ASTRONOMY AND ASTROPHYSICS. On the origin of polar radio brightenings at short millimeter wavelengths. S. Pohjolainen 1,2,

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1 Astron. Astrophys. 361, (2000) On the origin of polar radio brightenings at short millimeter wavelengths ASTRONOMY AND ASTROPHYSICS S. Pohjolainen 1,2, 1 Observatoire de Paris, DASOP, Meudon, France (pohjola@mesopy.obspm.fr) 2 Metsähovi Radio Observatory, Helsinki Univ. of Technology, Espoo, Finland Received 5 April 2000 / Accepted 10 July 2000 Abstract. Polar regions of the Sun are areas where coronal holes are most profound and where the heating and acceleration of solar wind plasma takes place. The observed and yet unexplained radio emission from these regions may be related to the origin of the solar wind flow. The recent analysis of radio brightenings near the poles at 87 GHz (3.5 mm) suggested structural counterparts like polar plumes, EUV and/or soft X- ray bright points, and diffuse EUV brightenings especially near the borders of coronal holes. Coronal holes themselves were mainly seen as radio depressed, with local radio brightenings inside them. The radio brightness temperatures for these types of features are now calculated by using the average temperature and density values from EUV and soft X-ray observations, assuming isothermal and optically thin plasma. The calculated values are in agreement with the observed ones. The calculations show some bright points and plume bases to be observable in mmwaves (calculated brightness temperature enhancements K, observed K), but that the observability depends strongly on density and loop geometry (line of sight source length) of individual sources. Coronal holes in general should be seen as radio depressions at 87 GHz (calculated brightness temperature drops K, observed drops K). Also a slight, 2.0% maximum, smooth limb brightening is now found to affect the radio observations inside R. This limb brightening is much less than reported in an earlier study using a similar telescope and same wavelength, which suggests a slightly different chromospheric model. The wide limb brightening does explain why some less intense EUV and soft X-ray features become observable in radio, as they get superposed on the smooth brightening. Comparison to previous results obtained at several radio frequencies suggest that some of the high-latitude (<70 degrees) radio brightenings at 87 GHz could be formed in the same atmospheric layer as the diffuse radio emission sources in the polar-cap regions and the diffuse radio emission sources inside equatorial coronal holes as they all show correlation to EUV emission sources, especially in the He II 304 Å ( K) line. However, the unexplained 87 GHz radio brightenings inside Present adress: Tuorla Observatory, University of Turku, Väisäläntie 20, FIN Piikkiö, Finland high-latitude coronal holes, the radio bright patches observed in polar-cap regions, and the bright compact sources seen inside equatorial coronal holes all seem to share the fact that they are not associated with any features seen in EUV. The best candidates for the formation of these radio sources are magnetic flux elements and density/temperature enhancements below the K layer. Key words: Sun: chromosphere Sun: corona Sun: radio radiation Sun: transition region Sun: UV radiation 1. Introduction Radio observations of the polar regions of the Sun have been of interest since the discovery of unexplained brightenings in the 1970 s (see summary in Kosugi et al., 1986). As enhanced radio emission has been observed inside coronal holes or regions nearby, a connection to magnetic field structures, heating mechanisms and solar wind has been suggested. Polar radio brightenings have not been studied much at short millimeter wavelengths, due to the lack of observations with good spatial resolution. Recent analysis of the Metsähovi 3.5 mm (87 GHz) radio maps from with comparison to SOHO EIT and Yohkoh SXT images (Pohjolainen et al., 2000 from hereon cited as the Metsähovi-EIT study), and from 1998 with comparison to TRACE images (Pohjolainen et al., 1999) showed both radio brightenings and depressions near the solar poles. The EUV and soft X-ray features that were correlated with radio brightenings included plumes (or bases of plumes), bright points, and diffuse structures. Coronal holes were mainly seen as radio depressed, but sometimes with unexplained local radio brightenings inside them. In this paper, the temperatures and densities for some EUV and soft X-ray features are listed from recent publications. The corresponding radio brightness temperatures are then calculated using the average values. It is then discussed whether these features could be causing the observed polar radio brightenings at short mm-wavelengths. Also observational restraints are briefly discussed. The results are then compared with previous results at other radio wavelengths.

