1 Introduction

The solar structures known as coronal holes (CH) are important for understanding the Sun's effects on the Earth, since they are the source of the high-speed solar wind (Krieger et al. 1973). These regions have lower coronal densities and temperatures than non-hole regions, which is presumably due to open magnetic field lines in the hole. CHs can be identified by a decrease in coronal line emission, for example the EUV Fe XII 195 Å line observed by the Solar and Heliospheric Observatory Extreme-ultraviolet Imaging Telescope (SOHO/EIT) instrument (Delaboudinière et al. 1995), or in emission from lines which are excited from the corona, such as the near infrared He 10830 Å line. One unexplained aspect of CHs, both near the poles and the equator, is the presence of bright regions in radio emission in some holes (Kundu & McCullough 1972; Wefer & Bleiweiss 1976; Evanov et al. 1980; Kosugi et al. 1986; Shibasaki 1998; Gopalswamy et al. 1999a,b; Nindos et al. 1999; Pohjolainen 2000). These radio enhancements may be related to the physical conditions in the hole which give rise to the fast solar wind.

These regions were first detected in polar coronal holes (PCH) and later in equatorial coronal holes (ECH), which often begin as PCHs and move to low latitudes. Brightenings in CHs have been observed at frequencies between 17 and 87 GHz. These enhancements may be classified as compact, 1$^\prime$ or less in extent, or diffuse, extending over the entire CH. Observations at 17 and 36 GHz made with 10-45 $^{\prime\prime}$ spatial resolution show both diffuse and compact enhancement within polar and equatorial CHs, with the mean emission level exceeding that of the quiet sun (Kosugi et al. 1986; Shibasaki 1998; Gopalswamy et al. 1999a,b; Nindos et al. 1999). Observations at 87 GHz show the presence of enhanced compact regions surrounded by depressed diffuse emission in CH's at a resolution of 1 $^{\prime\prime}$ (Pohjolainen et al. 2000; Pohjolainen 2000). At 10.7 GHz, observations made with a resolution of 1.25 $^{\prime\prime}$ show the mean polar CH levels are depressed relative to the quiet sun (Furst & Hirth 1975), but small unresolved enhanced emission sources might be present. Observations at 98 GHz made with a resolution of 17 $^{\prime\prime}$ show no polar CH diffuse or compact enhancements (Kosugi et al. 1986). To summarize, the mean CH emission level is depressed between 10.7 and 98 GHz, and compact enhanced regions have been observed between 17 and 87 GHz. High resolution observations outside this frequency range are required in order to determine the frequency extent of compact enhancements.

We consider CH radio emission at 17 GHz, since compact enhancements are strong at this frequency and the Nobeyama Radioheliograph observes the sun constantly at 17 GHz with 1 s time resolution (Nakajima et al. 1995). Full disk 17 GHz solar images at ten minute intervals are readily available from the Nobeyama Radio Observatory web server. At 17 GHz, the diffuse component is characterized by temperatures typically 1000 K above the quiet Sun (QS) level of 10000 K, while the compact component is as much as 3000 K above the background. The polar brightness enhancements are generally greater than the equatorial enhancements, due to contributions from microwave limb brightening. The true brightness temperature increase is in fact larger if the compact bright regions are unresolved.

Comparison of microwave images and longitudinal magnetograms shows that enhancements are correlated with regions of strong unipolar magnetic flux (Gopalswamy et al. 1999a,b, 2000) and with enhanced coronal emission in plume bases (Pohjolainen 2000). Enhancements are not found in areas of bipolar or mixed flux. Radio image sequences with ten minute cadence show the enhancements are highly variable on scales of ten minutes or less (Gopalswamy 1999b). Increased limb brightening has also been found in PCHs for some FUV emission lines (C III, O IV, and S V) (Wilhelm et al. 1998). The source of the enhanced radio emission has been investigated through modeling of the chromospheric, transition region and coronal contributions. We define the chromospheric temperature to be between 4000 K and 20000 K, the transition region between 20000 K and 500000 K and the quiet coronal temperature between 500000 K and 2000000 K, and the lower, middle and upper chromospheric temperature ranges to be 4000 K to 8000 K, 8000 K to 12000 K and 12000 K to 20000 K, respectively. Since the optical depth is proportional to the line-of-sight integral of $n_{\rm e}^{2}/T_{\rm e}^{3/2}$, where $n_{\rm e}$ is the electron density and $T_{\rm e}$ is the electron temperature, the CH contribution to the total 17 GHz emission must be small, approximately 1 to 2%, due to the low densities and high temperatures in the corona (Gopalswamy et al. 1998, 1999a,b). The contribution from the transition region between 20000 K and 200000 K must be negligible as well, since radio emission models excluding that region agree with the observed QS spectrum (Grebinskii 1987). Thus, the radio enhancements must be formed at temperatures below 20000 K, since temperatures above 200000 K are precluded due to low optical depth. This is consistent with the measured 17 GHz brightness temperature of 10000 K for the QS and peak temperatures of 12000 K to 13000 K for the compact enhancements.

In order to determine the cause of the enhancements, we have made a series of observations in a search for the physical layer where the enhanced radiation is formed. If the cause of the radio enhancement in polar and equatorial holes is a difference in temperature profile between the QS and CH regions of strong magnetic flux, the intensities of UV lines emitted at the radio formation height will also be affected, since the UV line excitation functions are temperature dependent. A correlation between radio and UV line intensity in an ECH radio enhancement would indicate the height and temperature of the increased 17 GHz emission. Previous observations have shown that some upper chromospheric emission lines, such as H I Ly$\alpha$, Ly$\beta$, Ca II K and Mg II K, which form at temperatures below 20000 K, are slightly more intense in CHs (Bocchialini & Vial 1996), but the characteristics of these lines in ECH radio enhancement regions have not been measured. Therefore, a series of UV ECH observations were made for the first time using the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) telescope and spectrograph (Wilhelm et al. 1997) on board the Solar and Heliospheric Observatory (SOHO) (Domingo et al. 1995) in several FUV chromospheric and transition region lines. The spectral line observations covered a formation temperature range of 8000 to 630000 K, which includes the lower chromosphere through the upper transition region to the lower corona. In this article, we present the results of a comparison between UV and microwave observations in these CHs. In addition, we compare radio images with visible H$\alpha$ images to include the lower and middle chromosphere in the enhancement layer search.