SOHO JOP 62 - OXFORD HELIUM ENHANCEMENT STUDY

Authors: Carole Jordan and Keith Macpherson (Oxford), Richard Harrison (CDS),
Bob Stern (EIT), Julia Saba (MDI), Philippe Lemaire (SUMER)

E-mail Addresses:
cj@astro.ox.ac.uk, kpm@thphys.ox.ac.uk,
harrison@solg2.bnsc.rl.ac.uk, stern@sag.space.lockheed.com
saba@mdisas.nascom.nasa.gov, lemaire@sumop1.nascom.nasa.gov

Progress:
Original Version:- April 1997
Updated Version:- September 23rd, 1997

Objective:
To investigate the systematic behaviour of He I and He II line fluxes in comparison with atmospheric models determined from other transition region lines.

Conditions necessary to run:
The presence of two `quiet' regions along the central meridian, and a coronal hole at either the north or south pole. Involvement of at least CDS and SUMER. Availability of synoptic full-disk EIT images close to time of CDS/SUMER sequences, plus synoptic full-disk MDI magnetograms preferred.

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.

ORIGINAL VERSION - April 1997:

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.

Operating Details:

(i) CDS Sequence HELIUM: 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.

UPDATED VERSION for proposed re-run: September 1997:

Pointing and Target Selection:

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.

At each location, run CDS sequence HELIUM twice, once in conjunction with SUMER Sequence 1 and once with SUMER Sequence 2. See Updated Operating Details below for specific details of CDS and SUMER sequences.

MDI and EIT to provide details on the overall type of regions studied.

No other individual runs of CDS-HELIUM on the solar equator required, unlike original operating procedure.

Operating Details:

(i) CDS: No change to CDS sequence HELIUM given originally. Sequence duration ~25 mins. Run twice at each of the 3 solar locations, firstly in conjunction with new SUMER Sequence 1, then repeated but this time with SUMER Sequence 2 (detailed below).

(ii) SUMER: Two sequences to run at each location, in conjunction with the two runs of CDS HELIUM study at each solar location.

Sequence 1:

Line list:
(1) Slit 7, 0.3" by 120" area. C III 977.0 on pixel 840
(2) Slit 7, 0.3" by 120" area. C III 1174-1176 with 1175.7 on pixel 550, He I 584 (2nd order)
(3) Slit 4, 1.0" by 120" area. C II 1334.5 and 1335.7 on pixel 800. Exposure time 115s, repeated 4 times, return full 1024 pixels of the detector.
Exposure times for Line lists (1) & (2): as long as possible to get good count statistics, but constrained by SUMER team restrictions on detector exposure.
No of exposures for Line lists (1) & (2): Repeated exposure of each line list for duration of 8 minutes, no of exp thus determined by precise exposure time used.
Total sequence duration: 3 x 8 mins = 24 mins to correspond with CDS HELIUM duration.

Sequence 2:

Line list
(1) Slit 7, 0.3" by 120" area. Si III 1206.5 on pixel 140
(2) Slit 7, 0.3" by 120" area. Si III 1294.5, 1296.7, 1298.9 and 1301.1 with 1298.9 on pixel 340
(3) Slit 4, 1.0" by 120" area. C II 1334.5 and 1335.7 on pixel 800. Exposure time 115s, repeated 4 times, return full 1024 pixels of the detector.
Exposure times for Line lists (1) & (2): as long as possible to get good count statistics, but constrained by SUMER team restrictions on detector exposure.
No of exposures for Line lists (1) & (2): repeated exposure of each line list for duration of 8 minutes, no of exp thus determined by precise exposure time used.
Total sequence duration: 3 x 8 mins = 24 mins to correspond with CDS HELIUM duration.

Practical Implementation: To minimise the number of slit changes required, we suggest the following sequence.

1st location: Start run of CDS-HELIUM with SUMER Sequence 1 in line list order (1), (2), (3).
Repeat run of CDS-HELIUM, SUMER Sequence 2 with line list order (3), (2), (1)

2nd location: CDS-HELIUM with SUMER Sequence 1, line list order (1), (2), (3).
Then CDS-HELIUM with SUMER Sequence 2, line list order (3), (2), (1)

3rd location: CDS-HELIUM with SUMER Sequence 1, line list order (1), (2), (3).
Then CDS-HELIUM with SUMER Sequence 2, line list order (3), (2), (1)

(iii) EIT: Full disk images in He II 304 plus coronal lines would allow us to study the larger structure around the regions studied in detail.

(iv) MDI: As before

kpm@thphys.ox.ac.uk

Last modified: Sept 23 1997