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Session 16 - Magnetic Fields I.
Oral session, Tuesday, July 01
Ballroom A, Chair: Edward DeLuca
It is common to model the Sun's corona as a force-free magnetic field undergoing slow changes due to motions of the photosphere. The occurrence of sudden energetic events, such as X-ray bright points, transient loop brightenings or compact flares, suggests that there are occasional departures from this quasi-static evolution which might involve magnetic reconnection. To understand these phenomena using full magnetohydrodynamics (MHD) it is typically necessary to consider simple magnetic configurations and employ numerical simulations. Recently, however, a type of model has been developed which treats a simplified physical system in much more complex geometries (Demoulin et al. 1993, Astron. Astrophys. 271, 292). These models simplify the physical model by attributing the coronal field to discrete photospheric flux concentrations (point charges) and considering primarily the field's topology.
While inherently less accurate than full MHD treatments, topological field models offer several advantages both as theoretical frameworks and as tools for interpreting observations. Discrete magnetic sources endow the field with topology and define sharp topological boundaries called separatrices and separators. Study of such a model shows that motion of the photospheric sources will induce current along the separators. It is possible to estimate the current carried and energy stored as a function of flux displacement (Longcope 1996, Sol. Phys. 169, 91). This leads to quantitative theoretical estimates for energies and frequencies of flaring or loop brightening and for average coronal heating rates. To model a specific active region, rather than generic field structures, a magnetogram is approximated using discrete sources. Preliminary results reveal the power of this technique as tool for interpreting observed coronal activity in complex active regions.
Program listing for Tuesday