2 350 S. Pohjolainen: On the origin of polar radio brightenings at short millimeter wavelengths 2. Brightness temperature determination At radio frequencies the brightness temperature T b of a compact source can be determined from the effective temperature T eff of the radiating electrons: T b = T eff (1 e τν ). (1) The optical thickness τ ν for thermal electrons (i.e. for thermal bremsstrahlung) can be written as n2 e τ ν = κ ν L 0.01 L (2) { ν 2 T 3/ lnT ln ν (T < K) 24.5+lnT ln ν (T > K) where κ ν (cm 1 ) is the absorption coefficient, L is the source length (cm), n e is the electron density in fully ionized plasma (cm 3 ), ν is the observing frequency (Hz), and T is the electron temperature (K). In the case of an optically thick source the brightness temperature is T b = T eff (τ ν 1), (3) and in the case of an optically thin source T b = T eff τ ν (τ ν 1). (4) The radiative transfer equation can be written as T b = T b0 e τν + T eff (1 e τν ) (5) when observing a (background) source with temperature T b0, through an emitting and absorbing medium with temperature T eff and optical depth τ ν. In the simplest case the plasma is assumed to be isothermal and have the same optical depth. The basics of radio astronomy can be found in e.g. Kraus (1986), and with emphasis on solar radio astronomy in Dulk (1985). For example, it is possible to calculate how much the brightness temperature is changed at frequency ν if the quiet Sun (at temperature T b0 ) is observed through a dark filament (with average temperature T eff, average density n e, and scale length L). In case the source does not fill the whole radio beam the temperature has to be scaled relative to the area covered. For example, if a filament with T eff only covers a solid angle Ω f (rad 2 )of the antenna beam area Ω A (rad 2 ), the true diluted effective temperature T eff will be T eff = Ω f T eff. (6) Ω A Quiet Sun emission at millimeter waves is mostly free-free emission of the hot plasma in the upper chromosphere and above. Millimeter wave radiation from the solar polar regions is generally assumed to be also due to Coulomb bremsstrahlung, as the regions are quiet and have low magnetic fields. No significant circular polarization has been detected in the polar emission either (Hiei, 1987). The observed quiet Sun brightness temperatures at mm vary slightly, see Table 1. Atmospheric attenuation can Table 1. Quiet Sun brightness temperatures near 3 mm λ T b Estimated Reference (mm) (K) height (km) Ulich et al., VAL, 1981 (summary) Nagnibeda & Piotrovitch, Irimajiri et al., Linsky, 1973 affect the total flux density measurements, and one method to overcome flux variations caused by atmospheric changes and instrumental defiencies is to normalize the quiet Sun flux to a fixed value, and give enhancements and depressions values relative to this. For example, the quiet Sun at 3.5 mm in the Metsähovi observations has been normalized to the average brightness temperature value of K. The radio source heights can reliably be determined only if the source is seen above the limb other estimates have mainly been done using the VAL (Vernazza et al., 1981) atmospheric model. 3. Temperature and density diagnostics 3.1. Coronal holes The radio depressions for the coronal holes in the Metsähovi EIT study were K below the quiet Sun level, with an average of 70 K. Kundu & Liu (1976) also observed a coronal hole at 3.5 mm and found that the brightness temperature inside the hole was in most places about K lower than the mean brightness temperature of the Sun. Coronal holes have been reported to have lower densities than the quiet Sun regions, although the value of the factor has varied from 2 (Doschek et al., 1997), to 2.7 (Gallagher et al., 1999), and 3 (Hara et al., 1994). The difference in the values can be caused by instrumental effects (these values were obtained using different instruments) or the different wavelengths (temperature ranges) that were analysed. For example, a recent SUMER-based study has suggested an isothermal plasma in the range R while the analysis of the same region using Yohkoh SXT found a temperature that increases with height (see Wolfson et al., 2000). The chromospheric coronal contribution to the brightness temperature at 3.5 mm can be estimated using the temperatures and densities given in Table 2. The source length L is estimated to be around km along the line of sight (average source height h = km, therefore the line of sight is longer near the poles, see Fig. 1) for both the quiet Sun and the coronal hole. Taking T = K and n e = cm 3 as the maximum values for the coronal hole from Table 2, we get the chromospheric coronal contribution T b = 12 K. The slightly higher temperature value 1.1 MK gives 1 K less. The less dense inter-plume values and the low values given by Doschek et al. would give only 1 K as the total chromospheric coronal contribution. However, it is well known that the network fills only a

