Physics of auroral phenomena : proceedings of the 35th Annual seminar, Apatity, 28 Februaru – 02 March, 2012 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2012. - 187 с. : ил., табл.

“Physics o f Auroral Phenomena", Proc. XXXV Annual Seminar, Apatity, pp. 21 - 25, 2012 © Kola Science Centre, Russian Academy of Science, 2012 Polar Geophysical Institute THEMIS OBSERVATIONS OF SUBSTORM INTENSIFICATION NEAR INNER EDGE OF THE PLASMA SHEET T.V. Kozelova, B.V. Kozelov (Polar Geophysical Institute, Apatity, Murmansk region, 184209 Russia) Abstract. We report a detailed analysis of explosive local magnetic field line stretching just before dipolarization observed by THEMIS-C satellite during the pseudo-breakup followed by local substorm of 6 Jan 2008. In the end of substorm growth phase THEMIS-C was located at r ~ 6.3 Re near the convection boundary of 10-keV electrons. Observed oscillations of fields and particles with period 50-60 s are consistent with the ballooning mode in the near-Earth magnetotail and with the near-Earth initiation CD model of substorm. The explosive stretching and following dipolarization observed by THEMIS-C satellite are a manifestation of the magnetospheric generator of the 3-D meridional current system of В type [ Bostrom , 1964] during non­ linear growth of ballooning instability when non-MHD processes are also turning on. 1. Introduction It is know that the return of magnetic field to more dipolar configuration, or 'dipolarization', is a well- known signature of the substorm expansion phase in the magnetosphere. There are two general ideas to explain the substorm dipolarization. One idea is the braking of bursty Earthward plasma flow from NENL, leading to magnetic flux pileup [Haerendel, 1992; Shiokawa et al., 1997]. The proposed mechanism is basically MHD process associated with sunward plasma flow. Another idea invokes a kinetic CD process in the near-Earth region during substorm onset, when substorm dipolarization occurs without significant plasma flow [Lui et al., 1999; Ohtani et al., 2000]. The current disruption (CD) is generally attributed to various forms of ballooning instability [Roux et al., 1991; Holter et al., 1995; Maynard et al., 1996; Kozelova et al., 2006], and the cross-field current instability (CCI) [Lui et al., 1991]. However the instability type dominating the onset process is still not clear. Here we consider a substorm event on January 6, 2008, which began at the inner boundary of the plasma sheet and was observed by THEMIS-C satellite at -6.3 Re. We evaluate the associated changes in the cross-tail current intensity using the line current model to simulate equivalent current perturbations. In section 4 we present a new mechanism for generation of the meridional current system (MCS) during process of explosive stretching. 2.1 Ground-based observations The evening of 6 Jan 2008 was characterized by long- lasting ground magnetic activity of 500-800 nT. At 17:58 UT, the first auroral arc appears near northward horizon of Loparskay (64.9°N; 113.6°E). After small intensification observed on this arc at 18:12 UT, the weak diffuse auroral azimuthal arcs were slowly moving equatorward. 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. This intensification forms azimuthally spaced auroral foldes which 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 7’0=18:34:19 UT, the breakup arc brightened and began to expand rapidly poleward. This process is traditionally called auroral breakup. The vortex formation and its spatial expansion constitute together the second stage of a breakup [Voronkov et al., 2003]. Figure 1 shows four ASC images from Loparskay all­ sky camera, two o f them are mapped in geographic coordinates. The THEMIS-C footprint marked in the maps was traced by TSY-98C model for the interval 18:30-18:44 UT. 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 and moved tailward and westward. This position is magnetically projected eastward of Loparskaya meridian, see Fig.l. Figure 2 presents magnitude of total magnetic field and its components, components of electric field and velocity of plasma flow. Hereinafter we use GSM reference frame system for magnetospheric data. Before 18:32 UT one can see slow stretching of magnetic field with velocity dBz/dt ~ 0.03 nT/s. Weak oscillations in Bt, Bx, Ex, Ey with period - 50 s were beginning after 18:32 UT. In 18:34 UT (marked as a moment ‘Г in figures) it was appears sudden drop of Bz (‘explosive stretching') with rate dBz/dt = 0.8 nT/s and duration approximately 15 s. After 18:34:30 UT the THEMIS-C observed a slow dipolarization with small- scale bursts of dBz>0 in moments ‘2’ and ‘3’. The most intensive burst in the moment ‘3’ was accompanied by large spike of Ey and changing of sign of meridional component of electric field. Oscillations with ~ 50 s period. The Ex oscillations are beginning simultaneously with the occurrence of the most equatorial auroral arc and - 2.5 minutes before To. We suppose that both Ex oscillations and the new arc may be a manifestation of FLR [Samson et al., 1992]. The larger-amplitude field line resonance can be 21