SOHO Joint Observing Programme 27

Diffuse Corona

Authors: J.L. Culhane (CDS), R.A. Harrison (CDS), J. Raymond (UVCS), P. Lemaire (SUMER) and R. Howard (LASCO)

Progress:

Draft Scheme				August  7, 1995
Distribution				August 12, 1995

Objective: Clarify the nature of the diffuse corona at and above the limb. Determine whether or not the plasma is contained in magnetically closed structures.Determine the plasma properties as a function of height using CDS, SUMER, UVCS and LASCO.

Conditions Necessary to Run: Minimum requirement is the operation of CDS or SUMER on a diffuse or quiet region of the corona at and above the limb. The region should as far as possible be separated from active regions, bright points and other manifestations of coronal activity. An appropriate region can be selected from Yohkoh or EIT full-sun images.

Scheme: CDS and SUMER should execute a series of radial spatial scans starting inside the limb and moving out into the diffuse structure with the aim of obtaining EM and T as f(h) for a series of line temperatures in the range 1 - 5.106K. Density diagnostic lines should also be chosen within this range so that the diffuse region can ideally be mapped and ne compared with values deduced from image based knowledge (Yohkoh, EIT) of the overall structure volume. Line widths should be measured and the inferred temperature compared with measured electron temperatures whenever the line profile statistics permit. LASCO and UVCS observations will be used to extend the height coverage of the regions and to provide a better delineation of the region volume.
Diffuse emission is pervasive in the corona. It is of interest to establish -

Whether or not the the structures are closed and looplike or unstable and quasi-open
What is the relationship of the structure to the underlying photospheric field configuration
Can detailed observations identify individual loops
Do the temperature, pressure and characteristic size values conform to loop scaling laws.

Assuming that the first observations provide useful data, future use of the JOP - preferably in coordination with ground based coronameter observations, could address questions related to the growth, evolution and decay of these structures. The interaction of diffuse material with coronal hole boundaries will be of particular importance.

Pointing: Based on Yohkoh and EIT inputs, ahomogeneous diffuse region should be selected with symmetrical structure in front of and beyond the limb. It should as far as possible be unrelated to activity and active regions. Radial scans should be made with height with CDS and SUMER.

Frequency: As often as well developed and stable diffuse regions are available at and beyond the limb.

Operations: Assuming observations of a comparatively large structure ~ 10 arc min in radial extent above the limb and ~ 20 arc min around the limb, the following should be used -

CDS
Spectrometer: NI
Slit: 5x240 arc sec
Area scanned: Up to five adjacent radial scans equi-spaced around the limb. Each radial scan should be progressively outwards in 60 steps of 10 arc sec. Each radial scan would occupy 250 minutes and so total coverage of the region could be spread if necessary over a period of days.
Lines: Similar to JOP 28 for Streamers. Lines provide a wide range of temperature sensitivity together with some density sensitivity. A He I line is observed to ensure co-registration. Lines are -
```
Ion		Wavelength ()	log Te		Comment

He I		522.20		4.3		Cool line for registration
Fe X		345.74		6.1
Fe XI		369.16		6.1
Fe XII (a)	338.26		6.2		Density pair with (b)
Fe XII (b)	364.47		6.2	      	 "      "     "   (a)
Fe XIII (c)	348.18		6.2	     	 "      "     "   (d)
Fe XIII (d)	359.64		6.2	     	 "      "     "   (c)
Fe XIV		334.17		6.3
Fe XVI		335.40		6.4
```
SUMER
TBD
Similar pointing coverage to CDS. Line selections to emphasize similar temperature range to CDS but with the aim of identifying non-thermal broadening in the emission lines.
LASCO
TBD
Extend observations to higher altitudes
UVCS
TBD
Extend observations to higher altitudes

Context Observations: Ground based coronameters and prominence monitors e.g. Mauna Loa, Yohkoh SXT.