PROPOSED JOINT SUMER/CDS OBSERVING PROGRAM FOR HELIUM 304 STUDIES OBJECTIVE: UNDERSTAND THE FORMATION OF THE HE II 304 LINE IN BOTH THE QUIET SUN (INCLUDING CORONAL HOLES) AND IN ACTIVE REGIONS It is clear that the He II 304 line is dominated by electron collisional excitation in the quiet Sun (Jordan, S. et al, ApJ, 1993, 406, 346), unless the line is produced in the solar chromosphere in regions far below the 'canonical' ionization equilibrium temperature. While the latter possibility has been advanced by Wahlstrom & Carlsson (ApJ, 1994, 433, 417), it is hard to explain how optically thick radiative transfer effects in their model - which is based upon investigating the 1640 line - could produce higher non-thermal 304 line widths in quiet regions than in active regions. We have observed this anticorrelation consistently with SERTS-91 and SERTS-93 (Davila, J. et al, in preparation). Working with codes developed by Vincenzo Andretta and Ivan Hubeny, the 304 line formation is now being tested by our group using the SERTS data and performing the non-LTE calculations with partial redistribution for a better line fit. So far, no obvious radiative transfer effects alone seem able to reproduce the SERTS observations. On the other hand, if the large line widths are indeed a signature of large nonthermal velocities, 'velocity redistribution' of the He II ions will expose enough of them to higher temperature thermal electrons to enhance the total line intensity (Jordan, S., in Solar Coronal Structures IAU Coll # 144, eds. Rusin, Heinzel, & Vial, Neografia, Slovak Republic, p. 415). Thus it may be possible to reproduce the total quiet-Sun 304 line intensity observed by this velocity redistribution, as proposed by Carole Jordan (1975, MNRAS, 170, 429; 1980, Phil. Trans. R.S.L., A297, 541). However, this is by no means the case for the more intense lines formed in the active regions. The situation in the active regions differs from that in the quiet Sun in at least two ways. First, the much greater strength of the magnetic field can be expected to inhibit all cross field motions, so the smaller nonthermal line-widths observed there are not surprising. Second, the more intense coronal radiation could compete with the collisional excitation mechanism to form the line. While the S. Jordan 1993 study also yielded collisional excitation dominance of the 304 line formation in ARs, the dominance was less pronounced than in quiet regions, and was not clearly out of the range of uncertainties in the analysis. Thus it behooves us to investigate the formation of 304 in both quiet Sun (including coronal holes) and in solar active regions. To the quiet-Sun observing program already proposed by Carole Jordan (program name HELEN (CDS) plus POP 22 (SUMER)), we would add comparable observations in selected active regions (ARs). The main difference between the two observing programs is that the AR program would require less observing time than estimated for the quiet-Sun program. One-third of the proposed quiet-Sun observing time would probably yield adequate intensities, but would exceed the telemetry rate if we wish, as we do, to recover all the GIS data from CDS. However, by observing for half the proposed observing time that was estimated in HELEN, we do not exceed the telemetry limitation, and we do wish to recover all the GIS data, as the highly ionized iron lines, in particular, give us a direct measurement of the coronal flux. The latter is important for assessing the photoionization- recombination contribution in the active region, where this mechanism could play an important role in the line formation. Finally, Dere and Mason (Solar Physics, 1993, 144, 217) have determined little differential behavior between nonthermal velocities in the quiet Sun and active regions in numerous EUV lines observed with HRTS. Why this should be true in lines like C IV and not He 304 is a further puzzle to be investigated. Clearly there is need to study the 304 formation in both the quiet Sun and in solar ARs. The total proposed program then consists of two parts. The quiet Sun part is identical to the program Helen (CDS) plus POP 22 (SUMER), already in the Blue Book and the Red Book. The desired parameters for the second part, the active region study, are identical except for the reduction in observing time by a factor of two. These parameters are given below. OBSERVING PROGRAM FOR THE ACTIVE REGION STUDY (CDS) Spectrometer: GIS Slit: 2x2 arcsec Raster Area: 30x30 arcsec Step (DX,DY): 2 arcsec, 2 arcsec Raster Locations: 15x15 = 225 Exposure Time: 15 sec Duration of Raster: 57 min (including overheads) Number of Rasters: 3 Total Duration: 2.85 hrs per target Line Selection: Full GIS output Bins across Line: N/A Telemetry Compression: None Pointing: Preselected Active Region (AR) Flags: Not to run during flags Solar Feature tracking: Unnecessary Frequency: Occasional runs on preselected ARs Product: 3 maps per run of the target AR Joint Observations with SUMER POP 22 modified as below OBSERVING PROGRAM FOR THE ACTIVE REGION STUDY (SUMER) Initial Pointing: slaved to CDS program above, or vice versa Slit: 1x300 arcsec Scan Area: 91.2x300 arcsec Step Size: full Number/Scan Locations: 