Three Criteria to Discriminate Between Elementary and Composite Coronal Loops
Markus J. Aschwanden Submitted: 2005-07-13 17:01
In this Letter we establish three simple criteria to discriminate between elementary
and composite loop structures, based on observations from high-resolution (TRACE)
and low-resolution (EIT) imagers. Both datasets contain triple-filter data with a
temperature diagnostic in the range of T=0.7-2.8 MK, which we use for
forward-fitting of a differential emission measure (DEM) distribution to constrain
the temperature range of each structure. From the TRACE dataset we find for the finest
structures a mean width of w=1.4±0.43Mm, a flux-to-background contrast of
c=0.18±0.13, and a temperature range of dT=0.07±0.10 MK,
which we identify as elementary strands, defined in terms of their thermal homogeneity.
The loop structures observed with EIT have loop widths w, flux contrasts c, and
temperature ranges dT that are all about an order of magnitude larger, and thus
clearly constitute composite structures, consisting of many loop strands.
These two contrasting observations resolve previous controversies about the
basic thermal structure of coronal loops and yield a simple discrimination rule:
Elementary loop strands (1) are near-isothermal (dT < 0.2 MK),
(2) have a small width (w < 2 Mm), and (3) have a faint contrast
(c < 0.3), while virtually all wider and higher-contrast loop features
are most likely to be multi-thermal composites and have a broad DEM.
Authors: Markus J. Aschwanden
Publication Status: ApJ Lett. (revised 2005-Oct-05)
Last Modified: 2005-10-05 10:26