I would like to apply for the MDI data corresponding to JOP053, JOP054 and JOP055 which we were running last november and december. As a justification of what we want to do with these data, I include copies of the scientific justifications from the observing proposal for these JOPs. Craid DeForest mentionned he would be happy to be the MDI contact person and work with me for these JOPs. I would be happy with that too. I'm writing this mail in the name of the complete Zurich team on these proposals, i.e. Sami SOLANKI, Katja STUCKI, Alen BRKOVIC, and Isabelle RUEDI. Many thanks in advances and best regards, Isabelle Ruedi ******************************************************************************** JOP053: Size and Brightness of Features in the Upper Solar Atmosphere --------------------------------------------------------------------- In the solar photosphere magnetic and thermal structures show substantial power at small scales (< 1''). In the upper chromosphere the magnetic field is considerably more homogeneous (e.g., Harvey and Hall 1971, R\"{u}edi et al. 1995) and covers the complete solar surface (e.g., Giovanelli 1980, Solanki \& Steiner 1990). The size of features and their brightness does not scale correspondingly, with considerable small-scale structure seen at all but possibly the highest transition-region temperatures. For example, SUMER has already observed features with sizes lying at the limit of its resolution (Lemaire et al. 1996). In addition, there is strong if indirect evidence for even much smaller scale structure (well below the spatial resolution of SUMER) in the transition region (e.g. Kjeldseth--Moe 1989). We propose to record the size, location and brightness of individual features at different temperatures and to reconstruct their geometry as far as possible therefrom. Although it is possible to determine filling factors of the radiating plasma using other means, the spatial distribution of emission can be mapped only by direct imaging. This geometry should provide constraints on the heating mechanism. Also, by observing at different distances from the limb a more accurate estimate of the centre-to-limb variation of the intensity in small-scale features is to be obtained. In the absence of seeing the MTF of the telescope and the pixel size (or the composite of slit width and scan step in the direction perpendicular to the slit) set the main limitations on the achievable spatial resolution. By deconvolving the known or estimated effective MTF (of telescope, spectrograph slit and pixels) from the signal it is possible to partially overcome this limitation. We propose to deconvolve images (initially at a single, possibly averaged wavelength) obtained by SUMER, CDS and EIT in order to obtain a better estimate of size and brightness of small-scale features. SUMER and CDS images are to be constructed by stepping the slit over the solar surface (rastering mode). In order to obtain maximum resolution, at least in the direction perpendicular to the slit the narrowest SUMER and CDS slits and smallest scanning step sizes are to be chosen. The deconvolution is to be carried out using various techniques, including recently developed wavelet-based techniques. Since SUMER and CDS can record more than 1 spectral line simultaneously it is in principle possible to construct deconvolved maps in multiple spectral lines and to form line ratios at the new, improved spatial resolution. In the case of SUMER the deconvolution can be carried out at every sampled wavelength in the line, thus allowing full line profiles to be reconstructed at the higher resolution. This means that it should also be possible to construct deconvolved maps of bulk and turbulent velocity. Except for the strongest lines in bright plage the line wings will, however, probably not be well reconstructed due to their low brightness. We propose to use magnetograms to help with the interpretation of the data. They should help determine the nature of the small-scale structures seen in the SOHO data and their association with photospheric magnetic fields. ******************************************************************************** JOP054: Empirical scaling laws for a range of temperatures ---------------------------------------------------------- Scaling laws, i.e. simple relations between fundamental properties of quasi-steady coronal loops (temperature, length, pressure and heating rate) have been theoretically postulated and partly tested using Skylab data two decades ago (Rosner et al. 1978). Recently Yohkoh has provided stringent tests of such scaling laws for high-temperature loops (e.g. Tsuneta 1996). Since the soft X-ray telescope (SXT) on Yohkoh is relatively insensitive to plasmas below approx 2--3 10^6 K the Yohkoh result must be considered incomplete. We propose to extend tests of scaling laws to lower temperatures, but also to directly confirm Yohkoh results at higher temperatures. In this manner we can check whether the same scaling laws apply to all loops, or if cooler loops exhibit different dependences between their various parameters. We propose to utilize the images in a group of spectral lines constructed by scanning the CDS NIS slit across the solar surface. In addition, we plan to use EIT images obtained in the three spectral lines of iron ions recorded by that instrument. We shall concentrate on the central half of the solar disk, in order to keep the geometry as simple as