Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsm51b1776z&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SM51B-1776
Physics
[2700] Magnetospheric Physics, [2724] Magnetospheric Physics / Magnetopause And Boundary Layers, [2740] Magnetospheric Physics / Magnetospheric Configuration And Dynamics
Scientific paper
From May to October in both 2007 and 2008, all five THEMIS spacecraft crossed the dayside magnetopause twice every day in the low-latitude region. During these crossings, they recorded a total of 3701 instances of bipolar magnetic variations in the magnetopause normal direction associated with enhancements of field magnitude. These signatures are interpreted as flux transfer events (FTEs) on the magnetopause and/or their perturbations in the background magnetosphere and magnetosheath. Plasma observations indicate that, when spacecraft traversed through FTE structures, the plasma velocity components tangential to the magnetopause were generally anti-sunward, consistent with the direction of the sheath flow. On the other hand, when the spacecraft were located within the magnetosphere and remotely sensed the perturbations related to FTEs on the magnetopause, the velocity tangential to the magnetopause was found to be either parallel or antiparallel to the sheath flow direction. Plasma properties, such as ion number density and temperature, known to vary with distance from the magnetopause can be used to infer whether measurements are made close to or farther from the boundary; it is found that FTE-associated flows close to the magnetopause are antisunward but that the flow direction reverses further in from the boundary. The velocity variations in the normal components for both kinds of events have the same bipolar structure with inward flow followed by outward flow on the magnetospheric side of the magnetopause. This pattern has the form of a flow vortex just inside the magnetopause associated with the FTE moving in an antisunward direction on or outside of the magnetopause. A 2-D magnetohydrodynamic (MHD) simulation code has been developed to understand the flow perturbations outside FTEs. Our simulation starts from the explicit solution of Farrugia et al. [1987] in which it is assumed that the plasma is inviscid and incompressible and no flow vortex is present. When we remove these two assumptions and impose finite viscosity near the FTEs, flow vortices develop. These flow vortices are comparable in size to the FTEs and reproduce our observations very well. However, the origin of this viscosity remains an open question.
Angelopoulos Vassilis
Auster H.
Galland Kivelson Margaret
Jia Yu
Khurana Krishan K.
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