Physics of auroral phenomena : proceedings of the 35th Annual seminar, Apatity, 28 Februaru – 02 March, 2012 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2012. - 187 с. : ил., табл.

E.E. Antonova el al. Transverse currents in the plasma ring In the conditions of magnetostatic equilibrium and isotropic plasma pressure transverse current jj. is determined in accordance with the relation: j ^ B x V p / S 2, (1) where Vp is the plasma pressure gradient, В is the magnetic field. Equation (1) indicates that surrounding the Earth plasma ring till the dayside magnetopause with plasma pressure gradient directed to the Earth (see Fig. 2) contains westward directed transverse current. Simultaneous measurements of pressure gradients and magnetic field give the possibility to determine current density. However, only nighttime current densities give the information about integral current. The shift of minimal value of magnetic field on the daytime field lines leads to current spreading along field line (see Antonova et al. [1999b,c]). Taking into consideration that the plasma pressure has a constant value along magnetic field line it is possible to evaluate current density at any point of the field line, if plasma pressure distribution at the equatorial plane is known, using one of the magnetic field models. Although such approach is not self consistent, it makes it possible to estimate current densities far from the equatorial plane. The calculations produced by Antonova et al. [1999b,c] using Tsyganenko-2001 model show that integral current at geocentric distances from 7.5 to IOR e is comparable with the nighttime current at the same geocentric distances. The centre of the averaged transverse current in these calculations was shift on ZeJf=±2.1RE from the equator. Such finding can mean, that surrounding the Earth plasma ring is the ring current domain, in which region till ~1RE is the region of ordinary ring current and the region at greater distances till magnetopause near noon is the region of high latitude continuation of this current named by Antonova [2003, 2004] the cut ring current (CRC). Transverse current splits into two branches in the dayside magnetosphere. The daytime part of CRC is situated comparatively close to the magnetopause and is located far from the equatorial plane. This could be the reason why it was not included into the traditional versions of Tsyganenko models and overstretching of these models. It is interesting to mention that current lines corresponding to CRC appear in the results MHD modelling (see Liemohn et al. [2011]). Plasma pressure gradients in the surrounding the Earth plasma ring as the source of Dst variation Numerous studies have been published on the relative contributions of different terrestrial and magnetospheric currents to the Dst index. Ring current was considered as the main source of negative values of Dst variation at the first stages. This point of view was later criticized (see the discussion in [Greenspan and Hamilton, 2000; Antonova, 2001] as some features of Dst dynamics (for 12 example, the decrease of IDst I index after substorm onset or effect of Iyemori and Rao [1996]) was difficult to explain using traditional ring current description. This leads to the suggestion that tail current produce large contribution in Dst formation. However many scientist (see Roeder et al. [1996]; Jordanova et al. [1998]; Greenspan and Hamilton [2000]) continued to support the traditional point of view. The existence of the surrounding the Earth plasma with earthward directed plasma pressure gradient at geocentric distances till the Earth magnetopause activate the attention to the problem of the sources of currents producing the Dst variation during magnetic storms. Greenspan and Hamilton [2000] chose 1 = 7 (L is considered as only the distance at the equatorial plane) as an outer boundary for the ring current because they consider that during disturbed times the nightside inner edge of the plasma sheet typically is located near that L value [Parks, 1991]. The existence of the surrounding the Earth plasma ring and CRC shifts the inner edge of the plasma sheet and tail current at larger L. That is why it is necessary try to evaluate the contribution of the ring current including its high latitude continuation in the Dst formation. The direct way of the clarification of the role of CRC in the Dst formation is the creation of the magnetic field model in which CRC is included. It was not possible until now to obtain the analytical expression for CRC magnetic field and create the corresponding model. However, it is possible to obtain the simplified estimation of the role of the whole ring current in the Dst formation if the radial distribution of the magnetospheric plasma pressure is measured. The value of the magnetic pressure in the external parts of the magnetic trap is comparable with plasma pressure. That is why instead of the Dessler-Parker-Sckopke relation (DPS), which predicts a linear dependence of the perturbation magnetic field at the surface of the Earth Д В on the total ring current kinetic energy Up, it is necessary to use expression, obtained by Carovillano and Maguire [1968]: AB /B s =(2Up+Ub)/3Us, (2) where Bs is the field on Earth’s equator, Us is the energy of the dipole field outside the Earth’s surface, Ub is the self-energy of ring current field. Expression (2) gives the possibility to calculate the effect of neglecting in DPS the fact that the magnetic field in which the ring current particles move includes the field that these particles produce. The ring current kinetic energy Up in DPS in accordance with (2) should be replaced by Up + Ub 12, where Ub is the self-energy of the ring current magnetic field. Calculation of Uh if the radial distribution of isotropic plasma pressure p(r) is known requires the solution using the sequential iteration method of the equation (see Vovchenko and Antonova [2010,2012] and ref. therein):