120 Dwell Time: 30 sec Duration of Scan: 60 min Number of Scans: 3 (corresponding to above CDS program) Number/Scan Mirror Settings: 1 Repointing: (corresponding to above CDS program) Total Duration: 3 hrs per target Line Selection: Si IV (1393.76, 1402.77, 1406.06) O IV (1397.27, 1399.77, 1401.16, 1404.81, 1407.39) Bins across Line: 1 spatial, 1 spectral Selection: 25 pixel spectral window per line for 8 lines Compression: None with 16 bit to 8 bit compression Reduction: None Cooperation: with above CDS program, same targets, run in sequence ********************************************* up-date of CDS program: 14.3.97: (Andretta) during the six days of our observing run we will have from SoHO (SUMER and CDS) 6 hours every day: either from 9:00 to 15:00 UT or from 10:00 to 16:00 UT. I guess that the best times for observing from Tenerife are in the morning, so those should be pretty good times. We are inclined for the second option: considering the average characteristics of the seeing at VTT, would that be OK for you? Here are the details of the CDS run: NIS(Normal Incidence Spetrograph): slit: 2"x120" raster area: 240"x240" exposure: 46s total duration of raster: 6082s lines: SiX 347, SiX 356, FeXVI 361, MgIX 368, HeI 537, CIII 538, OIV 555, HeI 584, OIII 599, HeII 304(2nd ord.), MgX 624, OV 630 GIS(Grazing Incidence Spectrograph): slit: 4"x4" raster area: 60"x60" exposure: 20s total duration of raster: 4793s mode: all 4 bands During each observing day, the likely sequence will be: GIS+NIS+GIS+NIS. As I mentioned earlier, the pointing will be decided on the basis of the SUMER slit position. *************************8 up-date SUMER: From JORDAN@stars.gsfc.nasa.gov Fri Mar 14 17:51 MET 1997 To: Our Ground-based Colleagues OBSERVING PROGRAM WITH SUMER FOR MAY 8-13 The SUMER observing program for May 8-13 will be similar to UDPs 1115 and 1129 in the SUMER database. However, small changes can be, and probably will be made depending on how quiet the Sun is during these days at West longitude 45 degrees, where SUMER is expected to be pointing. (We can move in elevation however.) Since we cannot raster, we plan to 'sit and stare,' with no rotation compensation, so at least a long narrow image will be built up. Since solar rotation will carry a given solar region between 7 & 8 arcsec projected on the plane of the sky in one hour - at 45 degrees longitude and the solar equator - this will give us a maximum of 48 arcsec in the azimuthal direction, to go with 120 arcsec (our slit length) in the vertical direction. Likely parameters of the final SUMER JOP-16 are: Slit: 1 x 120 arcsec, at the center of the detector for maximum sensitivity. Lines: He I 584.334 A and C III 1175.711 A, for profiles of the He I resonance lines and for density diagnostics at 100,000 degrees with the C III doublet components. Ref pixel for these lines is 400, to put the strong C III on the bare MCP. Width of window on detector: 25 pixels at about 24 pixels/A in first order (C III) and about 47 pixels/A in second order (He I), sufficient for maximum observed line widths and shifts in these lines. Integration time: 30 sec, more than adequate for good statistics with the 1 x 120 arcsec slit. If we need to go to the 0.3 x 120 arcsec slit because of unexpectedly high counts, this can be increased to 60 sec, since we are interested in high-resolution structure and relatively steady flows, not oscillations or explosive events (which we always see in C III and almost never in He I). Number of spectra to be taken (number of reps): Adjustable, but for six hours total observing time we will probably go for 700 reps at 30 sec each, which gives us enough time for data transmission and also noninterference with other follow-on programs. Rotation compensation will be off, as noted above. * * * * * The current JOP-16 plans envisions observations on the six days of May 8-13 during 09:00-15:00 UT. Please comment on this as you see fit. Best wishes for good observing (good weather on the ground!), Stuart Jordan/GSFC *********************** on last day, 13.5. for Tf only until 13.00 Tf-time possible! *************************** Preliminary program for VTT: instruments: spectrograph, slit-jaw imager lines: *) He I 10830 spectra (both He I components + either H2O or Si I - Si I prefered if target is still at disk center! *) Ca K spectra (center of line, includes K1, K2, K3, red and blue) *) + maybe Ca K nearby continuum spectra 2-3 cameras: AT1 F for Ca K AT1 E for He I another blue sensitive for Ca K continuum use 2x2 binning (or even 2x4) - spatial x spectral CaK slit-jaw images: Ca K: 0.6 A bandwidth, continuous video and single images from frame grabber target: quiet sun (according to SUMER slit) slit: width 120 mu (0.55 arcs) length: 93.8 arcs time: exposure time: 3 s (for He I and Ca K, maybe less for Ca K continuum) readout: 5 s => 8 sec/position scanning: 200 steps, 0.5 arcs each => area = 100 arcs, total time = 54 min repetition: max 6 for 6 h observing time with SoHO (will finally depend on seeing) calibration before and after SoHO program: dark current exposures (10), flatfield exposures (ca. 50, moving telescope) dataformat: fits-files, bytesize=16k, data on exabyte **********************