Physics of auroral phenomena : proceedings of the 33rd Annual seminar, Apatity, 02 - 05 March, 2010 / [ed.: A.G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 206 с. : ил.

“Physics o fAuroral Phenomena”, Proc. XXXIIIAnnual Seminar, Apatity, pp. 46 - 48, 2011 © Kola Science Centre, Russian Academy of Science, 2011 Polar Geophysical Institute STRENGTHENING EFFECT OF THE MAGNETIC FIELD DUE TO THE PRECIPITATION OF CHARGED PARTICLES M .A. Volkov {Murmansk State Technical University, 13 Sportivnaya Str., Murmansk, 183010, e-mail: mavol2006@yahoo.com) Abstract. During the expansion phase of the substorm in the nightside magnetosphere enhancement of the magnetic field and decrease the plasma pressure is observed simultaneously. Enhancement o f the magnetic field is recorded at first in the tail o f the magnetosphere, and then at more closer distances from the Earth. This process takes a few minutes and covers the region in the magnetosphere of 10 Earth radii. One o f possible explanations of this phenomenon is associated with cooling magnetic flux tubes caused by the precipitation of the energetic particles in the ionosphere. We have made evaluation of the effectiveness of this mechanism. Introduction The expansion phase of the magnetospheric substorm is accompanied by increase of the Bz component of the magnetic field in the magnetosphere tail and decrease of the plasma pressure f1]. This process is called dipolarization o f the magnetic field lines, it begins at the tail o f the magnetosphere and in a few minutes covers a length o f ~ 10 R e , approaching the Earth at a speed of ~ 300 km / s. The velocity o f the propagation o f dipolarization of the magnetic field lines is more than Alfven speed, but noticeably less magnetosonic and is equal to the electric drift velocity o f the plasma in the magnetosphere tail. One of possible explanations for this phenomenon is associated with the formation of plasma bubbles, the pressure of the plasma in the magnetic flux tube is less than the background, and the magnetic field, on the contrary, is more. Bubbles can appear during reconnection in the tail of the magnetosphere [2], or during the sudden weakening of the electric field of the magnetospheric convection at the late growth phase of the substorm [3]. The bubbles in these cases are connected with intensive field-aligned currents, flowing from the ionosphere. The electric field across such structure increased, and the bubble moves to the Earth. At the ionospheric level the moving of this structure towards the equator can be observed. Yet the auroral forms during the expansion phases of the substorms are moving toward the pole, but not toward the equator. The strengthening the Bz component of the magnetic field can also be caused by the magnetic flux tubes braking drifting from the tail of the magnetosphere to the Earth [4]. However, the mechanism of this braking is not entirely clear. In this paper we propose the explanation of the dipolarization magnetic field lines in the magnetosphere tail as a result of nonadiabatic cooling of the magnetic field tubes by precipitation of charged particles in the ionosphere. The description of basic processes The plasma is diamagnetic, so it tries to displace the magnetic field, if the plasma pressure is reduced, for example, due to the precipitation, the magnetic field in the tube increases. As the magnetic field is frozen into the plasma, the magnetic flux tube is compressed, and the pressure increases. A new configuration of the pressure distribution and magnetic field in the tail of the magnetosphere appears there. We have calculated the distribution of plasma pressure self-consistent with the magnetic field in the tail of the magnetosphere along the Sun-Earth line (axis X) for two cases, at the end of the growth phase, and after the process of dipolarization of magnetic field lines. The magnetic field in the tail of the magnetosphere is given by the magnetosphere model o f Tsyganenko 96 [5]. The effect of magnetospheric convection on the distribution of plasma pressure is not taken into account; the magnetic flux tubes are not considered drifting. In this case colatitude в of the magnetic flux tube before and after dipolarization does not change and we can calculate the kinetic energy o f charged particles in the tube 3/2/? F and entropy S = pV sn (p- the isotropic plasma pressure in magnetosphere, F-the volume o f the magnetic flux tube with unit magnetic flux in the ionosphere) before and after dipolarization. The decrease of the kinetic energy and the entropy should point to nonadiabatic processes and emptying magnetic flux tubes. According to observations [6] dipolarization in the tail of the magnetosphere and the auroral breakup are observed almost simultaneously at the same magnetic field lines. In the absence of convection equation of the balance pressure of the plasma and the magnetic field has the form: - + [rotB x B] = 0 ( 1 ), where /i0-the magnetic permeability of vacuum, S-the magnetic induction. In the projection on X (the X axis is directed to the Sun, Z-to the pole), equation (1) takes the form: d , „ d