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J. Karpen, S. K. Antiochos (NRL), P. MacNeice (Drexel U.)
We have developed a dynamic model of prominence formation in which steady but unequal footpoint heating causes a dynamic cycle of chromospheric evaporation, condensation, motion, and destruction [Antiochos et al. 1999a, 2000, ApJ; Karpen et al. 2001, ApJ]. We have performed 1D hydrodynamic simulations with varying geometries and other properties to determine the limits of this mechanism under solar conditions. In previous studies we identified three key parameters that dictate the existence and characteristics of this cyclic process: the ratio of loop length to heating scale height, the loop apex height, and the heating asymmetry. Here we discuss our latest calculations, in which we studied the role of the depth of field-line dips -- a feature common to most magnetic-field configurations proposed for prominences. In long fluxtubes with dips deeper than roughly f * Hg, where f measures the heating imbalance between footpoints and Hg is the gravitational scale height, condensations form, quickly fall to the bottom of the dip, and remain there while steadily accreting mass. Therefore, strongly dipped loops are not capable of supporting the observed counterstreaming flows along prominence spines. This places stringent limitations on flux rope models [e.g., Rust & Kumar 1994, SolPhys], as only the least twisted field lines close to the axis pass this test. For our shear-based model of prominence fields [Antiochos et al. 1999b, ApJ], a larger subset of field lines can support prominences formed by thermal nonequilibrium: for the case shown (f=0.25), fluxtubes longer than ~80 Mm, lower than ~100 Mm at the apex, or less deeply dipped than ~25 Mm meet the requirements.
This work was supported by NASA and ONR.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.