H For Pc5 with co-10'2s'1, P-10-4, Vz~20E, cos/~0.5, £ -2 0 mV/m, this estimate gives A/V/ j V~1.2%. Ion heating. The heating of the ionospheric ions during the plasma dragging through neutrals (having velocity Pn and temperature Та) can be estimated using the classical relation for the frictional heating [Lathuillere et al, 1986] M ATi - Tn = ^ r ( V < - V J 3k For a typical F-layer plasma the coefficient M„/3k~7.4*10'4. The background plasma velocity is produced by ion dragging by neutrals |Vi(0)|~|Vn|. During time intervals when Vi is anti-parallel to Vn a noticeable ion heating occurs. During the Pc5 event considered here, the oscillatory E-field component -20 mV/m imposed on the background field E<0)~30 mV/m corresponds to a convection velocity disturbances Vj~ 400 m/s. Assuming that Vi-Vn=300 m/s, Гп=103K, it follows from (4) that the ion Joule heating can cause А71~250°. The additional plasma heating shifts the ionization-recombination balance due to the temperature-dependent recombination coefficient p and causes plasma density variations. The coefficient P(7) of the dominant F-layer ion 0 + with 0 2 and N2 grows rapidly with temperature T, starting from Г-103K. Thus, periodic ion heating of F-layer plasma T+AT will cause the decrease of the recombination coefficient р+др, and consequently, the periodic plasma density variations N-N+AN . Linearization of the balance equation (1) yields 79