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 Phenom ena", Proc. XXXIV Annual Seminar, A patity, pp. 21 - 2 4 2011 Polar © Kola Science Centre, Russian Academy o f Science, 2011 Geophysical W У Institute THE DEPLETION AND DIPOLARIZATION OF THE MAGNETIC FLUX TUBES ABOVE AURORAL ARC M.A. Volkov (Murmansk State Technical University, 13 Sportivnaya Str., Murmansk, 183010, e-mail: volkovma(a)mstu. edu. ru) Abstract. The role of the hot magnetospheric particles (protons) in the formation of the electric fields and the field- aligned currents from the ionosphere to the magnetosphere have been studied. It has been shown that the gradient drift of the hot particles in these cases leads to the depletion of the magnetic flux tubes and increasing of the magnetic field in them. The induction electric field invoked of the magnetic field increasing leads to acceleration of the magnetospheric plasma which despite the energy spread begins to move as one unit. These phenomena occur above the auroral arcs and "inverted V” precipitations. The evaluations of the magnetic flux tube depletion and increasing the magnetic field in it have been made. The velocities of the magnetospheric plasma in the equatorial plane magnetosphere have been calculated at different distances from the Earth. Introduction The expansion phase of the magnetospheric substorm is accompanied by increase of the Вг (Z-to the pole, X and Y axes are directed toward the Sun and on the morning side ) component of the magnetic field in the magnetosphere tail and decrease of the plasma pressure [Runov et al., 2009]. This process is called dipolarization of the magnetic field lines, it begins at the tail of the magnetosphere and in a few minutes covers the length of ~ 10 /?E (the Earth radius), approaching the Earth at the speed of ~ 300 km / s. The velocity of the propagation of the magnetic field lines dipolarization is more than AlfVen speed, but noticeably less the magnetosonic one and is equal to the electric drift velocity of the plasma in the magnetosphere tail. These changes of the magnetic field and plasma may be result of the precipitation of charged particles in the ionosphere from the magnetic flux tube. This leads to the its cooling, reduce of pressure and increase of the magnetic field [Volkov, 2011]. However, the precipitating particles are mostly electrons whose pressure in the magnetosphere is negligible in comparison to the pressure of the ions. In this work we have been considered the possible mechanism of energy transfer from ions to electrons in the magnetic flux tube above the auroral arc. The most likely mechanism of the electrons acceleration over auroral arcs is appearance of the parallel electric field [Knight, 1973], the field-aligned electric field above the arc accelerates electrons, which precipitate into the ionosphere. Hot magnetospheric ions are also leaving the magnetic flux tube, moving across the magnetic field lines. As a result, the pressure and the magnetic moment of charged particles in the tube decreases, it leads to the increase of the magnetic field. Description o f the main processes Figure 1 shows possible distribution of electric potential and currents above the auroral arc. The ionospheric currents I] flow towards the electric field, but the magnetospheric currents direct against the electric field. The magnetospheric current IM is carried by hot ions and cold electrons move along the lines of the equal electric potential. The electric field makes negative work on the magnetospheric ions and the magnetic flux tube above the arc is cooled. Ii Fig. 1 Fig. 2 We assume that the field-aligned current flowing from the ionosphere is carried by cold electrons. In the magnetic flux tube with the field-aligned current arc the condition of charge neutrality is performed everywhere, i.e. 21