Summary of WGC Efforts

Although in some ways this group was well-focused, having the Elephant's Trunk Coronal Hole as its subject, in fact most of the members of the group were also involved in the other groups so it did not come together as a whole very often. Instead, having established aims and areas which needed to be worked on, we split into sub-groups for discussion and work, in between attending meetings of the other groups - all very confusing! However, I think it worked out well in that we made considerable progress towards achieving our aims.

Here is a list of people that are involved in the work of this group (not necessarily original members):

David Alexander	Yohkoh
Andy Breen	EISCAT IPS
Barbara Bromage	CDS
John Clegg	CDS/modelling
Giulio Del Zanna	CDS
Craig DeForest	MDI
Dana Dobrzycka	UVCS
Gopal Gopalswami	Nobeyama
Don Hassler	SUMER
Todd Hoeksema	MDI/modelling
Alan Lazarus	WIND
Jon Linker	modelling
Barbara Thompson	EIT
Harry Warren	SUMER
Zoran Mikic	modelling
Xue Pu Zhao	MDI/modelling
John Steinberg	WIND

I apologise if I have missed anyone out - please tell me if I have !
( Arik Posner and George Ho ?)

On the first day some specific aims and areas requiring work were identified. These were:

1. Relating Nobeyama oservations to CDS data. This work progressed well, concluding that the bright areas seen by Nobeyama were varying on a time scale of tens of minutes which might relate to brightenings seen in CDS data in transition region or chromospheric wavelengths. More data has been taken which will help to confirm this.
(Gopal, Del Zanna, Bromage)

2.Attempting to map the WIND observations back to the EUV observations of the ET hole. The aim is to map back to an image of the hole at the time that the fast wind seen by WIND might have originated. Mikic and Jon have already shown that the fast wind maps back to within their modelled coronal hole, based on magnetic field data from 26th August. However, we have still to match this to images (EIT.CDS) at around the correct time, which would have been a day or two earlier (?). During the workshop the question arose - why does the open field not appear to map back to the 'tip' of the Elephant's Trunk in the Mikic/Linker model? John Clegg will check if this is the case with his mag field modelling.
(Bromage, Thompson, Mikic, Linker, Clegg, Zhao, Hoeksema)

3. Compare intensity profiles/densities and temperatures within the hole taken by SUMER and CDS. Also relate to MDI data. This was during a period of high time resolution MDI/EIT observations. CDS has analysed this period. SUMER is still working on it. The EIT data has been looked at - there are one or two regions changing during this time. We have not looked at the MDI data yet.
(Hassler, Warren, Del Zanna, Bromage, Thompson, DeForest)

4.Comparison of CDS and Yohkoh temperature data. CDS try to do emission measure for MgIX? Use densities from SiIX to infer the volume. Maybe repeat for FeXIII? These will need to be done later - will take time. For now can compare Yohkoh temperatures with the MgX/MgIX ratio and perhaps the emission measure with the differential emission measure curve over that temperature range. Some of this work has transferred to group B. During the workshop an interesting cool feature was noted in the SXT data which will be investigated further.

5. Identify CIRs around the time of the ET hole. This topic I believe has transferred to group E (?).
(Breen, Posner, Galvin?)

6. Identify flow data in the ET hole. This was mostly to involve SUMER and possibly UVCS. This topic I believe also has transferred to group B.
(Warren, Strachan, Dobzycka)

Ensuing discussion generated one or two interesting topics:

What is a coronal hole? How do we define the boundaries? How does the structure of the ET relate to the associated prominence channels? How does this relate to the neutral line data produced by Pat Mackintosh? What does the 10830 data show us about the ET? Does the size (width) of the hole limit the speed of the fast wind emanating from it?

'Summary' Paper:

It was a little difficult to determine how much of the work on the ET hole should be covered by this paper. In the end an outline was generated, which hopefully covers the main points of interest. There was not really sufficient time to discuss this in detail so please continue to comment on/ complain about this!

The title : The Elephant's Trunk Coronal Hole

The paper should address the questions:

• What has caused/controls the hole?
• What is different about it compared to polar holes/polar hole extensions?
• What is the same?
• What is exciting about the observations/results?

  First section - Morphology
  Some sort of composite picture showing images on 26th August from EIT, Yohkoh,Nobeyama;mosaic in different wavelengths from CDS; series of sections of CDS OV data with Nobeyama contours on top, for 25,26,27th, showing variation from day to day.
  Bromage, Del Zanna, Gopalswami, Alexander, Thompson

  Second section - Context
  Footpoint synoptic from Xue Pu, surface picture from Mikic/Linker model showing surface of boundary between open and closed field, plus white light synoptic image from Mauna Loa with IPS source regions superimposed, CIR particle data. Zhao, Breen, Mikic/Linker, Posner

  Third section - magnetic structure
  Force-free field models showing the importance of prominence arcade and connection to active region. Evolution. Profile across the hole at different heights - 4 wavelengths CDS, 2 Yohkoh, Nobeyama, 4 EIT, all at 'the' CDS cut, inferred boundary structure. Maybe also some UVCS boundary info at 1.75 and 2.25 Rs.
  Clegg, Bromage, Del Zanna, Alexander, Gopalswami, Thompson, Dobrzycka

  Fourth section - rotation
  EIT (MDI). Show short-term differential rotation. Discuss longer term, with times of subsequent meridian passages.
  Thompson, DeForest, Bromage?

  Discussion -
  Include development, evolution, comparison with polar holes, relationship to fast/slow wind in model, relationship to CIR etc.

  Conclusions - summarising as answers to above questions.

  A draft abstract was also generated:

  THE ELEPHANT'S TRUNK

  The dramatic trans-equatorial coronal hole (the "Elephant's Trunk") which appeared at the end of August 1996, extending from the northern polar coronal hole to an active region just south of the equator, was observed by a large number of solar instruments. These include many on board the Solar and Heliospheric Observatory (SOHO) as well as some ground-based instruments such as the Kitt Peak National Solar Observatory and the Japanese Nobeyama radio telescope. Associated changes in the solar wind were seen by instruments on the Ulysses and WIND satellites, and by the EISCAT radar system in Scandinavia. We present a summary of these observations, describing the structure and evolution of the coronal hole. We compare the plasma parameters in and around the hole, determined by the various instruments. The expansion of the coronal hole with height in the atmosphere is estimated, giving an idea of the three-dimensional shape of the coronal hole boundary. Modelling of the magnetic structures which define and control the shape of the hole is discussed. Events seen in the solar wind data are linked to the Elephant's Trunk by MHD modelling of the magnetic field which extends from the Sun to the location of satellites recording in-situ data, indicating fast wind streams emanating from within the coronal hole and slow wind streams from the boundary regions.

  Achievements

  I personally found the workshop very stimulating with a lot of positive interaction and communication between people having different viewpoints on the Elephant's Trunk. I believe that the work achieved by our group during this week was sufficient to start the ball rolling on about half-a-dozen specific topics which will hopefully result in papers for the GRL issue, as well as the so-called Summary Paper. A very enjoyable week!

  Barbara Bromage