Physics of auroral phenomena : proceedings of the 34th Annual seminar, Apatity, 01 - 04 March, 2011 / [ed.: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 231 с. : ил.

Т. V. Kozelova, The next (third) intensification was the most intense and it was observed more southward of other ones (X ~ 5.5). The gradual initiation of the substorm was observed at stations IVA-MUO-PEL at ~21:05 UT. Further quick increase of the westward electrojet was observed at all stations from NOR to RVK at ~ 21:10 UT. This is associated with the substorm explosive phase. This evolution of ground magnetic perturbations is consistent with the behavior of aurora observed from Loparskaya and SOD. The bright western edge of WTS moved overhead of SOD at ~21:10 UT. At ~22:55 UT the last intensification observed at high-latitude stations LYR-BJN may be associated with a disturbance on the northern edge of auroral bulge. 4 Observations Fig. 2 presents the value (in relative units), orientation and location of currents dj (for dt = 30 s) estimated from the THA and THC in the interval 20:00 - 21:30 UT. At initial moment the THA was near inner edge of the plasma sheet, and the THC was located tailward and below the THA. The GSM coordinates of the satellites were rTHA= (-5.2, 1.6, -1.2) and /-THc = (-8.6, 1.6, -2.7) in Earth’s radii. Both spacecrafts were below the plane of the current sheet. From 20 UT to 21:05 UT (before the substorm onset 43'), the large scale magnetic field line stretching was observed both THA and THC. In this time the westward current djw was located at r~ 5-10.5 RE. Monotony of the field line stretching was violate near the moments 'tl' and 42', when enhanced djB appeared at r ~ 5-6 REin association with the first two intensifications. The largest changes of the differential currents occur during the intensifications 't3' and 44', which we will consider in details. Fig. 2. Magnetic field variations observed by THA and THC and deduced from them current disturbances. 4.1 The substorm The growth phase. At 20:50-21:00 UT the current dj^ , more intensive at -10 Re, presents the magnetic field line stretching observed both at гхнд = (-6.5, 1., -1.8)Re and rTHC= (-9.5, 1.7, -3.25)RE(Fig. 3). At the THA, the average energy of the low energy electrons increases (not shown) that is commonly interpreted as the passage В. V. Kozelov across of the Alfven convective boundary for hot electrons (on the inner edge of the plasma sheet). At 20:58-20:59 UT the d jE appears at ~ 5-6 RE simultaneously with increase of 1-7 keV electron flux. We suppose that this current may be the local dawnward inertia current near the breaking point of the earthward flow of the plasma. Fig. 3. The same as Fig.2, but for the substorm interval. Stage o f dj\\< increasing with low-frequency pulsation. At 21:00-21:05 UT the current djw increases locally near the THC projection on XY plane. In this time, the particle fluxes rapidly decreased at the THC, and the B l0, increases (not shown) that is typical for observations outside the current sheet. The drop of the particle fluxes is marked as horizontal bar in panel 3 of Fig.3. Thus, the increase o f the djw at ~10 Re results in the plasma sheet thinning. Besides, during this interval the magnetic and electric fields and particle fluxes were oscillated with period of 2 min (not shown). The appearance of the oscillations may be interpreted as a beginning of transition from the quasi-steady to more unstable (or turbulent) state of the plasma sheet. Then, short appearance of the djE (at ~ 10 RE) at ~ 21:01 UT (the moment 'ta') may be associated with the first local "current disruption. However large scale magnetic field line stretching observed both at the THA and THC continues and the djE sinks quickly. This evolution of magnetic perturbations in the magnetosphere is consistent with the behavior of aurora from Loparskaya and SOD. Before 21:01 UT a weak auroral arc was located in the northern edge of the Loparskaya field of view. At 21:01 UT a new arc appeared in the Loparskaya zenith. At 21:02 UT a new auroral spot appeared to the east of SOD zenith and moved to the south. Slow 'dipolarization'. In the interval 21:05-21:09 UT (from 'ta' to 'tb') the weak current yEplaced at 10.5 - 11 Re results in slow increase z-component of magnetic field at the THC. Slow (and nearly smooth) changes of В and E fields have no explosive character (slow 'dipolarization'). At ~ 21:05 UT the particle fluxes restore after the drop. We suppose that these field and particle variations could be associated with a local thickening (along the Z axis) of plasma sheet with small current reduction overhead the THC. 14