The weak field region in the magnetotail
At a distance of ~10 Re in the tail, the magnetic field changes from thin configuration to dipolar field dramatically during substorm. If the magnetotail is in vacuum, the magnetic field from the Earth's dipole will be in the order of 50 nT. The previous sampling by AMPTE/CCS and Geotail revealed that the strength is as small as 5 nT and the plasma beta can be as large as 50 near the magnetic equator of the thin plasma sheet before the substorm expansion phase onset [Figure 1].
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| Figure 1. The weak magnetic field region observed by Geotail in the near-Earth tail at distance of 12 Re from the Earth. Following the observation of the quasi-static thin/stretched magnetotail with the weak field, the substorm expansion phase onset was recorded by Geotail and Polar.
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The thinning of tail plasma sheet is associated with transfer of energy from the solar wind into the magnetosphere [Figure 2]. However, the governing process of the thinning is not understood definitively.
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| Figure 2. Chart for conventional view of the effect on the magnetotail by the energy transfer from the solar wind.
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Magnetotail models: empirical vs. MHD
The discrepancy has been known between the conventional view and adiabatic convective MHD models of the magnetotail. The conventional view is based on the observation-based statistical model. Figure 3 shows the key components of the adiabatic convective MHD model that has a broad magnetic field minimum in the near-Earth tail, which is not present in the empirical conventional view. This problem is also known as pressure balance inconsistency or the entropy crisis. Our paper [Saito et al., 2010] is concerned with clarifying the presence of such a magnetic field minimum.
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| Figure 3. Viewgraph showing the main points of the adiabatic convective MHD model.
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Formation-flight observation by THEMIS
In order to determine the formation of the magnetic field minimum, we need to determine the radial gradient. Apparently it seems difficult in particular when the plasma sheet is thin, even though we have multiple points it is unlikely we have more than two spacecraft in the magnetic equator.
However, our paper [Saito et al., 2010] introduces an alternative method to determine the radial gradient of the equatorial field. In the thin geometry, the latitudinal gradient is more pronounced, and field lines are connected to the magnetic equator. Using this model and analysis, we identify the magnetic field minimum that sustained for ~20 min [Figure 4].
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| Figure 4. Observation of the tail current sheet by five THEMIS spacecraft in formation flight.
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The physics governing the magnetotail
THEMIS in-formation flight provided unambiguous evidence for the magnetic field minimum in the magnetotail. An emergence of minimum B can be understood as a consequence of steady adiabatic earthward convection [Hau et al., 1989; Wolf et al., 2009]. At the minimum, the magnetic field strength is estimated to be as small as 2 nT. The plasma beta will be as large as 50 and the Larmor radius of the thermal ion will be 1 Re [Figure 5]. We envisage that the governing physics in the near-Earth tail will be described by the competition between MHD scale instability and kinetic stability effects.
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| Figure 5. Summary of the THEMIS observation of the magnetic field minimum in the magentotail and its implication in the tail dynamics.
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References
Hau, L.-N. (1992), Effects of steady state adiabatic convection on the configuration of the near-Earth plasma sheet: 2, J. Geophys. Res., 96 (A4), 5591-5596,doi:10.1029/90JA02619.
Hau, L.-N., R. A. Wolf, G.-H. Voigt, C. C. Wu (1989), Steady state magnetic field configurations for the Earth's magnetotail, J. Geophys. Res., 94 (A2), 1303-1316, doi:10.1029/JA094iA02p01303.
Saito, M. H., L.-N. Hau, C. -C. Hung, Y.-T. Lai, and Y.-C. Chou (2010), Spatial profile of magnetic field in the near-Earth plasma sheet prior to depolarization by THEMIS: Feature of minimum B, Geophys. Res. Lett., 37, L08106, doi:10.1029/2010GL042813.
Wolf, R. A., Y. Wan, X. Xing, J.-C. Zhang, and S.Sazykin (2009), Entropy and plasma sheet transport, J. Geophys. Res., 114, A00D05, doi:10.1029/2009JA014044.
Source
Saito, M. H., L.-N. Hau, C.-C. Hung, Y. -T. Lai and Y. -C. Chou (2010), Spatial profile of magnetic field in the near-Earth plasma sheet prior to dipolarization by THEMIS: Feature of minimum B, Geophys. Res. Lett., 37, L08106, doi:10.1029/2010GLO42813.
Biographical Note
Miho Saito is a postdoctoral fellow at NASA Goddard Space Flight Center. The work here is a collaboration work with Prof. Lin-Ni Hau, Mr. Chien-Chia Hung, Mr. Yen-Ting Lai, Mr. Yu-Chieh Chou, and Prof. Liu Chen, while she was a postdoctoral fellow at National Central University, Taiwan, R. O.C. Her research interest is explosive phenomena caused by magnetically confined plasma, particularly in space.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu
