RHESSI/SOHO/TRACE Workshop

Working Group 5 - Ribbons and Footpoints

This group will concentrate on the behavior of solar flares in the lower solar atmosphere, ranging from hard X-ray footpoints and white-light flares to "spreading moss" and regular moss. By focusing in this way we omit another very interesting aspect of "ribbons and footpoints," namely how they guide us to the evolution of coronal structure. That is the focus of Working Group 2, we believe, but we hope to have overlap sessions with this WG somehow.

Questions

1. How is the lower atmosphere of the flare heated?

2. How is white-light flaring distinguished physically from UV flaring?

3. How do we understand the horizontal structure seen within flare ribbons and moss?

4. What is the role of magnetic reconnection in the lower atmosphere?

Observations

We expect that footpoint/ribbon data will come from all three of the RST spacecraft (RHESSI/SOHO/TRACE). One striking new result from RHESSI suggests that the energetic ions precipitate into the lower atmosphere at different places from the electrons; both populations transport enormous energy fluxes, and yet they seem to affect the lower atmosphere differently.

Theory and modeling

The general theoretical idea is that the lower solar atmosphere is heated from above, either by particle bombardment in flares and flare-like phenomena; or by thermal conduction in more static situations. We do not know really if this covers all the possibilities, since there could be unknown physics in the footpoint regions too.

Because the data have improved strikingly, and we are seeing new things (moss) as well as old things better (white-light flares), it is time to re-consider some of the theoretical work and modeling. The standard models for footpoint evolution have been the 1D radiation hydrodynamics models. These have obvious potential shortcomings (no horizontal structure; assumption of small optical depth; tenuous assumptions about coronal behavior). All of this suggests that this workshop would be an excellent opportunity to review the performance of such models as a guide towards future improvements.

Events for study

Jake Wolfson's list of white-light flares?
Lyndsay Fletcher's list of well-observed ribbon events?

Literature

Relevant recent papers:

"Numerical simulations of impulsively heated solar flares," Mariska et al. (1989)
"TRACE and Yohkoh Observations of a White-Light Flare" Metcalf et al. (2003)
"On the Nature of the ``Moss'' Observed by TRACE" Martens et al. (2000)
"First Gamma-Ray Images of a Solar Flare" Hurford et al. (2003)
Fletcher et al. (2004)

H. Hudson, K. Schrijver