Nine solar radio maps, from 9 different days, observed in 1996-1997 were analysed for this study. The coordinates and fluxes of radio bright and depressed (relative to the quiet Sun level) areas near the polar zones were listed, as described in Sect. 2.5. Also some strong active regions at lower latitudes were given coordinates, mainly to check the accuracy of the radio coordinate determination.
The listed radio source locations were analysed in all EUV wavelengths, and all EUV features seen within the 1 arcmin radio beam were listed. The identification of EUV features was mainly based on coronal images (171/195/284 Å). The Fe IX/X line (171 Å) was usually checked first, and then compared with images at other wavelengths. The same procedure and listing was done with the soft X-ray images.
The classification of EUV and soft X-ray features was based on intensity, spatial scale, and general "lookout''. We determined the quiet Sun level in each of the images, and the enhanced/reduced brightness was determined relative to this. One effect of this method was that very few of the analysed locations were determined to be "quiet Sun'', as it was defined to be practically one intensity value. Also, as the radio beam area consists of 30-400 SXT or EIT pixels (depending on the observing mode), we did not try to give any "average'' intensity value for the EUV/soft X-ray features.
With this method each radio location could have more than one possible counterpart in EUV and soft X-rays (indicated by "/'' in Tables 3-11). The problem of multiple sources within the radio beam is clearly seen in the polar region EUV blow-up image, in Fig. 1. Tracking problems with radio antennas can sometimes cause displacements in the beam position, and therefore we also looked for strong emission sources nearby the assumed antenna beam position. The "nearby'' features are also listed in Tables 3-11. The radio maps are presented in Figs. 2, 5, ... 26, the soft X-ray images in Figs. 3, 6, ... 27, and the EIT images in Figs. 4, 7, ... 28.
The quiet Sun radio emission at 87 GHz (3.5 mm) originates from chromospheric heights, and has an estimated brightness temperature of 7200 K. The emission is mostly due to thermal bremsstrahlung. However, radio emission can also be produced at greater heights if the plasma density and temperature are high enough in, e.g., coronal loops. Besides thermal emission, other emission mechanisms may also be present. There has been some discussion on possible synchrotron sources and plasma emission connected with coronal holes (e.g. Shevgaonkar et al. [1988]).
In EUV coronal lines, the intensity is proportional to the emission measure, which is related to the temperature and the density. The iron lines are optically thin. With the iron lines different temperatures of the corona can be observed. The image appears darker where the temperature is lower (e.g., filament channels) or the density is smaller (e.g., coronal holes, where the plasma can escape along the open magnetic field lines). Fe XII is adapted to observe the "usual'' corona, while Fe XV observes the more hot temperatures. Structures in Fe IX/X always appear with narrow shapes (such as loops). This transition region line is very important for the analysis, as it shows the evolution of the topology and the morphology at intermediate temperatures between the photosphere and the corona. The He II line at 304 Å is an optically thick line. This chromospheric line is mixed with a coronal Si IX line.
The Yohkoh/SXT was mainly designed for observing flares and other transient phenomena. The quiet Sun emission, with changing features but low count rates, is therefore more difficult to analyse. Some studies of polar coronal holes have been made with the SXT (see, e.g., Foley et al. [1997]), but these have required longer than normal-mode exposure times and summing up of images over long periods. Coronal hole temperatures in soft X-rays have been found to be similar to the values for the nearby quiet corona, but with electron density values about 3 times lower (Hara et al. [1994]). We did not do any temperature or emission measure analysis because of the long image intervals and low count rates in some of the individual images.
One should also note that no comparison of features was done between the EIT and the SXT images. We compared only radio and EUV, and radio and soft X-ray emission sources in the given solar coordinates, but within the radio beam. Since the radio resolution is so different from the two others, comparison of a single location in high resolution images might give different results.
Quiet Sun structures are usually considered to be very stable, and not change much in time. Polar plumes may stay the same for hours, and radio depressions and brightenings have been used for, e.g., determining the solar rotation rate at high latitudes. However, when comparing the radio and soft X-ray images we found that the borders of coronal holes can change their position within some tens of minutes. On April 12, 1996, at 13:52 UT the radio brightening 3S was situated in the coronal hole border area, near a soft X-ray brightening, but in the later SXT image at 14:23 UT the radio position had moved inside the coronal hole area. On August 9, 1996, the radio bright position 4N moved from coronal hole border area to a diffuse bright area within 12 minutes, and on August 8, 1997, positions 1N and 6N moved from the coronal hole border area to inside the coronal hole, within about an hour. Therefore one should be very careful in comparing coronal hole border areas when the time differences between images are larger than the order of tens of minutes.
