Hi All,
I would like to pick up on some of the recent comments concerning turbulence. One the the "annoying observations" not considered by anyone so far is the FIP effect (abundance enhancement of Fe, S Mg, etc in the corona relative to the photosphere) and related coronal abundance anomalies such as the depletion of He and possibly Ne relative to H. In my opinion, these are all caused, at least in closed loops, by the action of the ponderomotive force as Alfven waves propagating in the loop reflect from the chromosphere at each end. In a recent paper (http://lanl.arxiv.org/abs/0901.3350) I argue that these Alfven waves must be generated in the corona. The amplitude required to get the fractionation right is 150 - 300 km/s, about one order of magnitude higher than current spectroscopic measurements of non-thermal line broadening, but high enough to give energy fluxes in Alfven waves commensurate with those required to heat the corona.
The high velocity amplitude suggests that only a small part of the emitting loop volume may be oscillating in this manner, which points to filamented loop structures. Nanoflares localized on particular field lines, or Alfven resonance are suggested as coronal heating mechanisms. Coronal emission line profiles should have a "pedestal" with width 150-300 km/s, similar to the observations of transition region line profiles by Hardi Peter (2001 A&A, 374, 1108). I would also suggest that magnetic energy release in the corona (perhaps as nanoflares) goes primarily to turbulence, which then ultimately damps as heat, presumably by phase mixing in horizontally structured conditions. Then the location of heating does not have to be the same as the location of initial magnetic energy release.
This approach is "orthogonal" to most of the discussion so far, arguing from the microphysics of element abundance fractionation as to how Alfven turbulence must behave. I hope that when source and damping terms are included in the non-WKB Alfven wave models, there will be several more constraints on coronal heating mechanisms. Does this seem reasonable? Or does anyone have a better idea of the origin of the FIP effect?
Martin
(John) Martin Laming Naval Research Laboratory, Code 7674L Washington DC 20375 laming@nrl.navy.mil tel: 202 767 4415 fax: 202 404 7997 cell: 240 893 5749
loops@solar.physics.montana.edu