Session C2: Coronal magnetic field reconstruction.

A. Physical difficulties:

1. the photosphere is not force free;
2. current sheets (and topology skeleton) are not force free.

B. Data limitations:

1. incomparable noise level in line-of-sight and transverse components;
2. 180 degree ambiguity in transverse magnetic field;
3. imbalanced flux in the regions of interest;
4. projection effect (if regions are away from disk center);

C. Technique challenges:

1. code is bit too slow;
2. if algorithms always converge;
3. if solution is unique (e.g. if solution depends on initial inputs, 
like initial configurations);
4. if codes tolerate noise;

D. Constrains from other observations:

1. Vector magnetic field observation on the chromosphere; (but the 
formation height has a large range, line information is complex, and
the noise might be bit high.)
2. coronal magnetic field measurements;
3. loop tracing; (but need to transform 2D images into 3D structure that contains
projection effect.)
4. tomography technique; (but need to assume the structures unchange for several days).

E. Some solutions:

1. Code is too slow: reduce resolution in vertical direction; reduce temporary 
resolution; more constrains on initial configuration; etc.
2. Noise, flux imbalance, etc in data: pre-processing the data;

F. Questions need to be answered:

1. Can we define a scope in which the NLFFF can be applied to? (Certainly some complex
active regions are beyond this scope)
2. Given the data we already have or we can foresee, what is a reasonable goal we
can achieve?
3. If we see differences between the modeled coronal structure and observations, then 
what we do next?

G. Summary:

We have seen certain level successes in modeling coronal magnetic field from the
NLFFF model; we are, of course, aware of tremendous challenges there. It is definitely
necessary and demanding to continue the effort of the NLFFF work group to rise
issues, to address challenges, to seek for solutions for the problems, and more
important, to learn somethings physically and technically during those practices.
It is also helpful to seek for further supports scientifically and financially from
the whole community by reporting the progress we made and rising the questions we
encountered to a larger group or the whole community.