Abstract
Abstract. Magnetic holes with relatively small scale sizes, detected by Cluster and TC-1 in the magnetotail plasma sheet, are studied in this paper. 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 Bz larger than Bx, 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 energy flux enhancement at a 90° pitch angle, which is quite different from the previously observed isotropic electron distributions inside magnetic holes in the plasma sheet. 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-1 orbit (~7–12 RE) are likely to be further developed than those observed by Cluster (~7–18 RE).
Highlights
Magnetic holes (MHs) were first detected in the solar wind (Turner et al, 1977)
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-1 orbit (∼7–12 RE) are likely to be further developed than those observed by Cluster (∼7–18 RE)
This result indicates that the plasma sheet magnetic holes seem to occur more often in a more dipolar magnetic field topology, which implies that these magnetic holes might have a close relationship with the dipolarization process
Summary
Magnetic holes (MHs) were first detected in the solar wind (Turner et al, 1977). They are observed as depressions in the magnetic field magnitude with durations of seconds to tens of seconds (Turner et al, 1977; Xiao et al, 2010; Zhang et al, 2009). Turner et al (1977) defined those magnetic holes with little or no change in the magnetic field direction as linear magnetic holes (LMHs) and Winterhalter et al (1994) found that the ambient plasma parameters of observed LMH trains, which are defined as at least two comparable linear magnetic holes in a 300 s interval, and LMHs were marginally mirror stable They suggested that LMHs are probably the remnants of mirror modes structures in the solar wind (see Winterhalter et al, 1995; Russell et al, 2008; Zhang et al, 2008, 2009). The characteristics of the background magnetic field of these magnetic holes and their scale sizes are shown as well
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