SOHO Joint Observing Plan 066: Loop Fine Structures

EIT/CDS/SUMER Loop Fine Structure Observing Program
G. Peres, R. Rosner, A. Poland, 

Purpose:  To observe isolated loop structures in coronal holes or on
the limb with SUMER, CDS and EIT to determine temperature and density 
both along and across the loop structure (i.e. along and across the
dominating magneting field), as well as the velocity field, 
as a function of time.

Observations made with SOHO coronal instruments have clearly shown the
presence of isolated loops in coronal (polar and non-polar) coronal
holes. Since they are isolated there is virtually no risk of confusion
with other nearby loops and no risk of superimposing plasma along the
same line of sight but from different loops.  Therefore such loops
offer a unique opportunity to observe and analyze the physics of loops,
believed to be the building block of the confined corona.

An important aspect of this program is to observe in the light of many
emission lines simultaneously so as to study the coronal plasma
conditions over a wide range of temperatures, from chromospheric to
coronal ones. This feature, among other things, allows to determine
whether any difference in morphology seen in the light of the various
lines, as well as time variability, is due to density or temperature

This observing program will help determining if the velocity flows
predicted by extant siphon flow or, alternatively, microflaring loop
models occur in real loops.

The capability of SUMER to achieve sub-arcsec angular resolution along
with very fine spectral resolution will help to study the fine
structure and, in particular, how plasma conditions change across the
loop structure. Therefore it is important to target loops observable
with such an instrument.

Observing Program:
1. In EIT:     Use synoptic observations to identify promising
               loops, e.g. the ones isolated 
               and evident against a dark background, for instance
               inside coronal holes or at the limb. Once the observing
               program begins, EIT takes small images of the loops at
               the fastest cadence possible in all four bands.

2. In SUMER:   if the loop fall in a place of the solar disk accessible
               to SUMER move the slit on it and use the 0.3"x120" slit,
               The line selection should cover a range from the
               chromosphere to the corona. Here we propose the
               following (redundant) list, arranged in order of
               increasing temperature of most efficient formation: C
               III 1335, C III 1175.7, C III 977.02, N III 989/991, S V
               796, N IV 765.14, N V 1238, O V 629.73, O V 1218, O VI
               1032/1037, N VII 770, Ne VII 895, Si VIII 1475, Mg X
               625, S X 1213, S X 1196, Fe XII 1242, Si XII 520.  The
               selection as well as the exposure times should comply
               with the requirements by SUMER, which may demand last
               minute selection. Such an aspect is particularly true
               for lines too bright or which fall in the wings of

3. In CDS:     Use the 2x240" slit, and cover an area 2'x4', make
               100 sec of individual exposures for a total duration
               of 1h 30m.  If the loop is too large to fit into a
               single exposure, move the image so the field of view is
               just above or just on the side of the previous one so as
               to cover the entire loop, and make another image, and so
               on until the entire loop is covered. Most loops,
               however, should be covered with a single image. This
               will give us a good overall view of the loop over a wide
               temperature range.  Use the following lines in CDS:  He
               I 584, He II (2nd) 607., O III 599.59, O IV 554, O V
               629, Ne VI 562, Ca X 557, Mg IX 368, Mg X 624, Fe XII
               364, Si IX 342/349, Si X 347.5/356, Fe XIV 334.2/353.8,
               Si XII 520

Coordination with the other instruments on SOHO:

UVCS - nearby outer coronal observations would be very useful if large
coronal loops are studied at the solar limb. Observations in the O VI
channel, in particular would allow to expand in the higher corona the O
VI observations made on disk.