5 Discussion

Quiet Sun radio emission at 87 GHz originates from the upper chromosphere, as free-free emission from a plasma at a temperature of about 7200 K. Thermal radio emission corresponds to the density and temperature of plasma, and the source height for radio emission can only be determined if the source is located over the limb. Also, radio brightening can be suppressed by cool dense material high in the corona, as is the case for the locations of H$_\alpha$ dark filaments. In our study a large number of coronal holes appeared as brightness enhancements in radio. However, many coronal holes appeared also radio depressed. Therefore coronal holes in general cannot be defined as radio bright at 87 GHz. Many of the localized intensity decreases (LIDs) seen in EUV also appeared radio bright - but not all of them. In many cases structural mixing was present, i.e., more than one EUV feature appeared within the radio beam. In some of the EIT maps it was possible to see a patchwork of small "holes'', and the radio enhancement peaks were located inside them. The observed diffuse bright soft X-ray emission over the EUV LIDs could be explained by hot coronal material, or by time differences in the observations, or by projection effects that could change the soft X-ray source positions.

Thermal radio emission is enhanced with growing density and/or temperature, and can be formed at any height in the solar atmosphere above the critical plasma density level. Therefore local dense and relatively hot (but cooler than needed for EUV lines), or less dense and very hot (possible to see in soft X-rays) regions would show out as radio brightenings. In our data a few locations were seen as dark coronal holes in the hot Fe lines, but when viewed in the chromospheric He II line, small local brightenings were found. Some of the coronal hole radio brightenings could therefore be formed at temperatures below the He II line. In a recent paper Gopalswamy et al. ([1999]) suggest that coronal holes are seen as radio brightenings only if there are flux concentrations present in unipolar magnetic regions. The authors connect microwave enhancements with a combination of a smooth component, which probably comes from network cell interiors, and other more compact sources. Unfortunately the spatial resolution is too poor in our case to be able to separate the possible different components. The magnetograms for our data set are to be analysed, to see if there is a difference between the radio enhancements and depressions for coronal hole-like areas. A study by Nindos et al. ([1999]), comparing 17 GHz radio brightenings with EIT features, did not find a one-to-one correlation between compact radio sources and bright EUV features either. They also suggested an association between the He II and diffuse polar cm-wave emission, which would mean that the radio emission comes from heights below the 80000 K layer, and not from the corona. The suggestion that in some cases the cm- and mm-wave radio emission has its origin at relatively low heights in the solar atmosphere has also been supported by the recent finding that 87 GHz radio bright regions are often associated with EUV, and even UV bright points, observed with the 0.5 arcsec pixel resolution of the TRACE satellite (Pohjolainen et al. [1999b]).

Our data set showed that bright points, and sometimes even plumes, can be detected at 3.5 mm wavelength. However, many of the plumes were too high in latitude to be visible in the radio maps (limit of about 70 degrees). The bright points that did show up well in the mm-radio maps were intense both in EUV and soft X-rays. Also some indication of emission below 80 000 K was detected inside the coronal holes.

It has been suggested that cm-wave radio brightenings near the solar poles and inside coronal holes - as they are most probably connected to temperature enhancements in the chromosphere - may be related to the origin of solar wind. Millimeter wave emission has its origin even deeper down in the chromosphere, and our finding of mm-wave coronal hole brightenings may be a further proof of heating processes taking place in the chromosphere, that are necessary for solar wind plasma flows.

Due to the low spatial resolution of the available mm-wave radio observations it is difficult to determine any single feature to be the cause for radio brightenings. Our study suggests that bright points and polar plumes, as well as unresolved EUV and soft X-ray brightenings could be associated with radio brightenings. We also discovered radio bright regions inside coronal holes, with no obvious features causing them. This suggests that a closer look with good spatial resolution will be needed in the whole spectral range - from UV to soft X-rays - combined with magnetograms, to discover the physical counterparts for the brightenings.

Acknowledgements

We thank the anonymous referee for his constructive suggestions and helpful comments that improved the paper significantly. Part of this work was done at Metsähovi Radio Observatory, during D.R.'s training period. S.P. is supported by the Academy of Finland Contract No. 42576. F.P-F was supported in his work also by private foundations. SOHO/ EIT was build by an international consortium involving ESA and NASA, under the supervision of J.P. Delaboudinière (PI). Yohkoh is a Japanese solar mission, with several internationally operated instruments. The SXT images were analysed using the Yohkoh Data Archive Centre (YDAC) at Mullard Space Science Laboratory. The alignment of multi-wavelength images for some of the figures was done with the help of Object-Based Methods for Analyzing Solar Images developed by D. Zarro.