Joint Observing Program proposal (JOP # 105) Title: Low corona diagnostics Authors: Jean Arnaud, Jean Vigneau (OMP) e-mail adresses: arnaud@obs-mip.fr,vigneau@obs-mip.fr Proposal status: First draft, April 16, 1999 Scientific Justification: More we observe in details the solar corona and more obvious it becomes that we are observing a very inhomogeneous and dynamics medium. It is important to get quantitative information on physical properties like densities (local density and filling factor), temperatures and, if possible, magnetic fields to improve our understanding of the physical process going on there. The SOHO instruments dedicated to the study of the solar external atmosphere provide an unique opportunity of addressing questions like: - The temperature distribution of the coronal material. - The coronal filling factor, its temperature dependance. - The coronal iron abundance. - The percentage of the coronal material at temperatures where Fe X, Fe XIII and Fe XIV ions, responsible of bright emission lines observed with ground based coronagraphs. - The variations of those physicals parameters with the coronal structures, the phase of the solar cycle. Proposed observations: The GIS and NIS spectrographs of CDS allow to observe many iron lines, from Fe VIII to Fe XVI. The covered temperature domain likely contains most of the coronal plasma. It is why we propose for this study to focus on CDS measurements of iron lines, having different sensibilities to temperature and density. K-corona measurements from the LASCO C1 or the Mauna Loa Observatory MarkIII are needed. The information on magnetic field topology from LASCO C1 green line polarization measurements would be very appreciated. EIT observation will allow to determine the targets for CDS observations they will also complement. TRACE observations would give interesting information on the small scale structure. To study the dependance of the physical parameters studied with the phase of the solar cycle, we propose to remake the observations about every six months. CDS operations: There are two options, depending on the structures visible on limb at the time of the observations : Option 1: Pointing: Bright coronal loops if present at heights of 90 to 120 arcsec above limb. GIS observation: Slit: 2x2 arcsec Exposure time: 60 sec Scan area: 16x60 sec area Scan duration: 4 hours Full GIS range, full data NIS observation: Slit: 2x240 Exposure time: 60 sec Scan area: 16x240 (or 240x240 if NIS observations only) Scan duration: 8 minutes (or 2 hours if NIS observations only) Line selection: Fe X: 345.7, Fe XI: 369.2, Fe XII 338.27, 364.47, Fe XIII 318.12, 320.8, 348.18, 359.64, Fe XIV 334.2, 353.8, Fe XV 321.8, 327.0, Fe XVI 335.4, 360.8 Option 2. In case option 1 targets are not present. Pointing: diffuse corona 90 to 120 arcseconds above limb. GIS observation: Slit: 8x50 arcseconds Exposure time: 15 seconds Scan duration: 22.5 mn GIS,8x50 slit, 144x250 area, 15 sec full GIS, full data NIS observations: Slit: 4x240 Exposure time: 100 seconds Scan area: 144x240 area (or 240x240 if NIS observations only) Scan duration: 1 hour (or 1 hour 40 minutes if NIS observations only) Line selection: Fe X: 345.7, Fe XI: 369.2, Fe XII 338.27, 364.47, Fe XIII 318.12, 320.8, 348.18, 359.64, Fe XIV 334.2, 353.8, Fe XV 321.8, 327.0, Fe XVI 335.4, 360.8 EIT observations: EIT Fe XII 195 line or Fe XV 284 line are to be used for CDS target choice. EIT observations in its four bandpasses as near as possible from the CDS observations. LASCO observations: CI observations of the K-corona and, if possible, of the 5303 A line of Fe XIV (intensity and polarization). Mauna Loa Solar Observatory (MLSO) observations: If no LASCO C1 K-corona observations, we will use MLSO K-corona. This imply that the SOHO observation have to be scheduled near the time MLSO is observing (about 17 to 21 UT).