RHESSI Paris workshop 28-Jul-04

Working group 4 notes

Mrozek: source heights
 * analysis following that of Aschwanden & Brown
Discussion of basic points of method (Demoulin, Hudson, Dennis, Benz)
 * re-checking the AB flare: forward method and CEAN give very different
  sources: centroid from RHESSI sw @ 60% and other contours
Discussion (Krucker, Dennis, Fletcher, Khan, Benz): do not use forward-fit now,
  back projection is more reliable
Discussion of AB boxcar smoothing (Hudson)
 * comparison of four image methods for 20-jun (?) flare with coronal
  and footpoint sources
 * 3-aug-02 19:04 event with TRACE overlay via correlation overlay
 * four 12-sec intervals, H vs E fits, show gradually increasing heights 
  with few-arcsec range
Discussion of model interpretation as density increase with time (Demoulin,
  Dennis, Mandrini, Fletcher, Khan)
 * height vs time at a given energy => velocity estimate, not incompatible
  with evaporation
 * energy of flattening of h-vs-e plot, statistical results: how to compare
  height scales for different events?
 * puzzling cases suggest step in density perhaps
Discussion of danger of running means and known image artifacts (Dennis)
 * 2D plot of source positions
 * comparison with Matsushita et al. 1992, Fletcher 1995 modeling 
Discussion (Khan et al): this work is a fundamental part of the RHESSI
  data analysis

Somov: coronal sources
 * 20 flares selected, for which 5 have upward velocity > 3 sigma: 
  10-30 km/s upward motion observed
 * collapsing trap with betatron (2nd invariant valid) and first-order
  Fermi energy gain
 * loss-cone behavior: Somov-Bogachev, Astr Letters 29, 621, 2003: exact
  match with energy gain and loss-cone growth
 * no need for shock structure, prediction of two components
Discussion (Fletcher, Demoulin, Hudson, Khan): is the trap dense enough
  to emit hard X-rays? (Veronig) some flares have high enough coronal
  densities to emit hard X-rays. (Benz) read my thesis! I don't think
  betatron gives enough energy. (Somov) start with SHTCL, main effect
  is not compression but instead the decrease of trap length - in the
  case of a termination shock, it might decrease to zero! (Hudson) is
  the SHTCL not clearly visible in soft X-rays if it is dense and hot?
Discussion II (Dennis): initial decrease of altitude (Somov) also seen
  in the HXT sample. (Dennis) we frequently observe downward early 
  motions at 20 keV.
 
Veronig: Nov. 3 event
 * Excellent RHESSI event with Moreton wave (H-alpha) with difficult but
  odd great-circle velocity fit
 * RHESSI images show nice FP+LT morphology 
 * apparent downward motion
 * second phase, soft-hard-hard, RHESSI shows footpoints
Discussion (extensive, too interesting for note-taking. What a great
  event!)

Benz: Nov. 3 radio data
 * two type II bursts and "afterglow"
Discussion: physical interpretation is complicated... 

Hudson: Nov. 4 white-light loops
 * talking so no notes

Somov: "bubble model"
 * large-scale reconnection in a large flare
 * reconnection in a vacuum and in a plasma
Discussion of current layer thickness (Hudson) deferred
 * powerful heating by waves of electrons in the current sheet
 * then "rainbow reconnection", as in Bastille Day flare
 * for which, topological model looking like an orangutan, based upon
  five effective sources
 * separatrix structure in a "topological portrait"
 * Somov et al., Astr Rep. 48, 246, 2004 
 * E = 30 V/cm 
 * Conclusion: it would be easy to criticize this simple model, but it
  may be adequate!
Discussion (Demoulin): no criticism, just appreciation. But... in the
  Bastille case, the N ribbon does not seem to follow the observation
  so well. So we probably need more sources.
  (Hudson) what is the electrical potential distribution? This is not
  a problem, the experts say!