V.P. Maksimov, D.V. Prosovetsky, V.V. Grechnev, B.B. Krissinel

1. On the relation of brightness temperatures in coronal holes at 5.7 and 17 GHz V.P. Maksimov, D.V. Prosovetsky, V.V. Grechnev, B.B. Krissinel Institute of Solar-Terrestrial Physics, Irkutsk, Russia K. Shibasaki Nobeyama Radio Observatory, Japan Abstract. From the analysis of simultaneous observations with the SSRT and NoRH we show that coronal holes are not uniform. In particular, in coronal holes small-scale features exist with anticorrelating brightness temperatures at 5.7 and 17 GHz. The features are disposed radially, which suggests radial heat transfer in them. We propose that the favorable heating mechanism within those features is dissipation of Alfvén waves. The contrast of coronal holes (CH) in radio emission depends on the observing frequency. At  > 8 cm, coronal holes have a high contrast and are well detectable against the background of the quiet Sun (QS) as areas of decreased brightness temperature. In the high-frequency domain, the contrast of coronal holes decreases, and the whole hole or some its parts can be either brighter (TCH > TQS), or darker than the quiet Sun (TCH < TQS), or indistinguishable from it (TCH  TQS). Based on simple model calculations, Krissinel et al. (2000) supposed the contrast diversity to be due to variations of the electron density and temperature. However, the frequency dependence of the contrast of coronal holes has not been particularly addressed in the literature. This is why simultaneous microwave observations at two remote frequencies, typical of the opposite contrast of coronal holes, with the Siberian Solar Radio Telescope (SSRT: 5.7 GHz, TCH < TQS) and the Nobeyama Radioheliograph (NoRH: 17 GHz, TCH > TQS) are promising for studies of coronal holes. Having studied several coronal holes at the two frequencies, we conclude that: 1. Microwave observations confirm that coronal holes are inhomogeneous (cf., e.g., Chertok et al. 2002). The inhomogeneities have various sizes and shapes. Some of them are identified with features observed in other emissions, and some not. 2. There are features within a hole and its vicinity whose brightness temperatures at both frequencies correlate: a) coronal bright points and diffuse brightenings, which are bright at both 5.7 and 17 GHz; b) filaments which are dark at both 5.7 and 17 GHz. 3. For the first time, we have found regions within coronal holes where, unlike features listed above, the brightness temperatures at 5.7 and 17 GHz anticorrelate. These regions are darkest at 5.7 GHz, but are not pronounced in either 195 Å, or H images We demonstrate our conclusions with consideration of a coronal hole in the southern hemisphere that passed across the solar disk during April 14–28, 1998. It is best to study it in detail during April 20–24, when it was close to the central meridian. To enhance the contrast of the coronal hole, we have averaged solar images observed with SSRT, NoRH, and SOHO/EIT in 195 Å channel (FeXII coronal line) during April 20–23 (one image per day), with their preliminary ‘derotation’ to the same time of April 22, 06:00 UT (see figure). The contour of the hole determined from the EIT image is superimposed over SSRT and NoRH images. Average brightness temperatures over the hole are <T5.7> = 15,000 K and <T17> = 11,000 K. Figures bellow show microwave images of the hole at 5.7 GHz (a) and 17 GHz (c) as well as in FeXII 195 Å channel overlaid with contours of brightness temperatures in SSRT (b) and NoRH (d) images. 5.7 GHz 5.7 GHz 5.7 GHz On April 20, three large (> 50) and several small (~ 25) dark patches with a decreased brightness temperature are seen within the hole at 5.7 GHz. To all but one of these patches there correspond bright patches at 17 GHz, although their shapes and locations do not coincide perfectly with the dark areas at 5.7 GHz. To estimate these features quantitatively, we plot in Fig. e (upper 5.7 GHz, lower 17 GHz) an E–W cross section of the hole including its vicinity (thin horizontal line in Fig. a–d). The figure shows a sharp decrease of T5.7 at the eastern edge of the hole. There is a dark patch with T5.7 = 14,600 K near this edge, to which there corresponds an enhancement up to 12,000 K at 17 GHz. In this region, the darker is the hole at 5.7 GHz, the brighter at 17 GHz it is. Just after this region, there is a diffuse brightening visible in EUV and at both radio frequencies. Further, 5.7 GHz image shows a plage region with T5.7 = 16,800 K beyond the hole followed by a decrease of TB at both frequencies up to T5.7 = 11,200 K and T17 = 7,400 K due to a dark filament well visible in EUV and Н images. Next, after the plage, the cross section meets the hole again, where anticorrelation is well pronounced within a small dark patch. This figure also shows the relation of T5.7 and T17 for a dark patch in the hole (f) and a portion of the filament (g). The presence of two curves (f) is due to asymmetry of the temperature distribution at both frequencies, i.e. sharper change at the left edge and more gradual change at the right one. Furthermore, we selected the portions referring to 5.7 GHz images, whereas 17 GHz features are smaller. The figures show the difference in the behavior of the brightness temperatures within the hole and filament. 