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Презентация PowerPoint - Nobeyama Solar Radio Observatory

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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 AlfvГ©n waves.
The contrast of coronal holes (CH) in radio emission
depends on the observing frequency. At пЃ¬ > 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 п‚» 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 demonstrate our conclusions with consideration
we conclude that:
of a coronal hole in the southern hemisphere that
1. Microwave observations confirm that coronal holes are
passed across the solar disk during April 14–28,
inhomogeneous (cf., e.g., Chertok et al. 2002). The
1998. It is best to study it in detail during April 20–
inhomogeneities have various sizes and shapes. Some of
them are identified with features observed in other
24, when it was close to the central meridian. To
emissions, and some not.
enhance the contrast of the coronal hole, we have
2. There are features within a hole and its vicinity whose
averaged solar images observed with SSRT, NoRH,
brightness temperatures at both frequencies correlate: a)
and SOHO/EIT in 195 Г… channel (FeXII coronal
coronal bright points and diffuse brightenings, which are
line) during April 20–23 (one image per day), with
bright at both 5.7 and 17 GHz; b) filaments which are dark
their preliminary �derotation’ to the same time of
at both 5.7 and 17 GHz.
April 22, 06:00 UT (see figure). The contour of the
3. For the first time, we have found regions within coronal
holes where, unlike features listed above, the brightness
hole determined from the EIT image is superimposed
temperatures at 5.7 and 17 GHz anticorrelate. These regions
over SSRT and NoRH images. Average brightness
are darkest at 5.7 GHz, but are not pronounced in either 195
temperatures over the hole are <T5.7> = 15,000 K
Г…, or HпЃЎ images
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 Г… channel overlaid with contours of brightness temperatures in SSRT (b) and NoRH (d) images.
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.
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 AlfvГ©n 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
AlfvГ©n waves.
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 AlfvГ©n waves.
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 SB RAS.
17 GHz
17 GHz
Contour levels in Fig. b: [12, 13,
14, 15, 16]пѓ—103 K (TQS 5.7 = 16пѓ—103
K dashed);
in Fig. d: [8-12]пѓ—103 K, step 500 K
(TQS 17 = 104 K dashed)
103 K
103 K
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.
5.7 GHz
On the next days, April 21 and 22, the overall
shape of the coronal hole and a horseshoeshaped 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.
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.п‚ў27, 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.
5.7 GHz
17 GHz
Contour levels in Fig. b: [10, 13,
15, 16]пѓ—103 K (TQS 5.7 = 16пѓ—103 K
dashed);
in Fig. d: [8-12,5]пѓ—103 K, step 500
K (TQS 17 = 104 K dashed)
103 K
Contour levels in Fig. b: [12, 13, 14,
15, 16]пѓ—103 K (TQS 5.7 = 16пѓ—103 K
dashed);
in Fig. d: [8-12,5]пѓ—103 K, step 500 K
(TQS 17 = 104 K dashed)
103 K
17 GHz
5.7 GHz
103 K
On April 20, three large (> 50п‚І) and several small (~ 25п‚І) 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 РќпЃЎ images. Next, after the plage, the cross
section meets the hole again, where anticorrelation is well
pronounced within a small dark patch.
103 K
5.7 GHz
References
Chertok I.M., Obridko V.N., Mogilevsky E.I. et al. 2002, ApJ, 567(2), 1225.
Gopalswamy N., Shibasaki K., DeForest C.E. et al. 1998, in ASP Conf. Ser. 140, Synoptic Solar Physics,
ed. K.S. Balasubramanian, J.W. Harvey & D.M. Rabin San Francisco: ASP), 363
Hollweg J.V., Jackson S., Galloway D. 1982, Sol. Phys., 75, 35
Hollweg J.V. & Johnson W. 1988, J. Geophys. Res., 93, 9547
Krissinel B.B. Kuznetsova S.M., Maksimov V. P. et al. 2000, PASJ, 52, 909.
Maksimov V.P., Prosovetsky D.V., Krissinel B.B. 2001, Astron. Letters, 27, 181
Moore R.L., Musielak Z.E., Suess S.T., An C.-H. 1991, in Mechanisms of Chromospheric and Coronal
Heating, Eds. P. Ulmschneider, E.R. Priest, R. Rosner (Springer-Verlag), 435
Nakariakov V.M., Ofman L., Arber T.D. 2000, A@A, 353, 741
Nindos A., Kundu M.R., White S.M. et al. 1999, ApJ, 527, 415
Zirker J.B. 1993, Sol. Phys., 148, 43
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