SOLAR CYCLE VARIATION OF MICROWAVE POLAR BRIGHTENING AND EUV CORONAL HOLE OBSERVED BY NOBEYAMA RADIOHELIOGRAPH

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1 Journal of the Korean Astronomical Society 5: , 217 August pissn: eissn: X c 217. The Korean Astronomical Society. All rights reserved. SOLAR CYCLE VARIATION OF MICROWAVE POLAR BRIGHTENING AND EUV CORONAL HOLE OBSERVED BY NOBEYAMA RADIOHELIOGRAPH AND SDO/AIA Sujin Kim 1,2, Jong-Yeop Park 1,3, and Yeon-Han Kim 1,2 1 Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 3455, Korea; sjkim@kasi.re.kr 2 University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Korea 3 School of Space Research, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin, Gyeonggi-do 1714, Korea Received December 6, 216; accepted July 25, 217 Abstract: We investigate the solar cycle variation of microwave and extreme ultraviolet (EUV) intensity in latitude to compare microwave polar brightening (MPB) with the EUV polar coronal hole (CH). For this study, we used the full-sun images observed in 17 GHz of the Nobeyama Radioheliograph from 1992 July to 216 November and in two EUV channels of the Atmospheric Imaging Assembly (AIA) 193 Å and 171 Å on the Solar Dynamics Observatory (SDO) from 211 January to 216 November. As a result, we found that the polar intensity in EUV is anti-correlated with the polar intensity in microwave. Since the depression of EUV intensity in the pole is mostly owing to the CH appearance and continuation there, the anti-correlation in the intensity implies the intimate association between the polar CH and the MPB. Considering the report of Gopalswamy et al. (1999) that the enhanced microwave brightness in the CH is seen above the enhanced photospheric magnetic field, we suggest that the pole area during the solar minimum has a stronger magnetic field than the quiet sun level and such a strong field in the pole results in the formation of the polar CH. The emission mechanism of the MPB and the physical link with the polar CH are not still fully understood. It is necessary to investigate the MPB using high resolution microwave imaging data, which can be obtained by the high performance large-array radio observatories such as the ALMA project. Key words: Sun: activity Sun: corona 1. INTRODUCTION The near-earth space environment is affected by the solar activities such as flares, coronal mass ejections (CMEs), and fast solar winds. These activities depend on the phase of the solar cycle: near the maximum, the strength and the occurrence frequency of flares and CMEs increases and coronal holes which are the source of the fast solar wind forms at all latitudes. Whereas, during the minimum, the frequency of flares and CMEs decreases and coronal hole tend to form at near the poles. There are several reports on various phenomena depending on the phase of the solar cycle, such as polar magnetic fields, polar crown filaments, surges near the poles, faculae, and so on (Selhorst et al. 23; Svalgaard et al. 25; Gopalswamy et al. 212; Shimojo 213; Svalgaard & Kamide 213; Altrock 214; Silva et al. 216). For instance, the polar field reversal and flux migration to the poles are considered an indicator of the following solar maximum (Svalgaard & Kamide 213) which is consistent with the dynamo model (Babcock 1961). The Nobeyama Radioheliograph(NoRH, Nakajima et al. 1994) has taken full-sun images at 17 GHz from 1992 July to present. It gives a unique chance to look into the solar cycle variation of radio brightness temperatures at all heliographic latitudes over 24 years. Corresponding author: S. Kim Shibasaki (1998) presented, for the first time, the microwave butterfly diagram using NoRH 17 GHz synoptic map and pointed out the appearance of polar brightening and its gradual expansion to lower latitudes during the declining phase of the solar cycle 23. Gopalswamy et al. (212) reported that the microwave polar brightening (hereafter MPB) is consistent with the strength of the polar magnetic field. They also found the tendency that the strength of the MPB and polar magnetic field during the solar minimum is related tothe strengthofthe followingmaximum. Onthe other hand, Gopalswamy et al. (1999) found that the microwave bright feature appears on the enhanced unipolar area inside coronal holes in the mid-latitudes. Those results imply that the MPB can be a good proxy for the following maximum and the photospheric magnetic fields below the MPB. However, the emission mechanism of the MPB and its physical association with the photospheric magnetic field is still not clear. In this paper, we present the microwave and the Extreme UltraViolet (EUV) intensity variation with latitude using full-sun images obtained by NoRH 17 GHz and Solar Dynamics Observatory/Atmospheric Imaging Assembly (AIA) 193 Å and 171 Å channels. Selhorst et al. (21) found negative correlation between the MPB in NoRH 17 GHz and the polar EUV intensity in SOHO/EIT 171 Å and concluded that it 125

