Physics – Plasma Physics
Scientific paper
Jul 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002jgra..107.1143m&link_type=abstract
Journal of Geophysical Research (Space Physics), Volume 107, Issue A7, pp. SMP 28-1, CiteID 1143, DOI 10.1029/2001JA009177
Physics
Plasma Physics
2
Space Plasma Physics: Discontinuities, Interplanetary Physics: Discontinuities, Magnetospheric Physics: Magnetopause, Cusp, And Boundary Layers, Solar Physics, Astrophysics, And Astronomy: Magnetic Fields, Magnetospheric Physics: Current Systems (2409)
Scientific paper
In order to answer why and how ``the minimum angle rotation of the magnetic field'' is realized in a current sheet with a sheared magnetic field, Taylor's helicity constraint, which is valid for low-β plasmas, is applied to a one-dimensional planar current sheet with a sheared magnetic field. A single constant helicity is defined for the total rectangular volume surrounding the current sheet and is shown to be gauge-invariant. The minimization of the magnetic energy with the constraint of the constant total helicity shows that the field is described by the constant α force-free equation and that the current sheet is a special class of tangential discontinuities with a constant field strength, or a ``perpendicular rotational discontinuity.'' The total rotational angle of the magnetic field across the current sheet is proportional to the ratio of the total magnetic energy/helicity in the force-free state. It is proposed that among an infinite number of force-free states the current sheet relaxes into a unique force-free state with the absolute minimum ratio of energy/helicity and thus into the absolute minimum energy state for a given constant helicity. Therefore, in the relaxed state the total rotational angle of the magnetic field across the current sheet is minimum and less than 180°. Although the present study of the relaxed state is applicable only to a tangential discontinuity, a qualitative resemblance of the model prediction with observations in situ and simulations of quasi-perpendicular rotational discontinuities suggests that the observed minimum angle rotation of the magnetic field in a current sheet with a sheared magnetic field is an emergence of plasma relaxation or self-organization in space plasmas.
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