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.

‘P hysics o f Auroral P h e n o m e n a P ro c . XXXVIII A nnua l Seminar, Apatity, pp. 44-47, 2 0 1 5 © Kola Science Centre, Russian Academy of Science, 2015 Polar Geophysical Institute DIAMAGNETIZM OF PLASMA AND FORMATION OF LOCAL DECREASES OF THE MAGNETIC FIELDS NEAR THE EQUATORIAL PLANE V.V. Vovchenko1, E.E. Antonova2,1 'Space Research Institute RAS, Moscow, Russia 2SkobeItsyn Institute o f Nuclear Physics Lomonosov Moscow State University, Moscow, Russia Abstract. We analyze the distribution of the magnetic field at the equatorial plane and plasma pressure using data of the AMPTE/CCE satellite at the equatorial plane and select the formation of local minima of the magnetic field. The main feature of such minima is the decrease of the magnetic field along the satellite trajectory accompanied by the increase of the field. Selected decreases of the magnetic field have the forms of magnetic holes. More than hundred magnetic holes are selected at all local times. Comparatively stable character of magnetic field decreases leads to the suggestion about quasistable character of observed magnetic holes. The formation of local minima of the magnetic field near midnight is traditionally explained as the result of the magnetic field distortion by tail current near its near Earth boundary. However the formation of magnetic holes can be connected with the local increases of plasma pressure in the magnetic trap in the conditions of magnetostatic equilibrium due to the diamagnetism of plasma. Action of such mechanism becomes rather probable due to observations of magnetic holes in all MLT sectors. We select the event 06 June 1985 when the increase of plasma pressure (pressure hump) was observed simultaneously with the magnetic hole. The nonlinear modeling of the magnetic field distortion by the observed pressure hump shows that the magnetic hole observed 06 June 1985 is the result of the diamagnetic effect of hot magnetospheric plasma. 1. Introduction Decreases of Bz component of the magnetic field near the equatorial plane are frequently observed near midnight and ordinarily interpreted as the magnetic field stretching in the tailward direction (see, for example, Petrukovich et al. [1999]). Saito et al. [2010] show using data of multipoint observations via NASA’s magnetospheric mission THEMIS that the decrease of Bz can have the local character and is accompanied by Bz increase with the increase of geocentric distance. Observed by Saito et al. [2010] local decreases of Bz can be considered as magnetic holes and had the quasistationary nature for ~20 min before dipolarization onset. Magnetic configuration with magnetic hole can be unstable. Therefore the result of Saito et al. [2010] leads to the appearance of multiple papers on the balloon mode development near midnight. Such development is considered as one of the possible mechanisms of substorm expansion phase onset. Constantly existing high level of plasma sheet turbulence at geocentric distances >10 RE (see Stepanova et al. [2009, 2011] and references therein) complicates the observations of magnetic holes and humps. That is why the analysis of CLUSTER observations made by Petrukovich et al. [2013] between 8 and 18 RE downtail selected only a few of events which exhibit clear quasistationary magnetic hump formation in the middle tail. However, quite time plasma distribution at geocentric distances <10Л£ is comparatively stable in comparison with plasma sheet proper. That is why it may be rather interesting try to find magnetic holes and humps in this region which in accordance with Antonova et al. [2013, 1014a] is the 44 high latitude part of the ring current. It is necessary to mention that Antonova et al. [2014b], Vorobjev et al. [2015] argue that most part of the auroral oval is mapped to the equatorial plane (more exactly to the surface of minimal values of the magnetic field at the magnetic field line) to the outer part of the ring current in contrast to the ordinary suggestion of its mapping to the plasma sheet proper. Decrease of the magnetic field in the comparison with not disturbed magnetic field and the field increase at larger geocentric distances is the characteristic feature of the magnetic field distortion by the ring current [ Chapman and Akasofu, 1968]. The increase of the magnetic field is formed at the outer boundary of the ring current simultaneously with the magnetic field decrease inside the ring current with the maximal distortion in the region of pressure maximum. Such feature corresponds to the observations of magnetic hole/hump structures by Saito et al. [2010]. Taking into account the results demonstrating the existence o f ring current continuation till geocentric distances -12 RE it may be very interesting to check the hypothesis about hole/hump structure formation by the ring current. The proof of such hypothesis requires simultaneous observations of magnetic field and plasma pressure. Comparatively good results of plasma pressure observations at the equatorial plane were obtained due to realization of AMPTE project. AMPTE/CCE data were used for the obtaining of global picture of pressure distribution at the equatorial plane during quite conditions [Lui and Hamilton, 1992; DeMichelis et al., 1999; Lui et al., 2004] and magnetic storms. Such data continue to be interesting till now due to