Physics of auroral phenomena : proceedings of the 38th annual seminar, Apatity, 2-6 march, 2015 / [ed. board: A. G. Yahnin, N. V. Semenova]. - Апатиты : Издательство Кольского научного центра РАН, 2015. - 189 с. : ил., табл.

Т. V. Kozelova and В. V. Kozelov increase of ion pressure gradient near the isotropic boundary for ions. Short (10-15 s) splash of low-energy (0.1-1 keV) electrons occurs during the Vv upsurge maximum. This electron splash was accompanied by short (localized) the magnetic field dBy variation with the change of dBy sign, which can be interpreted as the field-aligned current Ун transition by THE. After this 0.1-1 keV electron splash, the dipolarization at THE begins simultaneously with the electron pressure enhancement and the increase of electron Ecu, from Wcu, ~8 keV to Wcu, ~ 28 keV. Thus, we see similar structure of the phenomena observed by THE and THD, namely: sharp drop of the ion pressure and simulteneous sharp upsurge in the duskward ion drift velocity Vy. First, this structure was observed at THE at MLT=22.5, then - at more western THD at MLT=21.65. Judging from 60 s delay between THE and THD, this structure westward espanded with the velocity of 0.85 deg/min. This westward motion is slower than the azimuthal spreading of the onset arc brightening which occurs above APT ~ 30-40 s after t\ at AMD. Besides, expansion-phase auroras also expand poleward and can contact an arc along the auroral poleward boundary, forming a bulge region that expands westward as the westward traveling surge. This westward motion is slower than the azimuthal spreading of the onset arc brightening as in [Lyons et al., 2013]. 4. Discussions In this work the following important features of the particle and field variations in the magnetosphere during sharp drop of the ion pressure should be noted: sharp upsurge of the azimuthal flow (Vy) and splash of low-energy (0.1-1 keV) electrons. This phenomena associated with the drop of the ion pressure have not been observed before. *We suppose that the sharp upsurge of the azimuthal flow (Vy) during sharp drop of the ion pressure may not be attributed to the E * В drift only but to pressure gradient also. If the azimuthal flow is mainly associated with the pressure gradient, the westward flow enhancements can be interpreted as pressure gradient enhancements in the z direction (earthward when mapped along field lines to the equatorial plane) that could lead to enhanced field-aligned currents and thus to the growth phase arc intensifications. ♦Besides, short splash of low-energy electrons during the Vy upsurge maximum was accompanied by localized dBy variation with the change of dBy sign, which can be interpreted as the field-aligned current near THE. The whole complex of above-mentioned phenomena supports the suggestion of Lyons et al., 2003b based on particle divergence driven by magnetic drift, for an explanation of the plasma pressure reduction during the current wedge formation. Current wedge formation is expected to initiate in the region where the contours of constant P and V are not parallel, as illustrated in the left-hand panel of Fig. 5 (This is Fig. 3 from [Lyons et al., 2003b]), Here V = jds/B is the flux tube volume per unit magnetic flux evaluated along field lines from the equator to the ionosphere. During longitudinally localized reduction in P, the cross-tail current reduction occurs in the plasma sheet, giving dipolarization of the magnetic field, which is accompanied by an increase in B-_ and decrease in V. The decrease in P and in V will distort the contours of constant P and V as illustrated in the middle and right-hand panels of Fig. 5. This distortion of the contours is qualitatively as expected during current wedge forms. The particle divergence corresponds to a divergence in cross-tail current, which is related to field-aligned currents [Birmingham, 1992; Lyons et al., 2003b]. The component of the drift velocity Vd in the direction of VF gives particle divergence, and thus will give upward field-aligned currents. At Fig. 5 the asterisks note the expected sites of the satellites THE and THD. More favorable conditions for current д observation were for THE. 5. Conclusions We report a detailed case study of plasma pressure reduction during in the substorm expansion phase of substorm on December 19, 2014 using data from two THEMIS satellites in the evening sector magnetosphere. The following peculiarities of this event have been demonstrated: • Pre-onset auroral waves, auroral beading and the magnetic field oscillations with the quasi-period of 1.5-2.5 min and with quasi-period o f o f 30-40 s, observed at plasma sheet at 7-9 RE, suggest that current disruption during the substorm is caused by plasma instabilities such as the ballooning instability and the cross-field current instability [Roux et al., 1991]. • E- and В-field variations, observed 1 min before Texp, indicate on the appearance of SAW which may be associated with intensification auroral arc near THE (r—6.8 Re). • After Texp, the sharp lasted 30 s drop in the N) and Pxx of ion is observed simultaneously with sharp upsurge in the duskward ion velocity Vy. • Short (10-15 s) splash of low-energy (0.1-1 keV) electrons occurs during the Vy upsurge maximum was accompanied by localized dBy variation with the change of dBv sign, which can be interpreted as the field-aligned current near THE. The whole complex of above-mentioned phenomena has not been reported before. Our results support and supplement the suggestion of Lyons et al., 2003b based on particle divergence driven by magnetic drift, for an explanation of the plasma pressure reduction during the current wedge formation. Pressure gradient enhancements may also provide a free-energy source for exciting the drift ballooning mode instability, which may also cause cross-tail current diversion. ! Ь П С - Д ъ С . ь 4 1 Мурманская государственная областная универсальная научная библиотека 17