CDS OBSERVING SEQUENCE ====================== TITLE: Off-the-limb Large Scale Temperature and Density Study With ----- Iron Lines ID: STRUC -- CONTRIBUTORS: P.R.Young, M.J.Penn, R.N.Smartt, H.E.Mason ------------ SCIENTIFIC JUSTIFICATION ------------------------ The balancing of energy loss and gain processes is essential for determining the contribution of the (as yet unknown) coronal heating mechanism. For a plasma in thermal equilibrium two of these processes (thermal conduction and radiation) can be estimated from knowledge of the local temperature and density structure (see, e.g., Jordan & Brown, 1981) and, for the corona, these quantities can be accurately found by analyzing emission lines in EUV spectra. Off-the-limb temperature and density studies in the EUV were first performed by Kastner et al. (1974, 1976) using OSO-7 observations of the quiet sun in various iron lines. CDS allows these results to be significantly improved upon in terms of both spectral and spatial resolution, while the opportunity of obtaining simultaneous, high-quality ground-based images in the optical and infra-red coronal iron lines allows the study to be extended to greater heights. The temperature structure of the corona can be studied by looking at lines from several adjacent ionization stages of a single element: by comparing theoretical line intensities to observed intensities we can get an estimate of the amount of emitting material at any given temperature (i.e., the emission measure). In the corona, we have prominent emissions from Fe IX - XVI and so we focus on lines from these ions here. By using the Normal Incidence Telescope (NIS) of CDS in the wavelength range 310 - 380 AA we can cover emission lines of Fe X - XVII. The analysis of Kastner et al. (1974) suggests that these lines may be detectable out to 1.3 R_o. The 310 - 380 AA range also contains many density diagnostics which will enable the density structure of the corona to be mapped out. However, the majority of EUV diagnostics are only sensitive in the > 10^8 regime and thus the fall-off of density with height will be difficult to follow away from the inner corona. For this reason it is essential to obtain observations with the ground-based coronagraph at Sac Peak of the infra-red Fe XIII lines at 10747 and 10798 AA. These lines give a well-known density diagnostic (see Flower & Pineau des Forets, 1972, and Young et al., 1994) which is sensitive to densities below 10^8. This should allow determinations of the coronal density out to the limit of the coronagraph's range (i.e., 1.5 R_o). The coronagraph can also see down to about 1.02 R_o giving a good overlap with the EUV data. Routine observations of the Sun are made in the red and green coronal lines at Sac Peak and these will also be useful in correlating the EUV data with the ground-based data. In addition, matching EUV line intensities with optical and infra-red intensities will provide an opportunity for checking the photometric calibration of CDS. [It may be possible for the coronagraph to take images in the Si X 14304 AA infra-red line as well as the Fe XIII lines by using an imaging Fabry-Perot system and so we include the density sensitive Si X EUV 356/347 ratio in the CDS observations.] REFERENCES: Guhathakurta, Fisher & Altrock, 1993, ApJ 414, L145 Jordan & Brown, 1981, Solar Phenomena in Stars and Stellar Systems, p.199 Kastner, Rothe & Neupert, 1974, AA 37, 339 Kastner, Rothe & Neupert, 1976, AA 53, 203 Penn, 1994, ESA SP-373, Proceedings of the Third SOHO Workshop, p.185 Young, Mason & Thomas, 1994, ESA SP-373, Proceedings of the Third SOHO Workshop Coronagraph Operating Details ----------------------------- The observing sequence needs to performed when conditions are best for the coronagraph at Sac Peak, i.e., late Spring or Autumn. Thus we suggest either mid-late May or late September. Also, runs with the coronagraph need to be scheduled at least three months in advance. CDS operating details --------------------- As the EUV lines to be observed here are expected to be seen out to 1.3 R_o, it will be necessary to have an observing region greater than the size of the CDS telescope's field-of-view. This will require re-pointing of CDS during the observing sequence. It is suggested here to have a 5 arcmin x 4arcmin observing region, but this is subject to change depending on what type of region is being observed. Due to the fall-off in line emission of the EUV lines seen in Kastner et al. (1974), the observed region will be divided into three regions each having different exposure times proportional to their distance from the limb. The steps of NIS are expected to be made in an outward direction from the limb, the slit parallel to the limb. To reduce the total time for the sequence, a different slit size is used for the outer regions. The decreased spatial resolution at greater heights should not be a problem as most of the detailed structure is in the inner corona. The active region scan is expected to take less time than the quiet Sun scan and so we recommend 3 consecutive scans over the active region as it is likely to be more dynamic. The exposure times given below are provisional and await preliminary CDS observations. SPECTROMETER: NIS WAVELENGTH BAND: 308 - 381 AA LINES STUDIED: Fe XVII (4.0 10^6) 350.52* Fe XVI (2.5 10^6) 335.40* Fe XV (2.0 10^6) 327.03*, 321.79(d) Fe XIV (2.0 10^6) 334.17*, 353.83(d) Fe XIII (1.6 10^6) 348.18*, 320.80(d), 359.84(d) Fe XII (1.5 10^6) 364.47*, 338.27(d) Fe XI (1.3 10^6) 352.67* Fe X (1.0 10^6) 345.74* Si X (1.2 10^6) 347.40*, 356.03(d) 0-2.3 arcmins ------------- SLIT: 2" x 240" RASTER AREA: 140" x 240" STEP: 2" RASTER LOCATIONS: 70 EXPOSURE TIME: quiet 10s ; active 4s DURATION OF RASTER: 700s ; 280s NUMBER OF RASTERS: quiet 1 ; active 3 2.3-3.7 arcmins --------------- SLIT: 4" x 240" RASTER AREA: 80" x 240" STEP: 4" RASTER LOCATIONS: 20 EXPOSURE TIME: quiet 40s; active 16s DURATION OF RASTER: 800s ; 320s NUMBER OF RASTERS: quiet 1 ; active 3 3.7-5 arcmins ------------- SLIT: 4" x 240" RASTER AREA: 80" x 240" STEP: 4" RASTER LOCATIONS: 20 EXPOSURE TIME: quiet 80s ; active 32s DURATION OF RASTER: 1600s ; 640s NUMBER OF RASTERS: quiet 1 ; active 3 TOTAL OBSERVING TIME: quiet 3100s (52mins) ; active 3720s (63mins) * use for emission measure analysis (d) use for density analysis