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.

Т. V. Kozelova and В. V. Kozelov THEMIS-C, January 6. 2008 THEM1S-C, 6 January 2008 10000 2200.0 2000.0 18:20 18:30 18:40 UT Fig. 2. THEMIS-C data between 18:20 and 18:46 UT on Jan 6, 2008. ESA and SST ion fluxes. 2.2.1 Ion isotropic boundary. During the growth phase before 18:28 UT, the ion flux changes depend on the particle energies (Fig.2). The flux of the higher-energy (> 29 keV) ions is decreasing. These particles (a population 7 ’) are ‘old’ trapped particles on the closed drift paths [ Walker et al., 1976; Sauvaud and Winckler, 1980] at the region with quasi-dipolar magnetic field lines. The flux of low-energy (< 29 keV) ions is increasing or nearly constant. These particles (population ‘2 ’) are convecting toward the THC from the tail and associated with the increase o f the cross-tail current during the substorm growth phase. Nearly constant and maximum flux of 29 keV ions (Fig.2) and the isotropic pressure of the low-energy (< 29 keV) ions (Fig. 1) may testify about both the b2i boundary [Newell et al., 1996a, b] and the ion isotropy boundary (IB) [Sergeev et al., 1983]. Note that the geophysical significance of these boundaries is that they represent a good approximation to the earthward edge o f the tail current sheet [Sergeev et al., 1983; Sergeev and Maikov, 1988; Newell et al., 1998]. In the nightside sector they are usually observed at the distance range from 5 to 10 Re- Due to different physical processes, these boundaries are formed on the different time scales and sometimes may be quite sharp. In our event, before substorm onset, the b li boundary and the 29 keV- ion IB were observed at radial distance 6.3 R e . We suppose that these ions from population ‘2 ’ maintain the cross-tail current structure in the (quasi-) dipolar region o f the magnetosphere and play an important role in the substorm onset. This region in the magnetosphere can be a source for unstable pressure F ig.l. THEMIS-C data between 18:30 and 18:44 UT on Jan 6, 2008. From top to bottom: ESA electron energy flux (< 30 keV), density of ions (<30 keV), six components of pressure tensor o f these ions, electron (< 30 keV) pressure.