Physics – Plasma Physics
Scientific paper
Oct 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003aps..dppro1006b&link_type=abstract
American Physical Society, 45th Annual Meeting of the Division of Plasma Physics, October 27-31, 2003, Albuquerque, New Mexico,
Physics
Plasma Physics
Scientific paper
Most models of low beta space plasmas typically ignore finite beta effects, compressibility, and the possibility of non-conservative forces. Instead these models emphasize 2D concepts such as current sheets and x-points. Although current sheets have been successfully produced in lab plasmas, doing so involves imposing much more symmetry than normally occurs in nature. Structures observed in nature have 3D geometry and long filaments constitute the basic morphology (e.g., solar coronal loops, astrophysical jets). Obvious 2D current sheets are generally not observed in nature; regions where magnetic energy is converted into particle energy appear to be localized in 3D, for example as in a solar flare. Ongoing lab experiments at Caltech similarly show that MHD structures tend to be filamentary. It is argued that this universal tendency towards filamentary morphology (collimation) results from stagnation of magnetic flux convected by jet-like flows; these flows are driven by non-conservative MHD forces which arise whenever the current channel is not filamentary. Collimation involves flow stagnation, finite yet small beta, and compressibility. To the extent that the dynamics is lossless, structures should become filamentary even for arbitrarily small currents if one waits sufficiently long, like filling a bathtub with a trickle. Kink instability, a quite different phenomenon from filament formation, occurs later and can lead to interaction between adjacent filaments or to a filament segment detaching and forming a spheromak (plasmoid).
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