Doppler Shifts and Line Widths of Helium Lines and their Relation to the Line Formation

Last update: 1. April 1999

Proposed joint observations
of VTT/Observatorio del Teide/Teneriffe, SUMER, EIT, MDI, CDS/SOHO and TRACE
21 - 26 May 1999

Hardi Peter, HAO/NCAR, Boulder, USA (coordinator)
Wolfgang Schmidt, Reiner Hammer, Kiepenheuer-Institut, Freiburg, Germany
Michael Knölker, HAO/NCAR, Boulder, USA

If you have questions or suggestions to change the observing sequence, please contact me: hpeter@hao.ucar.edu Updated information on this campaign at http://www.asp.ucar.edu/~hpeter/projects/helium/planning.html

Overview

The planned observing sequence should give some new hints on the formation of the helium lines in the solar atmosphere by studying the spatial properties of the emission in these lines as well as in other chromospheric and coronal lines. Lines of neutral helium at 584 and 10830 Å will be observed with the spectrographic instruments SUMER/SOHO and the VTT at the Observatorio del Teide at Teneriffe. This will provide raster scans of 400'' x 300'' with full spectral information. The He II (304 Å) will be observed with EIT and CDS/SOHO. The influence of coronal back radiation can be studied from the imaging instrument TRACE that will provide movies in coronal (Fe IX/X) as well as transition region emission (C IV). In addition observations in other chromospheric lines and of the photospheric magnetic field (MDI/SOHO) will provide the possibility to study the spatial correlations between helium and other chromospheric emission. All together this provides a complete data set to address some aspects of the formation of the helium lines, i.e. to understand intensities, Doppler shifts and line widths in the quiet Sun and coronal holes as well as differences of network and inter-network.

Scientific rationale

Existing data: SUMER/SOHO He I (584 Å) full-disk scan

SUMER/SOHO observations of the full solar disk in He I (584 Å) clearly show the coronal holes in intensity, Doppler shift and line width (Peter 1999). In the coronal hole the intensity is reduced by a factor of 2, the line is blueshifted and much broader than in the quiet Sun. In the following those results of this observation are briefly summarized that are of interest for the proposed observations. The following figure shows the variation of these quantities in the southern coronal hole as a function of distance from disk center and the cosine of heliographic latitude. The observation was performed on March 3, 1999.
[more information on the SUMER full-disk data and the raster images]. Helium in coronal holes from Peter (1999) Comparing the variation for He I (584 Å) [red] and Ne VIII (770 Å) [green] two clear differences can be seen. First the line width of He I is strongly increasing in the hole towards the limb indicating that He I (584 Å) is not optically thin. The most striking result concerns the line position:the Doppler shift of Ne VIII is linearly decreasing with cos(theta) in the hole, which is consistent with a constant outflow in the coronal hole, i.e. the fast solar wind seen already in the transition region (line-of-sight effect). But in He I just the opposite trend is found! This is not consistent with a (solar wind) outflow seen already at the altitudes where He I is formed. The Doppler shift variation in He I would be consistent with a meridional flow. However, this seems unlikely as the plasma must be expected to be controlled by the magnetic field in the altitudes where He I is formed. Maybe an opacity effect plays a major role. Maybe models of the type of Andretta & Jones (1997) [including flows] or those of Carlsson & Stein (1997) [also for He I] have to be used to understand this effect. The proposed observations should provide a better observational picture for a comparison with the models still to be done. Another striking result from the full-disk scan is the cell-network relation as represented by the relation of Doppler shift and intensity. The following figure shows this relation in the quiet Sun (disk center, green and near the west limb, blue) and in the southern coronal hole (red). The intensities are scaled to the mean intensity in the respective regions to have comparable intensity scales. Helium cell-network relations from Peter (1999) While a clear cell-network relation is seen in the quiet Sun, the same relation is found in the coronal hole, but with a constant offset towards the blue! An explanation for this effect is still missing. The new proposed observations should concentrate on the spatial structures in the coronal holes and the quiet Sun to get a better basis for a future interpretation. This will mainly be achieved due to a deeper exposure to get a better signal-to-noise ratio.

Existing data: He I (10830 Å) observations

Ground based observations in the He I (10830 Å) line that are of the same type as the SUMER 584 Å data described above were performed by Dupree et al. (1996). In contrary to the He I (584 Å) results they found a center-to-limb variation of the line shift of He I (10830 Å) which is consistent with an outflow out of the coronal hole. They used the asymmetry of the line to derive a "line wing asymmetry parameter", which might be interpreted as being caused by an outflow. In the coronal hole they found a cell-network structure, which is comparable to the one found in He I (584 Å) as mentioned above. However, their interpretation of this relation by an outflow that is concentrated in the cell interiors does not seen to be unique. The proposed observation of He I (10830 Å) at the VTT/Teneriffe will have a 10 times better wavelength resolution that the observations of Dupree et al. (1996). Thus these new observations will provide a much better basis to study the Doppler shifts in He I (10830 Å) and the cell network relation.

New observations: He I (584 Å) and He I (10830 Å)

As mentioned above the proposed observations in both the 584 Å (with SUMER/SOHO) and 10830 Å (with VTT/Teneriffe) will provide a much better observational basis than what is available up to now. To achieve this aim raster scans of regions of approx. 400 x 300 arcsec should be performed with increased sufficient signal-to-noise ratio and increased spectral resolution (see observations section). These data will hopefully provide definite results on the spatial structures of the Doppler shifts and line widths of these two lines in coronal holes and the quiet Sun. Only this new observational basis will allow a detailed (statistical) comparison with new models to be performed on the formation of the helium lines, e.g. improved models of the Andretta & Jones (1997) or Carlsson & Stein (1997) type.

