The crossings of the magnetopause and low-latitude boundary layer by the THEMIS-B satellite in the spring of 2008 on the dusk flank under large negative X GSM (from –17 R E to –19 R E ) are studied. The parameters of plasma and magnetic field are analyzed from the data of ESA and MGF instruments. The changes in the total pressure, magnetic field pressure, and plasma pressure component during the transition from the magnetosheath to the plasma sheet (PS) are analyzed. The values of plasma pressures under the magnetopause at the edge of the PS for the considered events are determined. The applicability of the obtained results to the determination of the position of the boundary between the tail current and the ring current is discussed.

The thickness of the low latitude boundary layer (LLBL) is studied as a function of interplanetary magnetic field (IMF) using the data of THEMIS mission. The data from intersections of LLBL by Themis-A and -C satellites are analyzed. Solar wind parameters are provided by Themis-B satellite located before the bow shock. We use earlier developed method of LLBL thickness determination based on the analysis of the variation of plasma velocity in the layer perpendicular to the magnetopause. The database for the present analysis consists of 109 single satellite LLBL crossings where the values of LLBL thickness are obtained. The time shift of solar wind propagation from the spacecraft performing measurements outside the bow shock to the LLBL is taken into account. We analyze the dependence of LLBL thickness on IMF Bz and By using data of IMF measurements with 3s resolution and produce the 180s averaging of these data. Large scattering of the values of LLBL thickness and the weak dependence on IMF is demonstrated. Dawn–dusk asymmetry of LLBL thickness is not observed. The dependence of LLBL thickness on IMF clock angle is discussed.

The evolution of the spectral index of the omnidirectional differential flux of protons (within a range of 40 to 600 keV) in the magnetopause crossing near the subsolar point was analyzed. The work is based on the measurement data of the THEMIS international project as of July 18, 2007. A specific feature of the event under study is the possibility of determining the spectral index at energies >40 keV with a time resolution of 3 s. It is shown that the ion distribution functions, both outside and inside the magnetopause, can be approximated by kappa-like distributions with power low high-energy tail in the analyzed crossing. A high level of fluctuations of the spectral index of energetic ions was shown to have been observed near the magnetopause. The fluctuation level substantially reduces inside the magnetosphere. In this case, the spectral index has a value of ~6 on average and remains constant up to inner regions of the magnetosphere.

The role of a high fluctuations level in the Earth’s magnetosheath in plasma penetration into the magnetosphere and in the formation of the low-latitude boundary layer (LLBL) has been considered based on the events that occurred on November 1 and 5, 2007, using the THEMIS-A satellite observations. During the selected LLBL crossings the satellite was measuring behind the quasi-parallel and quasi-perpendicular bow shocks. The angle between the magnetic field direction in the solar wind and the normal to the bow shock (ΘBn) has been taken as a parameter reflecting the level of magnetic field and plasma paremeters fluctuations in the magnetosheath. It has been indicated that a thick LLBL is observed when angle ΘBn is small and the turbulence level in the magnetosheath is high. When angle ΘBn is large, the layer thickness decreases. The possible mechanisms by which a thick LLBL is formed are discussed.

Crossings of the magnetopause near the subsolar point are analyzed using data of THEMIS mission. Variations of the magnetic field near magnetopause measured by one of THEMIS satellites are studied and compared with simultaneous measurements in the solar wind by another THEMIS satellite. The time delay of the solar wind arrival at the subsolar point of the magnetopause is taken into account. 30 and 90s averaging of the magnetic field in the magnetosheath is produced. The results of averaging are compared with the results of measurements in the solar wind before the bow shock and foreshock. It is shown, that Bx component of the magnetic field near magnetopause is near to zero, which supports the possibility to consider the magnetopause as the tangential discontinuity. Comparatively good correlation of By component in the solar wind and near the magnetopause is observed. The correlation of Bz component near the magnetopause and IMF is practically absent, the sign of the Bz near the subsolar point does not coincide with the sign of IMF Bz in ∼30% cases.

We describe a method for determining the thickness of the low-latitude boundary layer (LLBL) of the Earth’s magnetosphere at the dayside near the equatorial plane based on the data gathered by a single satellite that traverses the layer and measures the plasma velocity. The method may be applied when the position of the magnetopause and the magnetosheath parameters fluctuate. The necessity of taking the presence of outer and inner LLBL regions into account is analyzed. The developed method is tested using the analysis results of two almost simultaneous close traverses of the magnetopause completed by the THEMIS mission satellites that provided relatively precise data on the LLBL thickness. It is shown that the developed method makes it possible to determine the LLBL thickness with an accuracy of ∼10%.

The nighttime region at geocentric distances ~7-10RE is ordinarily considered as the near tail region. However the results of observations including latest THEMIS mission clearly demonstrate the existence of surrounding the Earth plasma ring at these geocentric distances. The distribution of plasma pressure in the ring is near to azimuthally symmetric. Daytime compression of magnetic field lines and shift of minimal value of the magnetic field till high latitudes lead to splitting of daytime transverse currents in Z direction. As a result nighttime transverse currents in the surrounding the Earth plasma ring are concentrated near equator, daytime transverse currents are spread along compressed by solar wind field lines forming the cut ring current (CRC) which is the high latitude continuation of the ordinary ring current. CRC is supported by directed to the Earth plasma pressure gradients. The role of CRC in the development of magnetic storm and the creation of the Dst variation is analyzed. We stress that the development of partial ring current, which is one of the well-known features of magnetic storm, in the CRC region helps to eliminate paradox, appeared when it was suggested that tail current could have the considerable role in the Dst formation. The contribution CRC in the process of Dst formation during magnetic storms is evaluated for selected magnetic storms with known radial profile of plasma pressure. The magnetospheric substorm is one of the most extensively stydied magnetospheric phenomena for the most than 50 latest years. However the mechanism of observed energy explosion and localization of substorm onset continue to be widely discussed. We summarize the results of observations demonstrating the isolated substorm onset at geosentric distances smaller than 10RE. It is suggested that isolated substorm onset is localized in CRC.

There are strong experimental evidences of the existence of plasma domain forming a closed plasma ring around the Earth at geocentric distances ∼7–10RE. In this work, we analyze the main properties of this ring, using the data of the THEMIS satellite mission, acquired between April 2007 and September 2011. We also analyze the contribution of this ring to the storm dynamics. In particular, it is shown that the distribution of plasma pressure at ∼7–10RE is nearly azimuthally symmetric. However, the daytime compression of the magnetic field lines and the shift of the minimal value of the magnetic field to higher latitudes lead to the spreading of the transverse current along field lines and splitting of the daytime integral transverse current into two branches in Z direction. The CRC is the high latitude continuation of the ordinary ring current (RC), generated by plasma pressure gradients, directed to the Earth. We evaluated the contribution of the azimuthally symmetric part of the plasma ring to the Dst index for strong geomagnetic storms using the AMPTE/CCE radial profiles of plasma pressure published before, and showed that the contribution of the ring current including both RC and CRC is sufficient to obtain the observed Dst variation without the necessity to include the tail current system.