3 S. Pohjolainen: On the origin of polar radio brightenings at short millimeter wavelengths 351 Table 2. Temperatures and densities from recent EUV and soft X-ray observations Location T e (K) n e (cm 3 ) Estimated h (km) Instrument/year Reference CH SOHO/SUMER 1996 Doschek et al., 1997 CH/cell centre max SOHO/CDS 1996 Bromage et al., 1997 CH/network max SOHO/CDS 1996 Bromage et al., 1997 CH SOHO/CDS 1998 Gallagher et al., 1999 CH/interplume SOHO/CDS 1998 Young et al., 1999 quiet sun SOHO/SUMER 1996 Doschek et al., 1997 quiet sun/cell centre SOHO/CDS 1996 Bromage et al., 1997 quiet sun/network SOHO/CDS 1996 Bromage et al., 1997 equator SOHO/CDS 1998 Gallagher et al., 1999 BP SOHO/CDS 1996 Bromage et al., 1997 BP Yohkoh/SXT 1992 Hara et al., 1994 BP Yohkoh/SXT 1992 Kundu et al., 1994 BP Yohkoh/SXT 1997 Preś & Phillips 1999 BP Yohkoh/SXT 1997 Preś & Phillips 1999 plume SOHO/CDS 1998 Young et al., 1999 plume base SOHO/CDS 1998 Young et al., 1999 CH h1 Plumes quiet Sun h Line of sight Fig. 1. Geometry for observing the solar atmosphere at high latitudes: Polar coronal hole (CH) and the quiet Sun have scale height h and source length (radio path) L = h1 near the poles. If the same atmospheric layer is viewed near the center of the disk, the source length L = h. Note that L for polar plumes is not significant unless the plumes are seen along the line of sight (from the top). small fraction of the field (Zirin et al., 1991), so the temperature and density values for cell centres are the most significant. Taking T = K and n e = cm 3 for the quiet Sun, the chromospheric coronal contribution is now T b = 53 K. A less dense atmosphere, n e = cm 3, would give T b =45K. Again, the very low density value of Doschek et al. would give only about 3 K as the total chromospheric coronal contribution for the quiet Sun. The complex structure of the upper chromosphere and the transition region are not very well known, and treating these regions as homogeneous means simplifying a lot. However, as the radio emission is mainly sensitive to density variations and source lengths, local hot or very small features will not have much effect on the observed radio brightness. Therefore the average temperature and density values for the km atmospheric layer given by Gallagher et al. (1999) can be used in estimating the chromospheric coronal contribution in radio. The obtained numbers suggest an approximate K brightness temperature drop for coronal holes, compared to the nearby quiet Sun at 3.5 mm. The observed values, K drop with an average of 70 K drop, are slightly larger but not significantly considering the error margin (± 22 K) in the Metsähovi EIT study Bright points X-ray bright points (BP) that show up in cm- and mm-waves have been identified as a subset of He I dark points and bipoles (Webb et al., 1993), although some association with unipolar magnetic structures has also been found (Kundu et al., 1988). The morphological properties of coronal hole and quiet Sun region BPs have been found to be the same (Habbal et al., 1990). The typical diameter in EUV and X-rays is 10 50, with a lifetime around 8 hours. At 6 cm radio BPs have been reported to have lifetimes around 5 20 min, and diameters around 5 15 (Habbal et al., 1990, Fu et al., 1987). A simple loop model has usually been adopted for flux calculations (Kankelborg et al., 1996). Assuming a semicircle loop with a height h = km (approx. 30 /2) and diameter s = km, with a total length of km, the emitting volume would be around cm 3. But, as the radio flux depends only on the source length along the line of sight (L), seeing the loop in the center of the solar disk would only mean having L s = km, while seeing the same loop near the solar poles would give L km, see Fig. 2. A more realistic model for bright points is a system of loops, connecting two regions of opposite polarity. However, even in the case of multiple loops the radio source length remains approximately the same if the loops do not fall over each other. With average values like T = K and n e = cm 3, and length L =10 9 cm ( km) as the max-

