Stability and Uniformity of Electric Fields in the Magnetotail from the View of the Magnetosphere-Ionosphere Mapping Study

The plasma convection pattern in the polar region is known to depend systematically on the orientation of the interplanetary magnetic field (IMF). This dependence has been studied statistically by using radar and low-altitude satellite observations. Many papers have justified the presence of global plasma convection in the magnetotail corresponding to the low-altitude convection. We have examined the correspondence of the time variations between the low-altitude and magnetotail electric fields to study the process of momentum energy propagation in the magnetosphere. We compare electric field data from the Akebono and Geotail satellites obtained when they were in the midnight sector, and nearly conjugated by magnetic-field lines at different altitudes. We concentrate on variations with time scales ranging from several tens of minutes to several hours. The validity and spatial accuracy of the mapping between the magnetosphere and ionosphere are always questioned when we compare the electric fields at different altitudes. To answer the questions, we must know the stability and uniformity of the electric field in the real magnetosphere. In this work we examine the time variations of magnetic field using GEOTAIL data and evaluate the rot E in the magnetotail. In other words, we try the separation of temporal and spatial variations of electric fields, which has been believed to be impossible when we use data from a single spacecraft. At the same time the compressibility of the magnetosphere could be discussed. Regarding the time scales longer than several tens minutes, the electric field is uniform for much larger spatial scale than the spatial error of the mapping between the magnetosphere and ionosphere.