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.

“P hysics o f Auroral Phenomena", Proc. XXXIV Annual Seminar, A patity, p p . 1 3 - 1 6 , 2011 Polar © Kola Science Centre, Russian Academy of Science, 2011 vwMl Geophysical W У Institute CHANGES IN THE MAGNETOSPHERIC CROSS-TAIL CURRENT DURING SUBSTORM AS OBSERVED BY THEMIS T.V. Kozelova, B.V. Kozelov (Polar Geophysical Institute, Apatity,Murmansk region, 184209 Russia) Abstract. Magnetic field variations measured by the THEMIS at the midnight magnetosphere are studied to estimate the changes in the cross-field current during the substorm at 4-13 RE. The line current model has been used to simulate equivalent differential currents dJ. We found that the magnetic field and particle flux changes depend upon the position of the observing spacecraft relative to the region of current disruption, which develops discretely by creating new localized CD region outside previous activity. The dynamics of the CD region corresponds to the subsequent development of the ground activity in the given sector. 1 Introduction The changes of the magnetic field during a substorm in the near-Earth magnetotail follow well established pattern. The growth phase magnetic field line stretching are due to an increase in the intensity and/or earthward motion of the cross-tail current sheet. At substorm onset there is a reconfiguration of the magnetic field toward a more dipolar orientation. This reconfiguration is accompainied by an expansion of the plasma sheet and an injection of energetic particles. Russell and McPherron [1973] and Moore et al [1981] concluded that the magnetic field dipolarization and the injection of particles propagated toward the Earth with velocity ~ 10-100 km/s. Jacquey et al [1991] found that the current disruption (CD) was initially located at 6-10 R e and then CD expanded tailward with velocity ~ 320 km/s. Lui et al [1988] concluded that within the CD region (at 8.1 RE for that event) the magnetic field has the turbulent behavior and the complex magnetic field geometry. Magnetic field variations can be used to measure changes in the current distribution, even if the changes take place far from the spacecraft and on different field lines. This is in contrast to electric field and particle flux variations. Kozelova et al [1998] using the B-field variations found that both earthward and tailward displacements of the individual CD with velocity 200- 1000 km/s were observed by CRRES at distance 5-7 R e . Impulsive westward electric field (~ 24 mV/m peak) coincides with the local dipolarization, the increases of В-field oscillations and nearly dispersionless energetic particle injection [Kozelova et al., 2001]. In this time the eastward perturbation current appears ~ 1 RE tailward the CRRES. In this paper we will examine the ground magnetic variations and magnetic field perturbations from the THEMIS data to estimate the dynamics of initial current disruption (CD) site and the CD displacements at 4 -13 R e during the substorm intensifications. 2 Model for current perturbations in magnetosphere We assume that localized current perturbation in magnetosphere may be deduced from differential magnetic field perturbation dB(t)=B(t+dt) - B(t). The line current model has been used to simulate equivalent current dj, associated with this perturbation dB. We estimate the magnitude, orientation, and location of the current dj using the Biot-Savart law. The eastward differential current djE may signify the occurrence of the CD [Kozelova et al., 1998]. We expect that the initial disruption region and its dynamics can be determined by examining the spatial-temporal distribution of westward and eastward currents perturbations c//w and djE. 3 Ground observations During a selected event on 6 Jan 2008, two of the THEMIS spacecrafts A (THA) and С (THC) were located about 20° eastward the Scandinavian sector at ~ 20 UT and come nearer to Scandinavia at -22 UT. The observations made by the Scandinavian magnetometers show the several increases of magnetic activity (intensifications). We mark out four ones, Fig.l. At ~ 20:20 UT the first weak intensification began at high- latitude stations LYR-HOR (L ~ 13.8). At ~ 20:40 UT the second intensification began southward of the first one (at HOR-BJN). IMAGE magnetometer network 2000-01 -06 1 m in e i v w ig ii I I I I I I I I I I ' I Г""" I I____I__I____1__I____I__I____I__I—__1__I__ 20:00 20:30 21:00 21:30 22:00 223 0 23:00 Hour (UT) Fig. 1. Magnetic field perturbations at the Scandinavian magnetometers. 13