SM11C-01 08:00h
The Boundary Layer in Transition: Structural Dependence On IMF Orientation
We examine a very slow boundary layer crossing made by the Cluster spacecraft on December 7, 2000. During the crossing the IMF rotated from due south to due north in a discontinuous fashion. Evidence from Cluster shows that the boundary layer changes from an open, reconnection-driven configuration to a closed, Kelvin-Helmholtz-driven configuration. Using simultaneous data from ground-based radars and from FAST as well as from Cluster, we investigate the changes in the convection pattern, the destabilization of the Kelvin-Helmholtz mechanism, and the response of the boundary layer to discontinuous changes in solar wind and IMF.
SM11C-02 08:15h
Characteristics of the Magnetosheath Electron Boundary Layer Under Northward IMF: Implications for High-Latitude Reconnection
We analyze Cluster data to explore the statistical properties of the magnetosheath electron boundary layer, observed outside the high-latitude dayside magnetopause, under northward IMF. The presence of heated magnetosheath electrons coincident with the normal cool magnetosheath ion population in this layer is taken as the signature of magnetosheath field lines that have reconnected with the high-latitude magnetic field of the earth. We investigate the dependence of the presence and directionality of heated magnetosheath electrons on the dipole tilt angle and the IMF tilt angle. The recurrent presence of bi-directional heated electrons outside the probable magnetopause suggests that magnetosheath field lines may frequently reconnect in both hemispheres. We further investigate the occurrence of such bi-directional heated electrons outside the magnetopause as a function of the IMF clock angle orientation. The findings are discussed in the context of double high-latitude reconnection as a possible mechanism for low-latitude boundary layer, and eventually cold, dense plasma sheet formation under northward IMF.
SM11C-03 08:30h
Cluster observations of the boundary waves and magnetic reconnection at the dawnside magnetospheric flank
We will examine Cluster observations at the far tail closely equatorial magnetospheric flank during periods small positive $b\_z$. For several hours Cluster satellites had several quasi-periodic encounters of low density, high temperature magnetospheric-like of plasma and high density, low temperature magnetosheath-like plasma with a frequency $\sim$ 5 minutes. The velocity and magnetic field components show more rapid oscillations with frequency $\sim$ 2 minutes, which indicates sub-structure embedded in the large scale motion. The boundary normals showed significant fluctuations in $z$-direction, and bipolar variations of the normal component of the magnetic field were observed together with local pressure enhancements. In addition the satellites observed several intervals satisfying the Walen relation and the de Hoffman Teller frame, often with much higher or lower transformation velocity than that of the overall boundary. We will discuss the role of the reconnection inside the non-linear Kelvin-Helmholtz vortices and local flux transfer events to account for these observations
SM11C-04 08:45h
Kinetic Alfv\`en Waves and Plasma Heating and Transport at the Magnetopause
Large amplitude transverse ULF waves with frequencies below the ion cyclotron frequency are nearly always found at the magnetopause and they contain most of the observed wave energy. These transverse waves are consistent with kinetic Alfv\'en waves (KAWs) that are produced by mode conversion of upstream compressional MHD waves at the magnetopause. The KAW activity should be larger for southward IMF than for northward IMF conditions, which is supported by observational study of levels of ULF wave activity (amplitude and polarization) as a function of magnetic shear, Alfv\'en velocity gradient, and magnetosheath wave activity. Particle interaction with KAWs with wavelength on the order of ion gyroradii can cause perpendicular heating of ions via stochastic processes and parallel heating of electrons via Landau damping. Moreover, particle interaction with KAWs causes significant magnetosheath particle transport across the magnetopause with a diffusion coefficient $D \sim 10^9 \rm m^2/s$, which is sufficient to maintain the observed density gradient across the magnetopause. Because large amplitude transverse ULF waves are nearly always found at the magnetopause, the wave-particle interaction process is inevitable, and we expect a continuous mixing in energy spectra of magnetosheath and magnetospheric particles in the LLBL. Although the magnetic reconnection process would predict a similar particle transport rate via direct entry along reconnected field lines, it predicts substantially different energy spectra in LLBL with two distinct particle components of magnetosheath and magnetospheric origins: the magnetosheath ions will have velocities on the order of Alfv\`en velocity and electrons form beam-like distributions. The difference in particle energy spectra should be useful for devising experimental tests that can distinguish the direct particle entry due to reconnection from the diffusive particle entry due to wave-particle interaction.
