Title: 
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Oxford Helium Enhancement Study (SoHO JOP)

Lead Investigator:
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Carole Jordan, Keith Macpherson, Dept of Theoretical Physics, Oxford University, UK, Email:kpm@thphys.ox.ac.uk

Team Members:
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Richard Harrison (CDS)
Philippe Lemaire (SUMER)
Bob Stern (EIT)		     

Local Coordinator:
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Julia Saba

Lead Programmer:
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Technical Summary:
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To investigate the systematic behaviour of He I and He II line 
fluxes in comparison with atmospheric models determined from 
other transition region lines. 
        
Pointing and Target Selection:
        For the Joint CDS/SUMER observations, three locations should be 
        observed along the central meridian; a quiet region at (or near) Sun 
        centre, quiet region at a mid-latitude, and the third region near the 
        limb, to include a coronal hole. The direction away from Sun-centre 
        (N or S) should be decided to give the best coronal hole coverage. 
        MDI and EIT to provide details on the overall type of regions studied.
        CDS to observe two further quiet regions on solar equator, midway to
        limb and near the limb, without SUMER support.

Proposal Text:
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Scientific case:
        The overall scientific case is as set out in our original proposal 
        HELEN which has appeared in the CDS Blue Book, and as study 8.1.2.5 
        (POP 22) in one version of the SUMER Red Book. Briefly, Jordan (1975, 
        MNRAS, 170, 429), found that lines of He I and He II appear to have 
        higher fluxes than expected from models made using other transition 
        region lines formed at similar temperatures, and proposed that this 
        enhancement could arise through excitation by electrons with 
        temperatures higher than those expected in ionization equilibrium. The 
        enhancement factor was found to be less in coronal holes than in the 
        quiet Sun. The first objective is to examine this behaviour at the 
        higher spatial resolution of the CDS and SUMER instruments, by 
        observing several quiet Sun regions, from Sun-centre to near the limb, 
        and a coronal hole. With the benefit of the actual CDS spectra, we 
        propose to use the NIS spectra rather then the GIS spectra. To obtain 
        the same results, the NIS spectra require far shorter exposure times, 
        cover the same range of transition region temperature, provide the 
        He II (304 A) line in second order, as well as the He I lines. The 
        SUMER spectra required are the same. I.e. we need to obtain spectra 
        around the wavelengths of the Si IV lines, which are transition region
        lines formed around the same temperature as He II under `normal' 
        circumstances. Such spectra also include the O IV] lines from which the
        electron density can be measured. The latter are essential for 
        quantitative modelling.
 
        In a later paper Jordan (1980, Phil. Trans, Roy. Soc. London A297, 
        541) discussed how the enhancements might depend on measurable 
        transition region parameters (electron pressure, temperature gradient,
        non-thermal velocities) and stressed that dynamic effects must be taken
        into account. The second aim is to test the correlations proposed by 
        using also the widths of the Si IV and O IV] lines observed with SUMER.

        Observations with the EIT will give the type of structure in and 
        around the field of view to be observed. Coincident MDI spectrograms
        would give information on the magnetic field which has so far not been 
        available in the context of the helium lines.

Operating Details:

        (i) CDS:  NIS 2" x 240" slit, at 20 raster locations, to cover 40" 
        raster width. Exposure time, 70 s at each location. 30 pixel width for 
        each line. 15 lines, (where blends occur central wavelength of required
        region is given) as follows: He I (537.030 A), He I (584.334 A), He II 
        (303.783 x 2 = 607.566 A), O III (525.795 A), O III (599.598 A), O IV 
        (553.330 A), O IV (554.514 A), O IV (608.390 A), O V (629.730 A), 
        Ne IV (543.891 A), Ne V (572.34 A), Ne VI (562.80 A), Mg IX 
        (368.071 A), Mg X + O IV (609.79 A), Mg X (624.95 A).  

        (ii) SUMER: Slit 4, 1" by 120" area. Reference pixel 1: 600 on detector
        A; spectral window 25 pix. Line centering to cover 1387.44 to 1408.52 
        on KBr to provide Si IV lines plus O IV lines (including O IV 1407.39).
        Precise exposure time to be determined but will be much less than for
        CDS exposures.
 
        (iii) EIT: Images in He II and Fe IX/X would be useful for determining 
        type of region studied in detail.

        (iv) MDI: Either full disk magnetogram or the high spatial resolution 
        MDI mode images would allow relation between magnetic field and helium
        enhancement to be studied for first time.

	

See Also:
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