4 352 S. Pohjolainen: On the origin of polar radio brightenings at short millimeter wavelengths Line of sight (equator) BP L s Line of sight (polar region) qsl BP qsl Fig. 2. Observations of high latitude bright points with source length (radio path) L. If the loop (or a system of loops between two regions of opposite polarities) is viewed from the top (e.g., near the solar disk center), the source length L is equal to the loop diameter s. imum line of sight, the brightness temperature excess would be about 30 K. This is just visible at 3.5 mm, and with a half shorter source length (for example loop seen from above in the center of the solar disk, see Fig. 2) a BP like this would not show up. The values from Table 2, T = K and n e = cm 3, would give BP brightness temperatures around K. However, the hot and less dense BP value given by Preś et al. would only give a small 3 K excess in radio brightness. This shows that, most of all, BPs need to have high density in order to show up in radio. The detection of BPs can also be affected by the dilution effect of the radio beam (see Sect. 2 in this paper). The Metsähovi EIT study found local intensity decreases (LID) in EUV all over the solar disk, and if such areas happen to fall inside the radio beam together with a BP, the cold and less dense larger area will dilute the radio intensity of a small BP, and the brightness temperature of the source will remain below the quiet Sun level. This is illustrated in Fig. 3. In the Metsähovi EIT study some of the EUV and X-ray BPs had bipolar regions at their base that were spatially small and close together. These did not show any radio brightness enhancements, contrary to the BPs that had large and wellseparated bipolar regions at their base. Some examples of BP observability are presented in Figs. 4, 5, and 6. It is also possible to compare the observability of BPs at 87 GHz with observability of BPs at other radio wavelengths: If the emission mechanism for BPs is thermal bremsstrahlung from optically thin loop systems, the ratio of the brightness temperatures at two radio wavelengths should be the ratio of the square of the wavelengths (Kundu et al., 1988). We assume here that the emission arises from a same temperature and density plasma (Eqs. 2 and 4). For example, using the 6 cm (5 GHz) brightness temperature observations of Kundu et al. (1988), T b K, T b (3.5 mm)/t b (6 cm) = , we get the 3.5 mm BP brightness temperatures to be around K. Using the peak brightness temperatures obtained at 17 GHz (Kundu et al., 1994), T b K, T b (3.5 mm)/t b (1.8 cm) = 0.038, we get that the BP brightness temperatures at 3.5 mm should be around K. true brightness temperature LID observed brightness temperature Fig. 3. Dilution effect when observing a source smaller than the radio beam: effective temperature of the source is proportional to the area covered within the beam. For example, bright points (BP) usually have sizes around 5 50, while the Metsähovi beam at 87 GHz is about 60. If the BP is located inside a local intensity decrease (LID) or coronal hole area, the observed temperature enhancement can go undetected if it is below the quiet Sun level (qsl). The arrrows indicate enhancement (upward) and depression (downward) relative to the qsl. The EUV and soft X-ray BPs that did show up in the Metsähovi EIT study had brightness enhancements around K above the quiet Sun level. The brightness temperature calculations here, using simplified models and average temperature and density values from EUV and soft X-ray observations, give brightness enhancements around K. It seems evident that the observability at 87 GHz depends very much on loop geometry and source density along the line of sight Plumes Single plumes located near the solar poles have too short source lengths to be visible in radio (see Fig. 1), although their temperatures and densities correspond to those of the quiet Sun (Table 2). A large group of plumes, however, located along the line of sight inside a coronal hole, could rise the observed brightness temperture at least to the quiet Sun level. The values given by Young et al. (1999) for a plume base are similar to BPs. The source length depends again on geometry, but for a plume base with T = K and n e = cm 3, and source length L =10 9 cm ( km), the brightness temperatures would be in the range of K. The plume base does not necessarily fill the whole radio beam, so a dilution factor Ω source /Ω A should be applied (as with BPs). Assuming a plume base with a brightness temperature of 390 K, and the plume base fills about of the 1 diameter beam, we get a K rise in the observed brightness temperature, with respect to the surrounding brightness temperature. This is in agreement with the observations in the Metsähovi EIT study, that showed brightness temperature rises around K. But, the observability of a plume base de-

