TEMPERATURE AND DENSITY STRUCTURE OF THE SOLAR CORONA

A. Fludra, D. Alexander, D. Biesecker, J. Newmark, J. Raymond,
  C. Schrijver, and others....

Following the CDS observations of coronal holes and streamers made
during the Whole Sun Month-1 (Fludra et al. 1999a), a regular
observing programme has been initiated by CDS, focusing on
measurements of coronal line intensities and density-sensitve line
pairs that provide a measurement of coronal electron temperature and
density in the quiet sun and above polar coronal holes (Fludra et
al. 1999b).

The goal of this JOP is to enhance the CDS observations with data from
other instruments and derive an accurate empirical model of the radial
dependence of electron temperature and density in the quiet sun corona
and coronal holes, from the limb up to 3 solar radii. The emphasis
will be on distances close to the limb, up to 1.5Rsun, covered by the
CDS, SUMER, Yohkoh/SXT, UVCS, EIT and Trace, although extension to
greater distances will be sought by using the polarised brightness
data from LASCO and Mauna Loa.

A series of above-the-limb measurements will be made during the Whole
Sun Month-3. The targets will be quiet sun and polar coronal holes (if
present). Thereafter, periodic observations approximately once per
month will monitor the changes in temperature and density
distributions with the solar cycle, level of activity and type of
observed structures.

The two components: quiet sun/streamers and polar coronal holes will
be observed independently, with the polar coronal holes becoming more
rare at the current stage of the cycle.

 
Instruments' contributions:


CDS: a number of coronal lines provide the temperature information,
     and Si IX and Si X line ratios provide the density diagnostics.
     Existing NIS studies (CORONAW and STRE4W) will be used.
     They will be supplemented by GIS studies.  

Yohkoh/SXT: long exposure full-disk observations. SXT is capable of
     providing a measurement of a quiet sun signal and is valuable in
     sampling higher temperatures than CDS. Fludra et al. (1999a) have
     found a good agreement between the CDS and SXT observations,
     although some remaining differences indicate a need for a more
     detailed model.

SUMER: Si VIII density-dependent lines measurements above the limb.

EIT: several sets in 171, 195 and 284 bands

TRACE: images in 171 and 195 bands above the limb.   
     It is anticipated that Trace (and EIT) will find this
     collaboration useful as it provides a test of the resonant
     scattering contribution to their wavebands (Schrijver and McMullen,
     1999).

LASCO: white light polarized brightness measurements with good
      signal-to-noise. 2 or 4 sets per day. 

UVCS (input from John Raymond):
`` 3 days of observations when we could place the VLD
aperture on a streamer, do pB sequences with EUV
spectra (probably full detector spectra; we would
have to spend a couple hours at each height to let
any drifts die away).  We would not require that
the star be behind the streamer, as it is just a
positional reference.  We would require that the
streamer chosen be on the same limb, and preferably
not too far away.  

2 days getting full spectral coverage at 1.5 Rsun
on the streamer or coronal hole of choice.  Probably
5 grating positions.  This would give electron
temperatures.

2 days getting the O VI doublet ratio as a density
diagnostic at the highest possible spatial resolution.
Single row binning and high grating position.  Once 
in a streamer and once in a coronal hole.  Perhaps 
this could be done during the star observations, but
we would want to do it at 1.5 Rsun.''



References:

Fludra, A., Del Zanna, G., Alexander, D., and Bromage, B.J.I., 1999a,
Electron Density and Temperature of the Lower Solar Corona,
J. Geophys. Res. (WSM special issue)

Fludra, A., Del Zanna, G., and Bromage, B.J.I., 1999b, EUV
Observations above Polar Coronal Holes, Space Sci. Rev. (in press)