2 126 Kim et al Latitude Figure 1. Microwave butterfly diagram constructed by NoRH 17 GHz full disk images. A colorbar in the top is the color index of brightness temperature (T b ) was due to the depression by EUV polar coronal holes. It was demonstrated by Silva et al. (216) using high resolution 171 Å data obtained by the SDO/AIA. Since the AIA 193 Å channel, which is one of the seven EUV channels in the AIA, is sensitive to temperatures higher than the AIA 171 Å channel, CHs in the AIA 193 Å have higher contrast than those in the AIA 171 Å. The results of both channels are compared to find clear correlation between the MPB and the CHs. This also provides a chance to look into the difference in latitudinal distribution of the CH between two EUV channels. 2. DATA We have used the 17 GHz NoRH daily images taken at 3: UT (12: JST, noon of Japan) from 1992 July to 216 November. The data was retrieved from the NoRH public server ( nagoya-u.ac.jp/iccon) that provides fits data every 1 minutes. After sorting out the bad quality data for rain/snow or faulty operation, we collected a total of 84 images. We corrected following two factors for each image: the B angle which is changing of the tilt angle of the solar rotation axis to the ecliptic plane to a maximum of 7.3 degrees and the size of the solar disc which changes with the distance between the Sun and the Earth. The heliocentric coordinates of each image are converted to the 2-D synoptic map which is the plot with uniform angular width in latitude and longitude. We selected the meridional part between -2 to 2 degrees in longitude, and then extracted a brightness temperatures (T b ) for each latitude. One year average is applied to avoidthe seasonalvariationsofthe Bangle and beam-pattern (Shibasaki 213). The AIA instrument onboard the SDO has 7 EUV channels which are sensitive to the temperatures from the upper chromosphere to the corona (Lemen et al. 212). We selected the two channels, 193 Å and 171 Å, which has the temperature response peak at around 1 MK and.6 MK, respectively. We used a total of 1962 daily images for each channel obtained from 211 May to 216 November. The data was retrieved from the Korean Data Center for SDO (KDC, We processed each image with the same process described for NoRH data above. Consequently, we obtained the microwave and the EUV butterfly diagrams which are the intensity maps in latitude as a function of time (Figures 1 and 3). 3. RESULTS 3.1. Microwave Butterfly Diagram Figure 1 is the microwave butterfly diagram constructed from the brightnessmap ofnorh 17 GHz. It showsthe Tb distribution over latitude for 24 years from the minimum between the SC22 and the SC23 to the SC24. The brightness in the mid-latitude (between S4 and N4 degrees) is attributed to the active regions that radiate the gyro-resonance emission by the strong magnetic field of sunspots and the thermal free-free emission in the chromosphere and the corona (Shibasaki 1998). On the other hand, the MPB near the pole (above S5 and N5) is pronounced during the minimum between the SC22 and the SC23 and between the SC23 and the SC24. The MPB appearance in the north and the south was asymmetric in that the MPB in the south pole is stronger than and precede that in the north pole (Gopalswamy et al. 1999, 212). In order to trace brightness variation over the poles

3 Solar Cycle Variation of Polar Brightening and Coronal Hole GHz Pole and Mid-Latitute NoRH 17GHz Tb Latitude SC22 SC23 SC Figure 2. Mean values of T b at the poles (S7 S82 and N7 N82, upper curve) and at mid-latitudes (S45 N45, bottom curve). and the mid-latitudes respectively, we plot the mean values of T b in both areas in Figure 2. In the figure, one can find that: (1) the MPB and the T b in the mid-latitude show anti-correlation, (2) the peak of midlatitude T b in the maximum phase follows the minimum of polar T b, (3) the level of the polar T b in SC22/23 is higher than that in SC23/24, (4) the level of midlatitude Tb in SC23 maximum is higher than that in SC24. The results (3) and (4) imply that the polar brightness reflects the maximum level of solar activity of the following cycle (Gopalswamy et al. 212) 3.2. Coronal Hole and Polar Brightening AIA 193 AIA 171 The butterfly diagrams of the microwave, the AIA 193 Å, and the AIA 171 Å are displayed in Figure 3. The intensity variation pattern in the mid-latitudes is similar at all wavelengths, while the intensities in both poles is anti-correlated between microwave and EUVs. The EUV intensity depression appears in both poles and it is clearer in the AIA 193 Å than in the AIA 171 Å. Figure 4 shows the normalised intensity for the north pole (top) and the south pole (bottom). In the north pole, the microwave brightness increases continuously from 215 August and the EUV intensity drops steeply at the same time. In the south pole, the microwave brightness increases continuously from 214 January and the EUV intensity drops steeply from 213 November in the AIA 193 Å and from 214 July in the AIA 171 Å. Even though there is an asymmetry between the south and the north pole in intensity variation, it is clear that the intensities between microwave and EUVs are anticorrelated at each pole: the brightness increases in microwave when it decreases in EUV of 193 Å and 171 Å channels. Such EUV intensity depression reflects the presence of the coronal hole, which is due to the low density of the coronal plasma. -5 Figure 3. Butterfly diagrams of NoRH 17 GHz, AIA 193 Å, and AIA 171 Å channels (Top to bottom) from 211 May to 216 November. Dashed lines indicate latitudes of 7 and SUMMARY AND DISCUSSION We have examined microwave and EUV butterfly diagrams to investigate the cycle variation of polar brightness in each channel. As a result, we found that the MPB coincides with the intensity depression in the AIA 193 Å and 171 Å. There was a time difference between the MPB and EUV intensity drop in the south pole, but it was slight in the AIA 193 Å. In fact, both channels of the AIA are able to detect coronal holes, but the contrast of a coronal hole in the AIA 193 Å channel is higher than that of the AIA 171 Å. It is because the AIA 193 Åchannel is sensitive to a higher temperature