5 S. Pohjolainen: On the origin of polar radio brightenings at short millimeter wavelengths 353 Fig. 4. Left: SOHO EIT Fe IX/X at 12:30 UT, middle: Yohkoh SXT Al.1 at 12:39 UT, and right: SOHO MDI at 13:17 UT. The overplotted radio map (contours) was measured at 12:55 13:03 UT, on August 14, The plotted field of view is The radio beam (i.e. resolution) is 60. The BP is at distance 0.6 R (N40W17), whixh is not inside the limb brightening area. The magnetogram shows a small bipolar region in the location of the BP. No radio brightening is seen. Fig. 5. Left: SOHO EIT Fe IX/X at 12:30 UT, middle: Yohkoh SXT Al.1 at 12:39 UT, and right: SOHO MDI at 13:17 UT. The overplotted radio map (contours) was measured at 12:55 13:03 UT, on August 14, The plotted field of view is The radio beam (i.e. resolution) is 60. The BP is at distance 0.8R (N38W55), which is inside the limb brightening area. A faint bipolar region is seen in the magnetogram. No significant radio brightening is seen. Fig. 6. Left: SOHO EIT Fe IX/X at 12:30 UT, middle: Yohkoh SXT Al.1 at 12:39 UT, and right: SOHO MDI at 13:17 UT. The overplotted radio map (contours) was measured at 12:55 13:03 UT, on August 14, The plotted field of view is The radio beam (i.e. resolution) is 60. The BP is at distance 0.7R (S40E10), which is slightly limb brightened. A large, strong bipolar region is seen in the magnetogram. Intense radio brightening is also present.

6 354 S. Pohjolainen: On the origin of polar radio brightenings at short millimeter wavelengths E arcmin W 1.7 % Fig. 7. Radio intensity (A/D converter counts) across the solar disk in East-West direction. Data points are 0.6 arc min apart, each collecting emission from 1 arc min diameter area. The white line in the corresponding SOHO/EIT image represents the data point path near the center of the disk. The artificial limb darkening in the radio scan inside 0.92 R 1.00 R is clearly seen the vertical lines in the lower plot represent the optical East ( E ) and West ( W ) limb. The approximate value for the limb brightening is marked on the right, as percentage relative to the estimated quiet Sun level. The EIT image and the 87 GHz radio data are from April 12, pends again very much on the size and geometry of the source, as with BPs. 4. Limb brightening Limb brightening is expected at millimeter waves, as the length of the line of sight increases as the distance increases from the disk center. At 3 mm, the coronal contribution should be negligible, and the smooth-chromosphere models (spherically symmetric and uniform) predict radial brightening towards the limb, as well as a limb spike (Belkora et al. 1992). Observing instruments can also cause artificial limb darkenings and brightenings. For example, the observational limit in the Metsähovi maps is 0.92 R, due to the artificial limb darkening effect. The intensity drop inside the true solar limb (artificial limb darkening) and the intensity increase outside the limb (artificial limb brightening) is caused by convolving a disk source with a gaussian beam and sidelobes (Lindsey & Roellig, 1991). Therefore the maximal view towards the poles is about 70 degrees in latitude (depending on the B 0 angle), if deconvolution methods are not used. The observations of limb brightenings have given varying and sometimes even contradictory results. Strong evidence for limb brightening at cm-waves has been presented by Shibasaki (1998), who analysed solar maps from the Nobeyama Radioheliograph from 5 years (cleaned interferometer data). He found that the gradual brightening starts from about 50 degrees latitude. He suggests that the polar-cap brightenings are therefore the sum of a limb brightening and brightenings fixed to the polar region. Gomez-Gonzalez et al. (1983) used the Yebes 14-meter radio telescope at 3.4 mm wavelength (1 beam size similar to Metsähovi) to examine limb brightening with the method of Sun-Moon scans. The technique was used to eliminate the effects of convolving the source with the antenna radiation pattern. No limb spikes were seen, but a rather wide and smooth slight brightening inside the solar disk was discovered significant from 0.6 R measured from the center of the disk. A maximal brightening intensity of about 5% was observed (center to limb variation using average scans that represent the quiet Sun component), but in fact the observations (11 days in 1982) from different days seem to give slightly different values. The observed smooth brightening near the limb in the Metsähovi maps is in the scale of % ( K if the quiet Sun is at 7200 K). Although the intensity of the limb brightening observed by Gomez-Gonzalez et al. does not match the intensity seen in the Metsähovi maps, a bright ring with a width of 4 6 just below the 0.92 R observational limit is present, and is in agreement with the 0.6 R starting distance.