17 GHz 17 GHz 17 GHz Contour levels in Fig. b: [12, 13, 14, 15, 16]103 K (TQS 5.7= 16103 K dashed); in Fig. d: [8-12]103 K, step 500 K (TQS 17= 104 K dashed) 103 K 103 K 103 K Contour levels in Fig. b: [12, 13, 14, 15, 16]103 K (TQS 5.7= 16103 K dashed); in Fig. d: [8-12,5]103 K, step 500 K (TQS 17= 104 K dashed) On the next days, April 21 and 22, the overall shape of the coronal hole and a horseshoe-shaped filament northward of the hole underwent minor changes in EUV emission only. Positions of small-scale inhomogeneities in 5.7 GHz images significantly changed. However, the relation of T5.7 and T17 shows the same features as on April 20. That is, the darkest regions at 5.7 GHz have the brightest counterparts at 17 GHz, and the brightness temperature variations at these frequencies anticorrelate. 103 K 5.7 GHz The relation between the brightness temperatures measured at 5.7 and 17 GHz is well fitted by linear regression equations T17 – T17 QS = –A (T5.7 – T5.7 QS) with A = (T17 max – T17 QS)/ (T5.7 QS – T5.7min) > 0. Here T17max and T5.7min are peak brightness temperatures achieved within the coronal hole at the corresponding frequencies. Note that always T17max  T17QS +1500 K < T5.7min T5.7QS – 3000 K: the deficiency of the coronal material in a hole appreciable at 5.7 GHz is not sufficient to decrease the brightness temperature down to its value at 17 GHz. Note also that the correlation coefficients calculated separately for inclined parts of the anticorrelating features exceed 0.85 by the absolute value. 17 GHz Discussion To explain the enhanced brightness of coronal holes at 17 GHz (at the chromospheric level), considered were various mechanisms of additional heating, developed previously to explain the nature of coronal bright points (e.g., Gopalswamy et al. 1998). However, none of those heating mechanisms at the chromospheric level provides simultaneous cooling in the corona. More appropriate seem heating mechanisms associated with excitation and dissipation of Alfvén waves. However, they meet some problems due to difficulties of their excitation and dissipation (see, e.g., Zirker 1993 and references therein). Many papers considered how to overcome them (see, e.g., Hollweg et al. 1982; Hollweg & Johnson 1988; Moore et al. 1991; Nakariakov et al. 2000). In summary, existence of features where the brightness temperatures at 5.7 and 17 GHz anticorrelate and heat seems to be transferred radially suggests that the favorable heating mechanism in those features is dissipation of Alfvén waves. On April 23, the hole becomes more contrast at 5.7 GHz and resembles the shape of the hole in EUV better. The brightness temperature distribution is more complex than on the previous days. In the central part of the hole, T5.7 < 14,000 K, so, one could expect anticorrelation of T5.7 and T17, but it is not well pronounced. However, if the 5.7 GHz curve is shifted to the right by 0.27, then good anticorrelation appears. The reason for that is not a coalignment problem, since the diffuse brightening region shows perfect coincidence (Fig. e), but in height difference of the features observed at 5.7 and 17 GHz. That is, the coronal hole’s feature observed at 5.7 GHz is higher than that one observed at 17 GHz, and its projection is closer to the limb. Hence, those features are disposed radially. Contour levels in Fig. b: [10, 13, 15, 16]103 K (TQS 5.7= 16103 K dashed); in Fig. d: [8-12,5]103 K, step 500 K (TQS 17= 104 K dashed) 103 K 103 K Conclusion 1. Microwave observations confirm that coronal holes are inhomogeneous. The inhomogeneities have various sizes and shapes. Some of them are identified with features observed in other emissions, and some not. 2. The features within the hole and its vicinity that show correlation of their brightness temperatures: a) coronal bright points and diffuse brightenings, which are bright at both 5.7 and 17 GHz; b) filaments which are dark at both 5.7 and 17 GHz. 3. For the first time, we have found regions within the coronal hole where, unlike features listed above, the brightness temperatures at 5.7 and 17 GHz anticorrelate. Those features are disposed radially. 4. Favorable heating mechanism in those features is dissipation of Alfvén waves. 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Phys., 148, 43 Acknowledgments This study is supported by the Russian Ministry of Education and Science under grant NSh-477.2003.2, the Federal Program ‘Astronomy’ and Lavrentiev's young scientist grant of SBRAS.