SOHO Joint Observing Plan No. 5: Composition of Dynamic Layer Above Solar Limb Authors: J.-C. Vial, J. Fontenla, K. Boccialini, M. Rovira, E. Tandberg-Hanssen Purpose: To determine the nature, i.e., the composition and dynamics, of the low-altitude inhomogeneous layers seen above the solar limb. These layers are observed in spectral lines originating at temperatures in the range 10^4-10^5 K (e.g., Fontenla, J.M., Reichmann, E.J., and Tandberg-Hanssen, E. 1988, ApJ, 329, 464). Depending on the type of observations used, the layer has been interpreted in terms of spicules, cloud layer, macro-spicules or -most recently- as an X-ray absorption rim (Golubb et al 1995). UV observations reveal that the layer resides above the bright rim seen in e.g., CIV lines. This material is mixed with plasma at coronal temperatures and from crude estimates should be in a low plasma-beta domain. Observational evidence indicates that the layer is highly dynamic, and wave motions have been invoked in some interpretations. We shall explore whether Alfven-type waves may be involved and study observational diagnostic of transport of magnetic energy and momentum through the layer from the chromosphere into the corona. From the observed velocities, these waves are probably non-linear, have mixing of modes (they are not pure Alfven), and produce substantial dissipation. Thus, observational studies are crucial for evaluating the energy transport. SUMER data will be used to determine the height structure of the layer and its relationship to underlying solar disk features, e.g., filament channels, emerging fields, thereby associate the properties of the layer with magnetic field conditions. Our plan calls for observation of the layer over at least four different areas of the solar surface, viz. the quiet Sun, coronal holes, simple bipolar active region and active region of complex polarity. Lines from a variety of ions will be used, including, but not limited to, H I (Ly alpha), NV, SiIII, CIV, SiIV. These data will be used to model distributions of multi-thermal and multi-velocity material to explain the observed line profiles, keeping in mind expected strong departures from local ionization equilibrium. Observing program: I.A LIMB EMISSION MEASURE Operational Sequence Initial pointing selected limb position at the slit center Slit 1x120 arcsec^2 Scan Area 10 arcsec Step Size 0.76 arcsec Resulting Number of Scan Locations 14 steps Dwell Time 60 s Duration of Scan 14 MIN Number of Scans 2 Number of Scan Mirror Settings 3 Repointing none Total Duration 14 MIN Line Selection 1/ CIV (1548A AND 1550A), NE VIII (770 & 780) 2/ SI IV (1394 & 1401), O IV (1401 & 1404) 3/ NV (1238 & 1242), MG X(625), O V (630) Bins Accross Line 25 Estimated Reduction Factor * Selection 1/ 4x25x120 2/ 4x25x120 3/ 4x25x120 * Compression BYTESCALE 1 * Reduction Co-operation Requirements NONE I. B LYMAN ALPHA EMISSION AT LIMB Operational Sequence Initial pointing selected limb position at the slit center (SAME AS ABOVE) Slit 1x120 arcsec^2 Scan Area 10 arcsec Step Size 0.76 arcsec Resulting Number of Scan Locations 14 steps Dwell Time 10 s Duration of Scan 2 MIN 20 S Number of Scans 2 Number of Scan Mirror Settings 1 Repointing none Total Duration 4 MIN 40S Line Selection Ly alpha (1216.7A) (ON THE BARE MCP) Bins Accross Line 50 pixelS Estimated Reduction Factor * Selection 1x50x120 * Compression NONE * Reduction Co-operation Requirements NONE I. C LIMB EMISSION MEASURE Operational Sequence Initial pointing selected limb position at the slit center (AS ABOVE) Slit 1x120 arcsec^2 Scan Area 10 arcsec Step Size 0.76 arcsec Resulting Number of Scan Locations 14 steps Dwell Time 60 s Duration of Scan 14 MIN Number of Scans 2 Number of Scan Mirror Settings 4 Repointing none Total Duration 19 min Line Selection 1/ Ly beta (1026A), O VI (1032 & 1036) 2/ SI XII (499), N III (992), C III (977), LY GAMMA (973) 3/ LY DELTA, LY EPSILON, LY 6, LY 7 (926) 4/ LY CONT (912), C II (904), NE VII (895), LY CONT (880) Bins Accross Line 25 pixelS Estimated Reduction Factor * Selection 1/ 4x25x120 2/ 4x25x120 3/ 4x25x120 4/ 4x25x120 * Compression BYTESCALE 1 * Reduction Co-operation Requirements NONE