7 S. Pohjolainen: On the origin of polar radio brightenings at short millimeter wavelengths 355 E W 1.3 % arcmin Fig. 8. Same as in Fig. 7 for August 9, Some examples with East-West data paths from the rotated solar maps in the Metsähovi-EIT study are presented in Figs (the full disk radio maps are shown in Pohjolainen et al., 2000). In many of the radio maps active regions, coronal holes and bright points make it difficult to determine the true value for the limb brightening. 5. Polar-cap and high latitude radio brightenings anything in common? Polar radio brightening is a general term for enhanced radio emission near the solar poles, which includes 1) polar-cap brightenings which appear as a large number of bright patches superposed over areas of enhanced level of emission at latitudes > 65 degrees (e.g., Kosugi et al., 1986, Nindos et al., 1999), and 2) local radio bright regions at latitudes < 70 degrees (Riehokainen et al., 1998, Pohjolainen et al., 2000). There is not much overlap in the latitude ranges in 1) and 2), and analysis of both has been difficult due to instrumental limitations. For example, the polar cap diffuse emission and bright patches have been observed at 17 GHz (Nobeyama Radioheliograph) and at 36 GHz (Nobeyama 45-m single dish), but not at 98 GHz (Nobeyama 45-m). Local brightenings inside coronal holes at high latitudes have been observed at 87 GHz (Metsähovi 14-m single dish). Local brightenings inside equatorial coronal holes have been observed at 17 GHz (Nobeyama Radioheliograph) and at 36 GHz (Nobeyama 45-m single dish), but not at 98 GHz (Nobeyama 45-m single dish). These differences could simply be caused by the different instruments and different software, they could be the result of observing at different periods of the solar activity cycle, or they could just reflect the different physical features found in different coronal holes. Taking into account these limitations and the published results, it also seems evident that the brightenings are not restricted to a certain frequency range as suggested before. The high-latitude sources have been seen both inside and outside coronal holes. Coronal holes can exist in both latitude ranges. Polar radio brightenings in general are therefore related to the question whether coronal holes are radio bright, or if there are local radio brightenings inside coronal holes. Some studies of radio emission from equatorial coronal holes exist (Kosugi et al., 1986, Gopalswamy et al., 1998, Gopalswamy et al., 1999). These studies have shown that the radio enhancement inside coronal holes is not uniform, but inhomogeneous with larger less-intense brightenings and small intense point-like brightenings. A study by Shibasaki (1998) suggests that the diffuse, enhanced level of radio emission in the polar-cap region is due

8 356 S. Pohjolainen: On the origin of polar radio brightenings at short millimeter wavelengths 1.0 % E W arcmin Fig. 9. Same as in Fig. 7 for August 13, to limb brightening, with the bright patches superimposed on it. On the other hand, Nindos et al. (1999) suggest that the diffuse emission areas at polar caps could be associated with He II 304 Å features. No correlation was found between the radio bright patches and the EUV features. Also, the strong dependence on the solar tilt angle suggested a small scale height for the radio features. At latitudes lower than the polar-cap, the radio bright locations have been associated with bright points, plumes, diffuse structures and coronal hole boundaries seen in EUV and soft X-rays (Pohjolainen et al., 2000, Gopalswamy et al., 1999). Regions of white-light faculae and/or intense magnetic fields (Riehokainen et al. 1998, Kosugi et al. 1986) have also been found to be associated with radio brightenings. Some of the local radio brightenings inside coronal holes at latitudes < 70 degrees showed association with the He II 304 Å features in the study by Pohjolainen et al. (2000). This suggests that the diffuse radio brightenings in the polar-cap zone and the local brightenings inside coronal holes at lower latitudes could be associated. The few bright compact radio sources seen inside the equatorial elephant trunk