SM11C-05 09:00h
Cold Dense Plasma Sheet Formation During Northward IMF
A cold, dense plasma sheet (CDPS) is often observed when the IMF has been northward for an extended time period, usually several hours. One such period occurred October 22/23, 2003. During that period the IMF was strongly northward for approximately 36 hours, while Cluster II observed a cold dense plasma sheet. We present detailed global simulation results for this event that show how the CDPS develops due to lobe reconnection, which causes IMF flux tubes to be captured and subsequently convected into the tail, forming the CDPS.
SM11C-06 09:15h
Evolution of phase space densities from the dayside to nightside magnetosphere during a prolonged northward-IMF period: Cluster-II, GEOTAIL, and LANL-MPA comparison
It is observationally known that the plasma sheet becomes much cooler and denser than usual under prolonged northward interplanetary magnetic field (IMF) conditions [e.g., {\it Terasawa et al.}, 1997]. However, the mechanism responsible for the formation of CDPS is still far from understood. The Kelvin-Helmholtz (K-H) instability driven by the velocity shear at the magnetopause has been proposed as a possible mechanism of magnetosheath plasma entry through the LLBL [{\it Fujimoto and Terasawa}, 1994; {\it Otto and Fairfield}, 2000; {\it Hasegawa et al}., 2004]. Double lobe reconnection, i.e., reconnection of a magnetosheath flux tube with lobe field at the high-latitude magnetopause in both hemispheres, thereby becoming closed, is also an important candidate process for the dense, thick LLBL formation during northward IMF periods [e.g., {\it Song and Russell}, 1992]. On the basis of evolution of electron and ion phase space densities (PSDs) from the dayside to the nightside magnetosphere observed by Cluster II, GEOTAIL, and LANL-MPA spacecraft during a northward IMF interval on March 16, 2002, we examine the relative importance of the K-H instability and double lobe reconnection for formation of the CDPS. This event corresponds to one of the CDPS events at geosynchronous orbit during prolonged northward IMF periods [{\it Thomsen et al.}, 2003]. The heated electron signature observed by Cluster II indicates that formation of closed flux tube(s) through lobe reconnection in both northern and southern hemispheres indeed took place during the event. Comparison of PSDs between the newly closed flux tube observed by Cluster II and the dusk-flank LLBL by GEOTAIL indicates that double lobe reconnection is responsible for formation of the outer-LLBL. On the other hand, GEOTAIL observed wavy structures in the dusk LLBL, and comparison with results from numerical simulation of the K-H instability [{\it Matsumoto et al.}, 2004] suggests that the observed structure is consistent with the non-linear phase of K-H vortices. The evolution of PSDs in the LLBL suggests that the wavy structure is responsible for the selective plasma entry into the inner LLBL region. The newly-entered plasma from the magnetosheath coexists with hot plasma sheet ions in the LLBL region. Comparison between high-energy populations at GEOTAIL and LANL-MPA locations indicates that the effect of the magnetic drift is prominent for the high-energy population. Taking account of these results, the formation mechanisms of the CDPS under northward IMF condition will be discussed.
SM11C-07 09:30h
Quiet-time mass and energy transport in GUMICS-4 global MHD simulation, 2: Tracing mass and energy within the magnetosphere
During southward IMF, reconnection at the frontside magnetopause has been demonstrated to control the rate of energy, magnetic flux and plasma transfer from the solar wind to the magnetosphere. During northward IMF, the reconnection locations move to high latitudes and the energy transfer efficiency becomes much smaller. However, plasma transfer still takes place; recent results have suggested Kelvin-Helmholtz instability at the magnetopause flanks to be one energy and mass transfer mechanism during northward IMF. Here we trace the mass and energy flow trajectories within the magnetosphere using the GUMICS-4 global MHD simulation. We examine how the plasma sheet density and temperature are determined by the mass and energy transfer routes. Finally, we investigate the extent at which the plasma sheet characteristics depend on the solar wind parameters and the mass and energy entry sites and processes at the magnetopause.