The radio locations included 81 brightness peaks and 23 depression centers. For half of the radio locations (51/104) it was possible to determine a sole counterpart from the EIT images. For the rest of the cases two or possibly more different features were present within the estimated radio beam. We would have to know the temperatures and densities of each of these features to be able to determine how much they contributed to the radio emission.
The EUV regions listed in Tables 3-11 were classified as:
Active regions (AR) were classified with the help of NOAA active region listings from the Solar Geophysical Data. Also large EUV and radio bright regions with complex loop structure were classified as ARs, even if they were not listed by NOAA. These regions were much larger than bright points and much more intense than localized intensity enhancements (see below). The tops of the hot loops in ARs usually show thermal radio emission. During long duration events and flares, also footpoint brightening is observed at cm wavelengths (Hanaoka [1994]).
Bright points (BP) are bright individual features sometimes seen in
coronal holes. Bright points can also be seen over diffuse bright
structures, all over the solar disk. The sizes of BPs are
usually
.
Simultaneous EUV and soft X-ray observations have shown that
the regions of peak emission are not always cospatial, suggesting
a complex structure of small-scale loops, at different temperatures
(Habbal et al. [1990]). EUV and soft X-ray bright points
have been found to be associated with magnetic bipolar regions,
but cm-radio observations have shown some association between
radio bright points and unipolar magnetic structures (Kundu et al.
[1988]).
EUV bright points have not been studied much at radio wavelengths.
Polar plumes (PP) are bright individual features seen in polar coronal holes (DeForest et al. [1997]), and sometimes in coronal holes that extend to equatorial heights (Allen et al. [1997]). They can also appear in groups. The 3-D structures of polar plumes are very well seen with EIT. Polar plumes have not been detected previously at millimetric radio wavelengths. A study by Nindos et al. ([1999]) discovered centimeter radio emission arising between the plumes in polar regions.
Localized intensity enhancements (LIE) are brighter than the quiet Sun and less bright than ARs. They are also larger than BPs. They exist outside coronal holes and active regions, and include sometimes diffuse areas like the borders of coronal holes. Magnetograms for these areas give no definite polarities. The brighter features could be loop structures that are not seen due to the EIT spatial resolution. Temperatures given by EIT for these structures are between 1-1.6 MK.
Localized intensity decreases (LID) are small - a few arcmin diameter - regions that are less bright than the quiet Sun. These small "holes'' can be found all over the solar surface. The existence of small coronal holes has been suggested previously by Bohlin ([1976]). Coronal holes (CH) are seen as dark regions in EUV and soft X-rays, and as bright regions in the He I absorption line. The density and/or temperature is usually lower than in the surrounding corona. There has been some debate on whether CHs can be identified as open field line areas (Obridko & Shelting [1999]), but at least for statistical purposes this definition can be used. Coronal holes are usually seen as depressions at centimeter wavelengths, but several studies have shown radio bright coronal holes (Gopalswamy et al. [1999]; Brajsa et al. [1996]).
The radio brightness enhancements (81) correlated with the following EUV features: active regions (3), polar plumes (3), localized intensity enhancements (17), localized intensity decreases (12), coronal holes (9), and mixtures of features (37).
The radio depressions (23) correlated with the following EUV features: localized intensity enhancements(2), localized intensity decreases (13), coronal holes (2), and mixtures of features (6).
The structures seen in the Yohkoh SXT images were listed similarly, but to avoid confusion between the EUV and soft X-ray features we named some of the soft X-ray features differently. Brightness enhancements and less bright regions in soft X-rays are usually spatially larger (reflecting also the instrument resolution) than in EUV, and therefore we dropped the term "localized''. The soft X-ray features were listed as follows:
The radio brightness enhancements (81) correlated with the following soft X-ray structures: active regions (4), bright points (8), enhanced brightness (39), reduced brightness (5), coronal holes (16), and a mixture of features (9).
And the radio depressions (23) correlated with the following soft X-ray structures: enhanced brightness (2), reduced brightness (9), coronal holes (8), quiet Sun (3), and a mixture of structures (1).
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