Dynamics of the LLBL Thickness Based on THEMIS

Low latitude boundary layer (LLBL) can be found at low latitudes on the whole dayside magnetosphere and along magnetospheric flanks. This layer is filled by magnetosheath-like and magnetospheric plasmas. The LLBL plays a crucial role in the solar wind-magnetosphere coupling. We used measurements of the five THEMIS spacecraft that crossed the dayside LLBL on June 3, 2007. Simultaneous observations of the magnetic field and plasma parameters at several points of the LLBL and monitoring of the adjacent magnetosheath allow us to determine the LLBL thickness under different upstream conditions and to estimate the way of the plasma entry into the LLBL. For this, we apply the ion density-temperature (n-T) plots that allow us to investigate the LLBL profile. Introduction The existence of the low-latitude boundary layer (LLBL) as the internal magnetospheric boundary at low geomagnetic latitudes was first revealed via spacecraft observations more than 30 years ago [Hones et al., 1972; Eastman et al., 1976]. This layer is defined as a region where magnetosheath plasma penetrates into the magnetospheric side of the magnetopause and identifies itself as a tailward flux with properties intermediate between those in the magnetosheath and the magnetosphere. The LLBL was detected along the flanks of the geomagnetic tail by Hones et al. [1972] and Akasofu et al. [1973] but further observations have shown the presence of the LLBL along a whole dayside part of the magnetopause covering almost all local times [Eastman et al., 1976]. In spite of the crucial role of the LLBL in the solar wind-magnetosphere coupling as mass, momentum and energy channel, its basic parameters and their relations to upstream conditions still require more investigations. Due to the highly structured and dynamic character of this region, single spacecraft observations couldn’t resolve spatial and temporal effects. Therefore, multispacecraft projects such as Interball, Cluster and THEMIS are the main instruments for clarifying this problem. Several mechanisms have been proposed as candidates for the formation of the LLBL in order to explain how the solar wind enters into the magnetospheric boundary layer. Based on early observations, local and nonlocal plasma transport mechanisms for filling up the LLBL were distinguished. The local entry mechanism realizes through the transport of the magnetosheath plasma to the magnetopause via diffusion [Eastman et al., 1976; Eastman and Hones, 1979]. In the nonlocal entry mechanism, the magnetosheath plasma penetrates to the magnetosphere in the vicinity of the cusp region through the turbulent convection and goes down to low latitudes along magnetic field lines [Haerendel et al., 1978]. This process is accompanied by magnetic reconnection near the cusp. Also, the impulsive penetration of the solar wind plasma to the magnetospheric field lines has been proposed as a possible mechanism for the LLBL formation in order to explain the existence of the magnetosheath-like plasma on the magnetospheric side of the magnetopause [Lemaire and Roth, 1976]. The impulsive penetration is expected to be important only at the dayside magnetopause, while the other mechanisms also act along the flanks of the magnetopause [Scholer and Treumann, 1997]. It was proved that the LLBL can be found on both closed and opened field lines. The formation on open field lines under southward IMF is well described in terms of dayside reconnection [Luhmann et al., 1984] which produces the boundary layer of mixed particle populations of magnetospheric and magnetosheath origin. Dayside reconnection under southward IMF (interplanetary magnetic field) allows direct plasma entry across open portion of the magnetopause. On the other hand, the role of reconnection for the LLBL formation under northward IMF is more complicated, therefore other mechanisms, such as diffusion and impulsive penetration, were considered to be more important. Recently, 159 WDS'08 Proceedings of Contributed Papers, Part II, 159–167, 2008. ISBN 978-80-7378-066-1 © MATFYZPRESS