5 research outputs found
Fluxon Modeling of Low-Beta Plasmas
We have developed a new, quasi-Lagrangian approach for numerical modeling of
magnetohydrodynamics in low to moderate plasmas such as the solar
corona. We introduce the concept of a ``fluxon'', a discretized field line.
Fluxon models represent the magnetic field as a skeleton of such discrete field
lines, and interpolate field values from the geometry of the skeleton where
needed, reversing the usual direction of the field line transform. The fluxon
skeleton forms the grid for a collection of 1-D Eulerian models of plasma along
individual flux tubes. Fluxon models have no numerical resistivity, because
they preserve topology explicitly. Our prototype code, \emph{FLUX}, is
currently able to find 3-D nonlinear force-free field solutions with a
specified field topology, and work is ongoing to validate and extend the code
to full magnetohydrodynamics. FLUX has significant scaling advantages over
conventional models: for ``magnetic carpet'' models, with photospheric
line-tied boundary conditions, FLUX simulations scale in complexity like a
conventional 2-D grid although the full 3-D field is represented. The code is
free software and is available online. In this current paper we introduce
fluxons and our prototype code, and describe the course of future work with the
code.Comment: 14 pages, 11 figures; also in press for JAST
Observations of the Sun at Vacuum-Ultraviolet Wavelengths from Space. Part II: Results and Interpretations
A new view of the solar outer atmosphere by the transition region and Coronal Explorer
The Transition Region and Coronal Explorer (TRACE) described in the companion paper by Handy et al. (1999) provides an unprecedented view of the solar outer atmosphere. In this overview, we discuss the initial impressions gained from, and interpretations of, the first million images taken with TRACE. We address, among other topics, the fine structure of the corona, the larger-scale thermal trends, the evolution of the corona over quiet and active regions, the high incidence of chromospheric material dynamically embedded in the coronal environment, the dynamics and structure of the conductively dominated transition region between chromosphere and corona, loop oscillations and flows, and sunspot coronal loops. With TRACE we observe a corona that is extremely dynamic and full of flows and wave phenomena, in which loops evolve rapidly in temperature, with associated changes in density. This dynamic nature points to a high degree of spatio-temporal variability even under conditions that traditionally have been referred to as quiescent. This variability requires that coronal heating can turn on and off on a time scale of minutes or less along field-line bundles with cross sections at or below the instrumental resolution of 700 km. Loops seen at 171 Ă… (Ëś1 MK) appear to meander through the coronal volume, but it is unclear whether this is caused by the evolution of the field or by the weaving of the heating through the coronal volume, shifting around for periods of up to a few tens of minutes and lighting up subsequent field lines. We discuss evidence that the heating occurs predominantly within the first 10 to 20 Mm from the loop footpoints. This causes the inner parts of active-region coronae to have a higher average temperature than the outer domains