[Loops] summaries on nanoflare debates in "coronalloopworkshops"

[Laming, John]

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 at nrl.navy.mil
tel: 202 767 4415
fax: 202 404 7997
cell: 240 893 5749