Physics – Plasma Physics
Scientific paper
Mar 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000ppcf...42..275p&link_type=abstract
Plasma Physics and Controlled Fusion, Volume 42, Issue 3, pp. 275-300 (2000).
Physics
Plasma Physics
2
Scientific paper
We make new applications of our previously proposed method for estimating the strain-rate tensor and vorticity vector in plasmas (concerning the local deformation and self rotation of the plasma fluid elements, respectively) solely from magnetic field time series. Here we use solar wind measurements of Ulysses spacecraft made in the outer heliosphere, on and off the ecliptic plane, during the period 1990-1998. The application results imply that the solar wind plasma is, to a good approximation, weakly incompressible , being nearly incompressible on the local magnetic field-normal plane, while expanding at maximum strain rate in the magnetic field direction. This property is theoretically expected, at least for low and intermediate beta plasmas, and supports previous arguments for two-dimensional MHD turbulence in the solar wind. In the magnetic induction equation the vorticity term is favoured, being at least an order of magnitude larger than the strain-rate term, thus explaining the magnetic field alignment of the minimum magnetic field variance and the random wandering of the magnetic field's vector tip on a sphere, both being well known, general features of the heliospheric magnetic field fluctuations. Further, the solar wind is found dominated by magnetized vortex sheet structures (MVS), on the tangential plane of which lie the, (not generally aligned) average vectors of magnetic field, vorticity and plasma velocity in the solar-corotating frame of reference. These coplanarity properties are shown to be consistent with a theoretically predicted force-free state, minimizing the total energy while conserving a generalized helicity function. The theory additionally implies that the (not directly measured by Ulysses) electric current density also lies on the MVS tangential plane, hence the MVS also constitute current sheets . The MVS spatial orientation implies that the MVS are wrapped in the form of magnetized vortex tubes with axes aligned to the average magnetic field. The vector couplings, characterizing the MVS, are found to weaken with increasing heliodistance and near the heliospheric current sheet.
Moussas Xenophon
Polygiannakis J. M.
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