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.

“Physics o f Auroral Phenomena", Proc. XXXVI Annual Seminar, Apatity, pp. 1 3 -1 6 , 2013 © Kola Science Centre, Russian Academy of Science, 2013 Polar Geophysical Institute BEHAVIOR OF IONS NEAR THE SUBSTORM ONSET FROM THEMIS OBSERVATIONS T.V. Kozelova, B.V. Kozelov (Polar Geophysical Institute, Apatity, M urmansk region, 184209 Russia) Abstract. The substorm associated slow and fast changes of ions near the Earthward edge of plasma sheet is examined using data from THEMIS-C during the late growth and early expansion phases on January 6, 2008. Near the equatorial plane at r~ 6-6.5 RE, the spacecraft observed both a signatures of the b2i and the 29 keV ion isotropy boundaries. The convection boundary of 10-keV electrons was embedded in this ion region. A few minutes before the substorm onset, simultaneously with an intensification o f auroral arc in same longitudinal sector, the oscillations of the E and В fields and particles with period -50-60 s start near this convection boundary. During substorm onset, the injection of higher-energy (81-157 keV) ions occurs simultaneously with the sharp pressure drop of ions with the energies less than 29 keV. The observed variations near substorm onset are consistent the standing Alfven waves coupled to slow magnetosonic mode. Our analysis supports the idea about the ballooning instability in transition region between the inner magnetosphere and active plasma sheet as associated with the initiation of substorm onset. 1. Introduction The Near-Earth Neutral Line (NENL) [Hones, 1979] and the near-Earth Current Disruption (CD) [Lui, 1991] models are two basic models of substorm onset and expansion. They are distinguished by the temporal sequence in the formation of the substorm current wedge (SCW) and NENL [Ohtani, 2004]. The debate about the first substorm process is continued up to now, because the detailed analyses of the field and particle fluctuations in the near-Earth tail may provide clues to understanding of basic substorm onset processes. A well-known signature of the substorm expansion phase, the 'dipolarization' is accompanied by the injections of the particles and high level of magnetic fluctuations. Sometimes, ~1 min prior to the sharp substorm dipolarization, the Bz magnetic component and the magnetic field elevation angle at r <9 RE suddenly decrease. This short interval before the dipolarization was named the explosive growth phase (EGP) [Ohtani et al., 1992]. Cheng and Lui [1998] proposed that the kinetic ballooning instability with a period of 50-75 s can cause EGP. Kozelova and Kozelov [2012] reported a detailed analysis of explosive local magnetic field line stretching just before dipolarization observed by THEMIS-C satellite during the breakup followed by local substorm of 6 Jan 2008. Basing on the simple line-current model, Kozelova and Kozelov [2012] proposed that observed magnetic field and particle variations can be a manifestation of the magnetospheric generator of the 3- D meridional current system with the driving electric field in the meridional direction during nonlinear growth of ballooning instability when non-MHD processes are also turning on. Here we continue the consideration of the substorm event on January 6, 2008, which was began by Kozelova and Kozelov [2012]. We further analyze the state of the magnetospheric plasma in the outer region of the trapped energetic ions and demonstrate the structure of the low-frequency waves observed by THEMIS-C at -6.3 Re prior to the local substorm onset. 2.1 Ground-based observations As was shown in [Kozelova and Kozelov, 2012], the evening of 6 January 2008 was characterized by ground magnetic activity of 500-800 nT. At -18:31:50 UT, the brightening began at the arc that was equatorward of all arcs previously existed during growth phase in the field of view of Loparskaya all-sky camera (64.9°N; 113.6°E in CGM coordinates). Azimuthally spaced auroral foldes move from the east to the west. This was the first stage of a (pseudo-)breakup, as it was discussed by [Voronkov et al., 2003]. At the moment 18:33:50 UT, the most equatorial auroral arc was brightened again and the auroral breakup began at the moment -18:34:19 UT. Then the vortex formation and its spatial expansion constitute together the second stage of a breakup [Voronkov et al., 2003]. For the interval 18:30-18:44 UT, the THEMIS-C footprint (traced by TSY-98C model) was located nearly auroral breakup region (see Fig.l from Kozelova and Kozelov, 2012). 2.2 THEMIS satellite observations Near substorm onset under consideration the THEMIS- C satellite was located at (X,Y,Z) = (-6.3, 2.17, -1.8) RE in GSM coordinates. The THEMIS-C satellite observations are shown in Fig. 1-3. Fig.l presents the electron (from 6 eV to 30 keV) spectrum, ion density (from 5 eV to 25 keV), six components of the total ion pressure tensor and total electron pressure. Hereinafter the auroral breakup marked as a moment ‘Г. Remember here from paper Kozelova and Kozelov [2012], that the considered auroral breakup was accompanied by a sudden drop of Bz component (‘explosive stretching') with rate dBz/dt = 0.8 nT/s and duration approximately 15 s. After this stretching the THEMIS-C observed a slow dipolarization with small-scale dipolarization bursts of dBz >0 in moments ‘2 ’ and ‘3 ’. Here we analyze the dynamic changes and the role of ions both in substorm physics and in a precondition for substorm onset in the premidnight sector of the magnetosphere. The details of the ion behaivior one can see also in Fig. 2, where the ion flux variations for energies 0.1 - 419 keV observed by ESA and SST detectors of THC are shown. 13