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Evidence for the flare trigger site and 3-D reconnection in multi-wavelength observations of a solar flare
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David Alexander Submitted: 2001-02-06 09:39
Based on a multi-wavelength data set and a topological model for the magnetic field, we argue that a M1.9 flare which occurred on 3-May-99 shows evidence of 3-d coronal reconnection in a spine-fan configuration. Images from the Transition Region and Coronal Explorer allow the detailed examination of the structures involved in the flare and pre-flare in the 171A (1MK) EUV passband, and the Lyman-alpha (10,000-20,000K) passband. Yohkoh Hard X-ray Telescope maps the position of non-thermal electron
precipitation and the Soft X-ray Telescope reveals pre-flare and flare heating on large and small-scales. While the flare appears to be driven by changes in small scale field close to the photosphere, near the interface between strong opposite magnetic polarities, the result is the disruption of large-scale field. We demonstrate how this observed activity on large and small scales, along with many other aspects of the flare, find a qualitative explanation in the three-dimensional reconfiguration of coronal magnetic field,
following a small-scale flux cancelation at the photosphere.
Authors: L. Fletcher, T. R. Metcalf, D. Alexander, L. A. Ryder and D. S. Brown
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Publication Status: ApJ, 554, 451, 2001
Last Modified: 2001-07-10 17:38
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Heating the atmosphere above sunspots
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David Alexander Submitted: 1999-11-04 19:03
We present our results of a hybrid model of sunspots and their overlying corona.
The two-layer model considers both the nonlinear, compressible magnetoconvection
beneath the photosphere and potential, or linear force-free, models of the
coronal fields. Heating of the plasma along the field lines is then treated
using quasi-static and steady-state models with the heating rate being specified
by the dynamics of the magnetoconvection. Two distinct magnetoconvection
scenarios are considered. The first describes magnetoconvection in a 2D
axisymmetric geometry and considers the time development of the overlying
coronal field. The second describes a 3D cylindrical geometry with a static
coronal field configuration. Both scenarios diverge from the standard practice
of assuming constant temperature and vertical magnetic field conditions at the
top surface. Instead, a radiative linear force-free field condition is adopted.
Extrapolation of the top surface boundary conditions results in a coronal field
configuration which is assumed to be filled with plasma heated to coronal
temperatures. The heating rate and thermodynamic behaviour of the plasma is
related to the sub-surface model by assuming that individual fluxtubes are
heated uniformly with the necessary energy being generated from the dissipation
of the Poynting flux entering the coronal volume. Radiation and conductive
losses are included. The combination of a sunspot model, whereby the surface
field is completely specified, with a coronal heating model, in which the plasma
parameters are specified for a given energy input, allows us to explore a broad
class of heating paradigms.
Authors: Alexander, D., Hurlburt, N. E. and Rucklidge, A. M.
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Publication Status: Proceedings of SOHO 8 (in press)
Last Modified: 1999-11-04 19:03
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