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.

Behavior o f ions near the substorm onsetfrom THEMIS observations gradient (or ballooning) modes [Roux et al., 1991; Hotter et al., 1995]. As was shown in [Kozelova and Kozelov, 2012], in the end of this substorm growth phase, the convection boundary of 10-keV electrons was embedded in this region near the ~ 29 keV ion isotropy boundary. 2.2.2 Substorm ion injection. During substorm onset one can see: (i) both the (<29 keV) ion density and pressure are sharply dropping (Fig.l). After this drop the ion pressure was not isotropic. The non-zero non-diagonal components of ion pressure tensor indicate existence of dissipation and violation of the frozen-in condition. Beside, (ii) the short-living injection of fresh higher-energy ions (81 -157 keV) (population '3 ’) occurs (Fig.2). Note that in this time, the THC observed the negative Ey component [Kozelova and Kozelov, 2012]. The decrease of the electron flux at the open drift paths (Fig.l) indicates that there was no source of electrons tailward the satellite in this time. Simultaneous sharp drop of the low-energy (< 29 keV) ion flux indicates that the old source of this ions (the dominant current carriers) is insufficient to provide the current required for the more tail-like magnetic field configuration. We suggest that the injection of higher-energy ions (81 -157 keV) observed in this time occurs as a result of serpentine cross-tail motion of the ions establishing the plasma sheet current system [ Speiser, 1965]. Note finally, that (iii) the anticorrelation of the high energy ions and electron fluxes appears. This is usually observed during passage of the WTS across the spacecraft meridian and can be associated with the interchange instability [Roux, 1985; Roux et al., 1991; Kozelova et al., 1986; Kozelova et al., 2006]. The substorm onset is a source of these fresh westward drifting energetic ions (protons). These particles with a sharp gradient of the particle flux in the direction of the gradient-B drift may excite the drift-wave resonance of magnetic field lines [Cladis, 1971] and support (increase) the particle flux oscillations on the transitional boundary from the inner magnetosphere to active plasma sheet. Thus, in the end of considered substorm growth phase, the THC was located at the region near the -29 keV ion IB, which become steeper and converge both with the convection boundary for ~ 10-keV electrons and the boundary of transition between different configurations of the magnetic field. This convergence leads to exposure one more boundary, namely, a boundary between adiabatic and non-adiabatic ion motion, which is very important for non-linear developing of plasma instability as was discussed by [Kozelova and Kozelov, 2012]. Just in this region the weak variations with period ~50-60 s are beginning simultaneously with the occurrence of the most equatorial auroral arc ~2 minutes before substorm onset [Kozelova and Kozelov, 2012]. 2.2.3 Oscillations with ~50-s period. In this section, we demonstrate the structure of the low- frequency waves observed by THEMIS-C prior to the local substorm onset on 6 January 2008. We use an approach, when the total magnetic field is sum of slow changes and more fast variations of magnetic field. The sliding 54s-smoothed values of the magnetic field, <B>, demonstrate slow changes of the field, while the deviations of measured magnetic field from these smoothed values present more fast fluctuations of the magnetic field (dBx, dBy, dBz). The same approach was used for the electric field. The Fig. 3 shows these more fast fluctuations В and E fields observed by THEMIS-C in the interval near the substorm onset under consideration. Note that the dBy component was more variable than other components and wherefore the bottom panel presents 18s-smoothed values o f this dBy component. On the Fig. 3, the letters к, I, m, n, p and q note the moments of the small peaks (enhancements) of ion pressure near the substorm onset. Themis C, Jan 6. 2008 Fig. 3. Low-frequency waves observed by THEMIS-C near the local substorm onset on Jan 6, 2008. From top to bottom: the fluctuations of the magnetic field dBt and its components. dBx and dBy are noted by solid lines. Dotted curves in panels 2 and 3 present dEy and dEx components of the electric field. From Fig. 3 one can see, that before substorm onset: (i) the magnetic field variations with period -50 s are dominated in the X direction, dBx>dBz>dBy (dBz not shown); (ii) The perpendicular fluctuations of the magnetic field and the electric field dBx and dEy are -90 degrees out of phase as for standing AlfVen waves; (iii) the ion pressure dPi oscillations and the magnetic dBt oscillations are -180 degrees out of phase as for the slow magnetosonic mode. We can deduce that the observed magnetic field variations before substorm onset are consistent the standing AlfVen waves coupled to slow magnetosonic mode. This conclusion corresponds with results [Holter et al., 1995; Kozelova et al., 2004; Saito et al., 2008a,b]. These oscillations may be excited by the ballooning instability. Thus, our analysis supports the idea about the ballooning instability near the inner edge of the plasma sheet as a mechanism associated with the initiation of substorm onset. 15