Search for UV Signatures Corresponding to Microwave Enhancement in Coronal Holes N. Gopalswamy (NASA/GSFC), P. Lemaire (MEDOC), K. Wilhelm (Lindau), K. Shibasaki (Nobeyama), A. Lara (Catholic U.), B. J. Thompson, J. B. Gurman (NASA/GSFC) and C. E. DeForest (Stanford) Received October 29, 1999 Updated November 24, 1999: MDI part more explicit. Instruments: SOHO (SUMER, EIT, MDI), Nobeyama radioheliograph Objective: To search for a signature corresponding to the microwave enhancement in coronal holes in the low temperature lines (10,000 - 15,000 K) using SOHO/SUMER observations. Scientific Justification: Microwave enhancement is an excess brightness temperature within coral holes as compared to the quiet Sun and is known for quite some time (Wefer and Bleiweiss, 1976; Kosugi et al, 1986; Gopalswamy et al, 1999). However, this is an enigmatic phenomenon with no satisfactory explanation. The enhancement is observed over a narrow spectral range (15-40 GHz) and occurs in both polar and low latitude coronal holes. After the advent of the Nobeyama Radioheliograph, the microwave enhancement has become an important tool to study the structure and dynamics of coronal holes (Gopalswamy et al, 1999). The enhancement is closely related to the enhanced unipolar magnetic flux within coronal holes and has a diffuse component and a compact component. The diffuse and compact components have a brightness temperature of 500 K and up to 2500 K respectively, above the quiet Sun. From our recent studies, we have concluded that the radio enhancement is confined to the upper chromosophere where the temperature is around 10,000 K, contrary to previous studies which suggested a transition region origin. One way to test is to look at line emissions in the coronal hole and quiet Sun at a few low temperature lines formed in the 10,000 - 15,000 K range. We expect an excess broadening of the profiles in the coronal hole as compared to the quiet Sun. Proposed Observations: We propose to observe a low latitude coronal hole when it is near the disk center. We choose the coronal hole based on EIT 195 A observations. We choose a set of lines for SUMER observations in the 10,000 - 15,000 K temperature range. The following lines seem to be suitable: 1. O I (1152.1 A): 10,000 K 2. C I (1253.47 A): 10,000 K 3. Fe II (1563 A): 12,000 K 4. Si II (1526.7 A): 13,000 K 5. Ly beta (1025 A): 15,000 K With SUMER, we shall obtain time profiles as well as raster images of an area consisting of both coronal hole and quiet Sun regions. If possible we shall obtain high cadence MDI magnetograms of the region of interest so that the variability of the field within the coronal hole can be studied in conjunction with the radio observations. Full disk magnetograms are generally preferred since they complement the full-disk radio observations. However, if the coronal hole fits entirely within the MDI high-resolution field of view, then high-cadence, high-resolution magnetograms are also desired (but not required) for the day or so when the coronal hole is near central meridian. The Nobeyama radioheliograph operates daily between 23 UT and 07 UT. The data quality is best around local noon which is 03:00 UT. Therefore, we ask that the coordinated observation be centered around this time. The Nobeyama images have a maximum spatial resolution of about 10" and a temporal resolution of up to 100 ms. We ask that we do this observing as many days as possible during the 5 week period starting in the beginning of October 1999. References: Gopalswamy, N., K. Shibasaki, B. J. Thompson, J. B. Gurman, and C. DeForest, Microwave enhancement and variability in the Elephant's Trunk coronal hole: Comparison with SOHO observations, JGR, 104, 9767, 1999 Kosugi, T. M. Ishiguro, and K. Shibasaki, Polar cap and coronal hole associated brightenings of the Sun at millimeter wavelengths, Publ. Astron. Soc., 38, 1, 1986. Wefer, F. L., and M. P. Bleiweiss, Observations of coronal hole associated features at wavelengths of 2.0 cm and 8.6 mm, Bull. Am. Astron. Soc., 8, 338, 1976. N. Gopalswamy Code 682.3 Bldg 26, Room G-1 NASA/GSFC Greenbelt MD 20771-0001 ph: 1-301-286-5885 fax: 1-301-286-0264