He II (304 Å)

The observations in the He I (584, 10830 Å) lines should be supported by observations in He II (304 Å). This will enable a further study of the line formation and a comparison of the spatial structures. As the expected Doppler shifts in the quiet Sun are quite small, it is still has to be decided if a observation with EIT in its 304 Å channel is sufficient of if also a CDS raster is of great use.

Coronal and transition region radiation

To be able to study the line formation of the helium lines the coronal back radiation has to be known. Thus supporting observations of TRACE in a coronal channel, e.g. in 171 Å (Fe IX/FeX, 1 million K), is of great interest. Similarly observations in the C IV channel of TRACE is of interest to get information on the transition region emission To get a better feeling on the spatial structures movies in the coronal and transition region lines are extremely helpful. As SUMER and VTT/Teneriffe will only provide one raster scan in the helium lines TRACE movies will be helpful to understand the structure and reconfiguration of the transition region and corona during the time of the rasters.

Relation to chromospheric emission

The spectrographs SUMER and VTT/Teneriffe will provide also information on chromospheric lines. This will enable to study the relation of Helium to other lines from the chromosphere and may give some hints where these lines are formed. In a way this would enable a spatial comparison similar to that performed for time series in He I (10830) and Ca K by Fleck et al. (1996).

Underlying photospheric magnetic field

Information on the photospheric magnetic field (provided by MDI/SOHO) will allow to study the connection of the magnetic structure and spectral features in the helium lines.

Planned Observations

Participating instruments

This is a MEUDOC campeign, i.e. the SOHO instruments will be coordinated from Paris.
Planers/contact persons for the time of the planned helium observations:
SUMER/SOHO: Philippe Lemaire lemaire@medoc-ias.u-psud.fr 17. Mai - 23. Mai
Jean-Claude Vial vial@ias.fr 24. Mai - 30. Mai
VTT/Teneriffe: Wolfgang Schmidt wolfgang@kis.uni-freiburg.de 19. Mai - 26. Mai
TRACE: Harry Warren hwarren@cfa.harvard.edu 18. Mai - 24. Mai
CDS/SOHO: Andrzej Fludra fludra@cds8.nascom.nasa.gov
EIT/SOHO: Frederic Auchere auchere@filament.nascom.nasa.gov 2. Mai - 29. Mai
MDI/SOHO: Craig DeForest zowie@urania.nascom.nasa.gov

Schedule for observations

Observational sequences and status

SUMER/SOHO:
  • Observed lines: He I (584 Å) [2nd order], O I (1152 Å).
    Transmit two 50 pixel windows.
  • Raster scan covering 400'' x 300''.
    1x300'' slit in N-S direction, 1'' steps in E-W direction.
  • Exposure time: 30 s.
  • Total time: approx. 3.5 hours.
  • Full spectral frame before and after sequence.
  • SUMER will be operated from Paris.
  • Raster scan is OK.
  • Time required is OK.
  • Minor problem: operation over weekend.
    The SUMER sequences for 22-25 May have to be send already on Friday 21 May.
VTT/Teneriffe:
Observatorio
del Teide
  • Observed lines: He I (10830 Å), H-beta.
  • Raster scan covering 400'' x 300''.
    ??x150'' slit in N-S direction.
    Meander scan: step 150'' down, ??''right, 150'' up, ??'' right...
  • Exposure time: 10 s.
  • Total time: approx. 3.5 hours.
  • Time reserved for whole week (May 20-26, 1999) for this sequence.
  • Observations will be performed by Wolfgang Schmidt, KIS/Freiburg/Germany.
TRACE:
  • Observed wavelength bands: Ly-alpha, C IV, Fe IX/X (171 Å)
    (for 1548 Å the band at 1550, 1600 and 1700 Å are needed)
  • Image region (8.5' x 8.5') covered by SUMER and VTT.
  • Movies in all wavelength bands. Cadence: 2-3 minutes.
  • Total time: up to 3.5 hours.
  • Will have to share observing time with a program of Observatoire Pic du Midi.
    Probably full support every other day, partly supported the other days.
    Final decissions will also depend on weather forcasts for Pic du Midi and Teneriffe.
CDS/SOHO:
  • Observed line: He II (304 Å).
  • Raster scan covering 400'' x 300''.
  • No CDS support during 24-26 May.
EIT/SOHO:
  • Observed wavelength band: He II (304 Å).
  • Full-Sun full resolution images (1024x1024 pixel)
    at 9:00, 10:30 and 12:00 UT.
  • In addition: synoptic program:
    full-field full-resolution images in all four wavelenghts
    at 01:00 UT 7:00 UT and 13:00 UT and 19:00 UT every day.
MDI/SOHO:
  • magnetogram
  • one or more full-Sun magnetograms during observation
  • MDI takes 15 full-disk magnetograms (2 arcsec pixels) per day.
  • Having about 2 of these during the observing sequence is sufficient.
  • As the resolution of the SUMER and VTT observations will be 2 arcsec no high-res field data are needed.
  • No special mode for MDI required, just ensure that some of the daily full-disk magnetograms are taken during the sequence.

Regions/field-of-view covered by participating instruments

Study three regions in coronal hole, quiet Sun at disk center and quiet Sun near the limb The following image should illustrate the location and the sizes of the observed regions.
(Of course they may change from north/south hole or east/west limb.)
This assumes that SOHO is back to its "original" orientation, i.e. that the SUMER-slit is in the N-S direction and that north is top in the EIT and MDI images. where to observe

References

Hardi PETER High Altitude Observatory / NCAR Phone: (303) 497-8324
P.O. Box 3000 Fax: (303) 497-1589
Boulder, CO 80307-3000 E-mail: hpeter@ucar.edu
U.S.A http://www.asp.ucar.edu/~hpeter/