coronal hole (Gopalswamy et al., 1999) were, however, connected more with unipolar enhanced magnetic flux elements than with bright point-like EUV emission arising from mixed polarity regions. Therefore the radio bright patches in the polar-cap zone could be connected with the bright compact sources inside equatorial coronal holes, and also with brightenings inside some high latitude coronal holes (the ones with no association to EUV emission). Further studies are clearly needed, with concentration on radio emission arising from coronal hole areas. As measurements near the poles are somewhat restricted (e.g., in radio maps and magnetograms), large equator-reaching coronal holes should be the main observational target in the future. As the main part of the millimeter wave radio emission arises from chromospheric heights, observations at temperatures below the He II line ( K) would be most useful. 6. Conclusions This paper has tried to find answers to the question why the polar regions of the Sun appear as radio bright. This question is linked to other unresolved problems like how is the corona heated and what is the origin of the solar wind. At short millimeter wavelengths we can observe the solar atmosphere at relatively low heights, and therefore gain new insight into the problem. At 3.5 mm coronal holes have appeared both radio depressed and radio bright. The observed temperature depressions within the coronal holes in the Metsähovi EIT study were in the range

9 S. Pohjolainen: On the origin of polar radio brightenings at short millimeter wavelengths % E W arcmin of K (±22 K), with an average drop of 70 K relative to the nearby quiet Sun level. The difference of the chromospheric coronal contribution to the coronal hole and the quiet Sun area is calculated to be around K. The larger observed values for temperature drops probably mean that coronal holes have lower brightness temperatures at 3.5 mm meaning that τ =1 corresponds to a lower height, i.e., we are observing a region lower in the solar atmosphere. Why do some coronal holes then appear radio bright? Possible candidates for causing local brightenings are polar plumes and bright points: Plume bases and some BPs should be visible at 3.5 mm, especially when superposed on a limb brightening. The calculated brightness enhancements over the quiet Sun are in the range of K, while the observed radio brightenings were in the range of K (±22 30 K). Inside coronal holes these sources should be very dense and spatially large to become visible. The observability estimates using previously measured quiet region BPs at 5 GHz (giving K at 87 GHz), and flaring BPs at 17 GHz (giving K at 87 GHz), are also in agreement with the Metsähovi-EIT observations. The published density values for one plume base are larger than for many BPs whether this is due to the different source heights observed by different instruments or structural differences in the sources themselves need further investigation. Fig. 10. Same as in Fig. 7 for August 27, A smooth limb brightening is now detected in the Metsähovi radio maps in the 0.6 R 0.92 R zone. This can contribute to the observed intensities for different features, as the determined quiet Sun level (one value) is not valid over the whole solar disk. Taking into account the smooth limb brightening and viewing angle effects (source lengths), one would assume BPs to be more observable in radio in the 0.6 R 0.92 R zone than near the center of the Sun. The data set presented in the Metsähovi EIT study, as well as the examples presented in this study, do not give clear support to this assumption. It seems that densities, viewing angles and magnetic field configurations play a more important role than limb brightening in whether BPs become visible or not in radio. The original radio EUV soft X-ray comparison was done by selecting the radio bright locations first and then comparing them with other wavelengths. In this way selection effects may play a part. For BPs it seems evident that not all, even the most intense ones, are visible in radio. One reason could be observing time differences if the BPs were flaring. On the other hand, due to the technical problems in radio observations, comparisons with very high-latitude features like polar faculae and polar plumes may be difficult. It is interesting to compare our results with the results obtained from polar-cap regions and equatorial coronal holes at other radio wavelenghts. It seems that some of the high-latitude

