Study Of Magnetic Structures In The Plasma Sheet And Their Relation With Plasma Flow

Magnetic hole is a kind of spatial structure which corresponds to a decrease in the magnetic field magnitude. Magnetic hole was first observed in the interplanetary space, and after that, there were many observations of magnetic holes in planetary magnetosheath, in the vicinity of comet cone and Io, and even in the terrestrial cusp. The scale size of these kinds of magnetic holes ranges from several to dozens of proton Larmor radii. The magnetic holes in the planetary magnetosheath are commonly believed generated by mirror instability, while there are many different kinds of mechanisms about magnetic holes in the interplanetary space. Recently, THEMIS observed one kind of magnetic hole in the plasma sheet. These plasma sheet magnetic holes were observed between two dipolarizationsThe study of plasma sheet magnetic holes might help us to further understand this kitid of magnetic hole. Because of their high correlation with dipolarization, this study could help us to understand the process of dipolarization. And the statistical study of large scale flapping movement of magnetotail current sheet, which correlates with the plasma sheet magnetic hole but show different features, will help us realize the features of current sheet and their interaction process with plasma ilow. Magnetic holes with relatively small scale sizes, detected by Cluster and TC-I in the magnetotail plasma sheet, and the features of large scale flapping movement of magnetotail current sheet are studied in this paper. We will also discuss their relationship with plasma flows.It is found that these magnetic holes are spatial structures and they are not magnetic depressions generated by the flapping movement of the magnetotail current sheet. Most of the magnetic holes (93%) were observed during intervals with B larger than B., i.e.. they are more likely to occur in a dipolarized magnetic field topology. Our results also suggest that the occurrence of these magnetic holes might have a close relationship with the dipolarization process. The magnetic holes typically have a scale size comparable to the local proton Larmor radius and are accompanied by an electron flux enhancement at90°pitch angle, which is quite different from the previously observed isotropic electron distributions inside magnetic holes. It is also shown that most of the magnetic holes occur in marginally mirror-stable environments. Whether the plasma sheet magnetic holes are generated by the mirror instability related to ions or not, however, is unknown. Comparison of ratios, scale sizes and propagation direction of magnetic holes detected by Cluster and TC-1, suggests that magnetic holes observed in the vicinity of the TC-1orbit (-7-12Re) are likely to be further developed than those observed by Cluster (-7-18Re).The distribution of magnetotail cunent sheet crossings position observed by Cluster show that the magnetotail current sheets in the dawn and dusk sides (Y>8RE) are found in the larger|Z|value than that in the midnight region (Y<8RE). This result indicate that the current sheets in the dawn and dusk regions are more like the corrugations in the surface of current sheet which with a large amplitude. The statistical results show that current sheet crossing events could be observed in both the geomagnetic active and quite times, which imply that the mangetotail current sheet crossings have no clear relationship with geomagnetic active processes. The Enter/Retreat events show a correlation with southward interplanetary magnetic field (IMF), and the averaged geomagnetic index show that these events occurred under a larger value than that of the magnetotail current sheet.