Working Group 1 (Coronal Mass Ejections) continued to pursue a main
theme from last year's workshop: observational tests of competing CME
initiation models. We focused our attention on two rather specific
topics that were identified last year and that appear in our matrix of
predicted observables (http://umbra.nascom.nasa.gov/shine/cme_models.html).
In question form, they are:
1. What is the role of photospheric magnetic field evolution in
a. Is there a gradual buildup phase involving the emergence, shearing,
or convergence and cancellation of magnetic flux, as envisioned in
"storage and release" type models?
b. Are there real-time changes in the photospheric field during the
eruption, as envisioned in "dynamo" type models?
2. What is the role of magnetic reconnection in the eruption process?
a. Does reconnection, as evidenced by strong heating for example, occur
before or after the eruption begins?
b. Is the reconnection located above or below the primary erupting
structure (sheared core field or flux rope)?
The two topics were introduced in the excellent introductory talk by
Zoran Mikic and were then addressed separately during the first two working
group sessions. These sessions consisted of lively discussion centered around
invited presentations. The discussion continued during the third and final
session, which included short presentations from three young CME modelers
(B.C. Low being young at heart!).
Most of the invited presentations were concerned with observations, and
it quickly became apparent that there is no easy answer to the above
questions. Movies of photospheric magnetograms reveal that many different
types of surface field evolution occur together during the period leading up
to CMEs: emergence, cancellation, shearing, convergence, etc. It is not
readily apparent which, if any, plays a dominant role in the energy buildup
and triggering of eruptions. Sizable magnetogram changes are not generally
observed while CMEs are taking place (at least not in the several examples
presented), but disagreement about what the dynamo models predict prevented
us from making a definitive assessment of their viability.
Similarly, the question of when reconnection occurs is a challenge, but
it is now possible to better identify effective methods to address this
issue. From the theoretical standpoint, the question hinges on whether
reconnection occurs early in the highly-sheared core region, early in a
region far-removed from the core, or if early reconnection does not occur at
all (i.e., eruptions due to some non-resistive process, with reconnection
only occurring after eruption onset). Based on the presentations at this
meeting, there is some evidence that CMEs show early evolution prior to
flare brightenings, based on comparisons with LASCO, EIT, and GOES data.
Also, there are cases where CME-associated dimmings in EIT are seen well
before other eruption signatures. These results may be implying that
eruptions are occurring at an early stage without a substantial reconnection
signature. Another view, however, comes out of H-alpha observations which
show Doppler motions in the neighborhood of filaments that are about to
erupt; these observations may suggest that reconnection episodes occur prior
to the onset of substantial soft X-ray emission.
The last working group session included discussion of a third topic--the
the pre-eruption coronal magnetic field structure--which also appears in the
matrix of predicted observables from SHINE 2000. The basic question is
whether a twisted flux rope is present before the eruption begins or whether
it forms during the eruption process. There was no concensus.
An outcome of our deliberations was a list of challenges for the coming
1. Develop QUANTITATIVE measures of flux cancellation, flux emergence,
and shear flow, and determine whether they are correlated with CMEs.
2. Investigate the location and timing of magnetic reconnection relative
to the onset of CMEs, using coordinated observations from a variety of
sources, including H-alpha, EUV, and X-ray images.
3. Determine observationally the pre-eruption coronal magnetic structure
(e.g., flux rope or sheared core field).
4. Model a real event based on time-dependent photospheric boundary
conditions from observations.
5. Determine whether CMEs are global phenomena (i.e., are a result
of evolutionary changes involving a major fraction of the photosphere
In summary, Working Group 1 had an enjoyable and productive time in