10 358 S. Pohjolainen: On the origin of polar radio brightenings at short millimeter wavelengths radio brightenings at 87 GHz could be formed in the same atmospheric layer as the diffuse radio emission sources in the polar-cap regions and the diffuse radio emission sources inside some equatorial coronal holes. They all show correlation to EUV emission sources, especially in the He II 304 Å ( K) line. Also, a common factor for the unexplained radio brightenings inside high-latitude coronal holes at 87 GHz, for the radio bright patches observed in polar-cap regions, and for the bright compact sources seen inside equatorial coronal holes, is that none of them seem to be associated with features seen in EUV. The best candidates for the formation of these radio sources are density/temperature enhancements below the K layer and magnetic flux elements. Acknowledgements. The author wishes to thank E. Valtaoja and K-L. Klein for useful discussions and helpful comments on the manuscript. Also the remarks given by the anonymous referee helped to improve the paper. Thanks are due to K. Wiik for assisting in plotting some of the diagrams. SOHO was build by an international consortium involving ESA and NASA. Yohkoh is a Japanese solar mission, with several internationally operated instruments. Part of this work was done while visiting at Tuorla Observatory, University of Turku. S.P. is supported by the Academy of Finland Contract No References Belkora L., Hurford G.J., Gary D.E., Woody D.P., 1992, ApJ 400, 692 Bromage B.I.J., Del Zanna G., DeForest C., Thompson B., Clegg J.R., 1997, ESA SP-404, 241 Doschek G.A., Warren H.P., Laming J.M., Mariska J.T., Wilhelm K., Lemaire P., Schuehle U., Moran T.G. et al., 1997, ApJ 482, L109 Dulk G., 1985, ARA&A 23, 169 Fu Q., Kundu M.R., Schmahl E.J., 1987, Solar Phys. 108, 99 Gallagher P.T., Mathioudakis M., Keenan F.P., Phillips K.J.H., Tsinganos K., 1999, ApJ 524, L133 Gomez-Gonzalez J., Barcia A., Delgado L., Planesas P., 1983, A&A 122, 219 Gopalswamy N., Shibasaki K., Thompson B.J., Gurman J., DeForest C., 1999, JGR 104, 9767 Gopalswamy N., Shibasaki K., DeForest C., Bromage B.J.I., Del Zanna G., 1998, In: Balasubramaniam K.S., Harvey J., Rabin D. (eds.) Synoptic Solar Physics. ASP Conf. Ser. 140, p. 363 Habbal S.R., Dowdy J.F. Jr., Withbroe G.L., 1990, ApJ 352, 333 Hara H., Tsuneta S., Acton L.W., Bruner M.E., Lemen J.R., Ogawara Y., et al., 1994, PASJ 46, 493 Hiei E., 1987, PASJ 39, 937 Irimajiri Y., Takano T., Nakajima H., et al., 1995, Sol. Phys. 156, 363 Kankelborg C.C., Walker II A.B.C., Hoover R.B., Barbee Jr. T.W., 1996, ApJ 466, 529 Kosugi, T., Ishiguro, M., Shibasaki, K., 1986, PASJ 38, 1 Kraus J.D., 1986, Radio Astronomy. Cygnus-Quasar Books, Ohio Kundu M.R., Liu S.-Y., 1976, Sol. Phys. 49, 267 Kundu M.R., Schmahl E.J., Fu Q.-J., 1988, ApJ 325, 905 Kundu M.R., Shibasaki K., Enome S., Nitta N., 1994, ApJ 431, L155 Lindsey C.A., Roellig T.L., 1991, ApJ 375, 414 Linsky J.L., 1973, Solar Phys. 28, 409 Nagnibeda V.G., Piotrovitch V.V., 1994, Solar Phys. 152, 175 Nindos A., Kundu M.R., White S.M., et al., 1999, ApJ 527, 415 Pohjolainen S., Portier-Fozzani F., Ragaigne D., 2000, A&AS 143, 227 Pohjolainen S., Riehokainen A., Valtaoja E., 1999, Proc. EPS 1999 Meeting, ESA-SP 448, in press Preś P., Phillips K.J.H., 1999, ApJ 510, L73 Riehokainen A., Urpo S., Valtaoja E., 1998, A&A 333, 741 Shibasaki K., 1998, In: Balasubramaniam K.S., Harvey J., Rabin D. (eds.) Synoptic Solar Physics. ASP Conf. Ser. 140, p. 373 Ulich B.L., Rhodes P.J., Davis J.H., Hollis J.M., 1980, IEEE TAP AP- 28, 367 Vernazza J.E., Avrett E.H., Loeser R., 1981, ApJS 45, 635 Webb D.F., Martin S.F., Moses D., Harvey J.W., 1993, Sol. Phys. 144, 15 Wolfson R., Roald C.B., Sturrock P.A., Lemen J., Shirts P., 2000, ApJ 529, 570 Young P.R., Klimchuk J.A., Mason H.E., 1999, A&A 350, 286 Zirin H., Baumert B.M., Hurford G.J., 1991, ApJ 370, 779