Minimum energy state and minimum angle rotation of the magnetic field in a current sheet with sheared magnetic field
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概要
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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.
- American Geophysical Unionの論文
American Geophysical Union | 論文
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