4 128 Kim et al. Normalized intensity Normalized intensity AIA 171 AIA 193 NoRH 17 GHz North Pole(N7 - N82) South Pole(S7 - S82) Figure 4. Normalized intensity for north pole (N7 N82) and south pole (S7 S82) estimated from butterfly diagrams of NoRH 17 GHz, AIA 193 Å, and AIA 171 Å channels in Figure 3. than that of the AIA 171 Å, so the relatively the cold and low density of a CH area are vivid in the AIA 193 Å. The magnetic field of the CHs is not strong enough to generate the microwave emission by gyroresonance of electrons (Dulk 1985). Thus, the thermal bremsstrahlung could be the most probable radiation mechanism of the microwave in the CHs. Brajsa et al. (27) calculated the microwave brightness assuming the thermal bremsstrahlung as the emission mechanism, and they found that the CH brightness in microwave is dimmer than the quiet sun level. On the other hand, Kosugi et al. (1986) found that the observed equatorial CHs had higher temperatures than the quiet sun level. Gopalswamy et al. (1999) also reported the microwave enhancements in the equatorial CH. However, such brightness was not uniform and was associated with the enhanced unipolar field pattern of the photosphere below the CH. Therefore, it seems clear that the MPB is intimately associated with the magnetic field strength in the pole, and the magnetic field strength mayplayaroletoenhancethe microwavethermalemission above the photosphere. Moreover, Ito et al. (21) reported that the total magnetic flux of the polar area is larger than that of the quiet sun in September 27. They used Hinode/SOT data obtained in the north pole which coincides with the MPB in Figure 1. Meanwhile, EUV butterfly diagrams revealed the intensity enhancement in the pole when the MPB is weakened. If it is related to the increase of the coronal plasma density in the pole, the physical link among the weak photospheric magnetic field (Gopalswamy et al. 212), the disappearance of the MPB, and EUV polar brightening would suggest a clue to solve the MPB mechanism. Even though the microwave polar brightening has been watched over 24 years thanks to NoRH continuous observation, still unknown is the emission mechanism of this phenomenon. Considering its cycle variation and close relationship with cyclic phenomena, it seems certain that the polar brightening is crucial information to judge the following solar maximum. Hence, it is important to keep the NoRH in operation till the next solar cycle to investigate the physical source of the polar brightening with high performance data of the next generation of instruments. ACKNOWLEDGMENTS Nobeyama Radioheliograph is operated by the International Consortium for the Continued Operation of Nobeyama Radioheliograph (ICCON). ICCON consists of ISEE/Nagoya University, NAOC, KASI, NICT, and GSFC/NASA. The SDO data were(partly) provided by the Korea Data Center (KDC) for SDO in cooperation with NASA, which is supported by the Development of Korea Space Weather Research Center project of the Korea Astronomy and Space Science Institute (KASI). This work is partly supported by the development of models for analyzing solar images and for predicting long-term solar activities, a project of Korean Space Weather Center of Radio Research Agency (RRA), and the KASI basic research fund. REFERENCES Altrock, R. 214, Forecasting the Maxima of Solar Cycle 24 with Coronal Fe xiv Emission, Sol. Phys., 289, 623 Babcock, H. W. 1961, The Topology of the Sun s Magnetic Field and the 22- Cycle, ApJ, 133, 572 Brajsa, R., Benz, A. O., Temmer, M., Jurdana-Sepic, R., Saina, B., & Wohl, H. 27, An Interpretation of the Coronal Holes Visibility in the Millimeter Wavelength Range, Sol. Phys., 245, 167 Dulk, A. G. 1985, Radio emission from the sun and stars, ARAA, 23, 169 Gopalswamy, N., Shibaski, K., Thompson, B. J., Gurman, J., & DeForest, C. 1999, Microwave Enhancement and Variability in the Elephant S Trunk Coronal Hole: Comparison with SOHO Observations, JGR, 14, 9767 Gopalswamy, N., Yashiro, S., Makela, P., Michalek, G., Shibasaki, K., & Hathaway, D. H. 212, Behavior of Solar Cycles 23 and 24 Revealed by Microwave Observasions, ApJL, 75, L42 Ito, H., Tsuneta, S., Shiota, D., Tokumaru, M., & Fujiki, K. 21, Is the Polar Region Different from the Quiet Region of the Sun, ApJ, 719, 131 Karachik, N. V., & Pevtsov, A. A. 214, Properties of Magnetic Neutral Line Gradients and Formation of Filament, Sol. Phys., 289, 821 Kosugi, T., Ishiguro, M., & Shibasaki, K. 1986, Polar-Cap and Coronal-Hole-Associated Brightenings of the Sun at Millimeter Wavelengths, PASJ, 38, 1

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