Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...450..422k&link_type=abstract
Astrophysical Journal v.450, p.422
Astronomy and Astrophysics
Astronomy
40
Magnetohydrodynamics: Mhd, Methods: Numerical, Plasmas, Sun: Chromosphere, Sun: Magnetic Fields, Videotapes
Scientific paper
We investigate the hypothesis that all chromospheric eruptions are manifestations of a common magnetohydrodynamic phenomenon occurring on different scales: the acceleration of chromospheric plasma driven by localized magnetic reconnection. Our approach is to perform 2.5-dimensional numerical simulations of shear-induced reconnection in a potential magnetic field with a central X-point above the photosphere, embedded in a model chromosphere with solar gravity and numerical resistivity. Calculations with two values of the footpoint displacement were performed by applying a localized body-force duration twice as long in one case as in the other; after the shearing was discontinued, the system was allowed to relax for an additional interval. For the stronger shear, the initial X-point lengthens upward into a current sheet which reconnects gradually for a while but then begins to undergo multiple tearing. Thereafter, several magnetic islands develop in sequence, move toward the ends of the sheet, and disappear through reconnection with the overlying or underlying field. During the relaxation stage, a new quasi-equilibrium state arises with a central magnetic island. We also performed a reference calculation with the stronger shear but with greatly reduced numerical resistivity along the boundary where the X-point and subsequent current sheet are located. This simulation confirmed our expectations for the system evolution in the ideal limit: the current sheet becomes much longer, without significant reconnection. For the weaker shear, a much shorter sheet forms initially which then shrinks smoothly through reconnection to yield an X-point relocated above its original position, quite distinct from the final state of the strong-shear case. After reviewing the dynamics and plasma properties as well as the evolving magnetic topology, we conclude that geometry, shear strength, and local resistivity must determine the dynamic signatures of chromospheric eruptions. Our model reproduces such fundamental observed features as intermittency and large velocities, as well as the approximately concurrent appearance of oppositely directed flows. We also find that reconnection in the vertical current sheet is more consistent with Sweet-Parker reconnection theory, while the rapid interaction between the magnetic islands and the background field better approximates the Petschek process.
Antiochos Spiro K.
DeVore Richard C.
Karpen Judith T.
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