possible. Since sufficient statistics are required, i.e. a statistically significant number of loops needs to be observed, these observations need to be repeated a few times (e.g. 1--2 times per week for 3 weeks). CDS ``images'' are to be obtained in spectral lines with different temperature, but also different density sensitivities. From a set of spectral lines with different temperature sensitivities within the range log T = 5.5--6.4 (Fe VIII 370.4 ang, Mg VIII 313.7 ang, Mg IX 368.1 ang, Fe XII 364.5 ang, Fe XIII 348.2 ang, Fe XIV 334.2 ang, Fe XVI 335.4 ang, Fe XXI 335.9 ang) lines we can determine the temperature of the plasma. The density in principle follows from the emission measure. However, the use of density-sensitive line pairs (Fe XII 338/364, Fe XIII 348/359, Mg 315/335) should allow the true density of the emitting plasma to be determined even if unresolved fine-scale structure is present in the spatial-resolution element. One of the parameters entering the empirical scaling laws is the length of the loop. Although the good spatial resolution of the SOHO instruments is an advantage, there may nevertheless be considerable uncertainty if the footpoints show little contrast relative to the surrounding plasma. In such cases it may sometimes help to use magnetograms to find the exact location of the footpoints. We propose to use MDI magnetograms for this purpose. As a bonus of this study we expect to present statistics of loop properties, such as the distribution of loop lengths, temperatures, densities, etc. We also expect to learn more about the variation of temperature and density along the loop. ******************************************************************************** JOP055: Coronal holes versus normal quiet sun --------------------------------------------- Coronal holes are enigmatic structures. Fundamental parameters of the coronal plasma such as temperature, density, solar wind velocity, etc. change drastically over a short distance at the edge of the hole and retain the new value over large parts of the solar surface. Coronal holes are best seen in X-rays at relatively high temperatures, where they appear very dark, i.e. their high-temperature plasma is at an extremely low density. At lower temperatures the contrast between coronal hole and the normal quiet sun decreases, although holes are still clearly visible in the He II line observed by EIT and a marked signature is present in count-histograms of transition zone and other chromospheric lines (Huber et al. 1974). Coronal holes also have a significantly lower differential rotation than the photospheric plasma (e.g. Sime 1986, Insley et al. 1995). This is intriguing since it implies that in some cases at least coronal holes move relative to the underlying photosphere and hence are probably not always bounded by the same field lines. If this is correct it would imply reconnection processes taking place at the coronal-hole boundaries. We propose to follow coronal holes and the adjacent quiet sun through a range of temperatures starting at normal coronal temperatures and going to increasingly lower temperatures until they are indistinguishable. Also, by comparing scans outside the limb with scans on the disk it should be possible to at least partially disentangle the various effects of height, horizontal spatial position and temperature, so that a better understanding of the coronal-hole phenomenon can be obtained. At solar activity minimum polar coronal holes are the largest. Hence, in order to avoid contamination by the brighter non-hole material off-limb observations are to be limited to polar holes. In addition, by combining CDS and SUMER off-limb observations of O VI lines (1032 ang and 1038 ang by SUMER and 173 ang by CDS) it may be possible to determine the density of the coronal hole material relative to the normal quiet sun. Furthermore, the O VI lines observed by SUMER can be followed by UVCS to much greater distances from the sun, so that combining SUMER and UVCS will allow us to map a coronal hole and its boundary right from the solar surface to a couple of solar radii. These joint observations will rely heavily on the intercalibration between the 3 instruments CDS, SUMER and UVCS. The magnetic field must hold the key to why certain parts of the quiet sun atmosphere form a coronal hole, while others do not. We plan to look for possibly subtle differences between magnetic structure under coronal holes and in the normal quiet-sun. Such an investigation will be two-fold. On the one hand we plan to look closely at the region near the coronal hole boundary. On the other hand we also propose to carry out statistical investigations of the magnetic fields generally found under coronal holes and compare with the results of similar studies of the field in non-hole quiet-sun. Care will be taken to compare like with like; for example only regions at similar distances from the limb will be compared. The planned investigation will be complementary to that of, e.g., Harvey et al. (1982). The main aims of this JOP are thus to determine the basic properties of coronal holes and their boundaries as a function of temperature and height. We also hope to determine which is the lowest temperature at which coronal holes are visible and search for (statistical) relations between the presence of a coronal hole and the underlying magnetic field. ********************************************************************************