Thin current sheets at R = -10- -13 RE
Formation and instability of thin current sheets (TCSs) in the near-Earth magnetotail is
the key element of substorm dynamics and a subject of fundamental plasma physics.
Multi-point observations are essential to study TCSs. Two-point observations of this
region with the ISEE 1 and 2 spacecraft revealed the existence of the current sheet with
thickness of 500 km [Sergeev et al., 1993].
Flapping motion of the magnetotail plasma/current sheet provides a possibility to scan the
current sheet interior and reconstruct the current sheet structure [Sergeev et al., 1998,
Runov et al., 2006].
THEMIS will provide 3-point measurements at geocentric distances 10 – 12 RE near the
nominal magnetic equator. Major issues to be addressed to the multi-point current sheet
probing in near-Earth magnetotail are
 Determination of the current sheet orientation;
 Estimation of the current sheet scale (half-thickness);
 Reconstruction of the current sheet structure, including plasma density,
temperature and current density distributions in the sheet (vertical profiles).
Techniques to determine the current sheet orientation includes
a. Minimum Variance Analysis (MVA). To be applied for each probe time series.
b. Multi-point timing analysis.
c. Triangulation of min|Bx| plane (local magnetic equator).
d. B-gradient/curl estimation (using s/c pairs or, if applicable 3-pint curlometer).
e. Particle remote sensing (finite proton gyroradius technique) [see Wilber et al.,
2004 and references therein].
Techniques to estimate the current sheet thickness
a. Integration of Vz during flapping [e.g., Sergeev et al., 1998, 2003]. High time
resolution and good quality of plasma velocity (moments) are required.
b. Three-point fit to the Harris function [e.g., Sanny et al., 1994, Nakamura et al.,
2002]. May be applied for non-flapping intervals.
c. B-gradient method. To be applied for flapping current sheet. The orientation of
the current sheet normal is required. Magnetic field difference between a pair of
probes and their separation are used to estimate magnetic field gradient along the
normal (nB). Then the current sheet thickness h = B(nB)-1dt [Runov et al.,
2005].
Radial separation of the spacecraft is important for multi-point method applications
(timing, triangulation, gradient-based techniques).
The planned P3, P4, and P5 probes separation during T3 (dR=1RE, dZ=600 – 3000 km)
will be optimal to resolve meso-scale (~ several minutes time scale and ~1 RE spatial
scale) current/plasma sheet dynamics. The radial separation will help to probe the current
sheet tilt (kinking) in the XZ plane. The T4 separation (dR=1000 km, dZ=100 – 500 km)
will be optimal for gradient estimations and probing of fine structures of the
current/plasma sheet. Since lengths of the magnetic field and plasma density gradients are
expected to be larger in tail-align direction than those in vertical (Z) direction, it is
essential to keep the radial separation of about 5 – 10 times larger than the vertical
separation.
References
Nakamura, R., W. Baumjohann, A. Runov, M. Volwerk, T. L. Zhang, B. Klecker, Y.
Bogdanova, A. Roux, A. Balogh, H. Rème, J.-A. Sauvaud, and H. U. Frey, Fast flow
during current sheet thinning, Geophys. Res. Lett., 29, 10.1029/2002GL016200, 2140,
2002.
Runov, A., V. A. Sergeev, W. Baumjohann, R. Nakamura, S. Apatenkov, Y. Asano, M.
Volwerk, Z. Vörös, T. L. Zhang, A. Petrukovich, A. Balogh, J.-A. Sauvaud, B. Klecker
and H. Réme, Electric current and magnetic field geometry in flapping magnetotail
current sheets, Ann. Geophys, 23, 1391, 2005.
Runov, A., V. A. Sergeev, R. Nakamura, W. Baumjohann , S. Apatenkov, Y. Asano, T.
Takada, M. Volwerk, Z. Vörös, T. L. Zhang, J.-A. Sauvaud and H. Réme and A. Balogh,
Local structure of the magnetotail current sheet: 2001 Cluster observations, Ann.
Geophys, 24, 247, 2006.
Sanny, J., R. L. McPherron, C. T. Russel, D. N. Baker, T. I. Pulkkinen, A. Nishida,
Growth-phase thinning of the near-Earth current sheet during CDAW 6 substorm, J.
Geophys. Res., 99, 5805, 1994.
Sergeev, V. A., D. G. Mitchell, C. T. Russell, and D. J. Williams, Structure of the tail
plasma/current sheet at ~11 RE and its changes in the course of a substorm, J. Geophys.
Res., 98, 17.345, 1993.
Sergeev, V. A., V. Angelopulous, C. Carlson and P. Sutcliffe, Current sheet
measurements within a flapping plasma sheet, J. Geophys. Res, 103, 9177, 1998.
Sergeev, V. A., A. Runov, W. Baumjohann, R. Nakamura, T. L. Zhang, M. Volwerk, A.
Balogh, H. Rème, J.-A. Sauvaud, M. André, and B. Klecker, Current sheet flapping
motion and structure observed by Cluster, Geophys. Res. Lett., 30,
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