• The Bulletin of The Korean Astronomical Society
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• v.36 no.2
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• pp.102.2-102.2
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• 2011

Sunspots usually appear in a group which can be classified by certain morphological criteria. In this study we examine the moments which are statistical parameters computed by summing over every pixels of contours, in order to quantify the morphological characteristics of a sunspot group. The moments can be additional characteristics to the sunspot group classification such as McIntosh classification. We are developing a program for image processing, detection of contours and computation of the moments using continuum images from SOHO/MDI. We apply the program to count the sunspot numbers from 303 continuum images in 2003. The sunspot numbers obtained by the program are compared with those by SIDC. The comparison shows that they have a good correlation (r=89%). We are extending this application to automatic sunspot classification (e.g., McIntosh classification) and flare forecasting.

• The Bulletin of The Korean Astronomical Society
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• v.34 no.1
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• pp.153.1-153.1
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• 2009

• Journal of The Korean Astronomical Society
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• v.38 no.2
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• pp.295-298
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• 2005

Recently a big progress has been made on the measurements of magnetic helicity of solar active regions based on photospheric magnetograms . In this paper, we present the details of Chae's method of determining the rate of helicity transfer using line-of-sight magnetograms such as taken by SORO /MDI. The method is specifically applied to full-disk magnetograms that are routinely taken at 96-minute cadence.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.60.2-61
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• 2017

A Convolutional Neural Network(CNN) is one of the well-known deep-learning methods in image processing and computer vision area. In this study, we apply CNN to two kinds of flare forecasting models: flare classification and occurrence. For this, we consider several pre-trained models (e.g., AlexNet, GoogLeNet, and ResNet) and customize them by changing several options such as the number of layers, activation function, and optimizer. Our inputs are the same number of SOHO)/MDI images for each flare class (None, C, M and X) at 00:00 UT from Jan 1996 to Dec 2010 (total 1600 images). Outputs are the results of daily flare forecasting for flare class and occurrence. We build, train, and test the models on TensorFlow, which is well-known machine learning software library developed by Google. Our major results from this study are as follows. First, most of the models have accuracies more than 0.7. Second, ResNet developed by Microsoft has the best accuracies : 0.77 for flare classification and 0.83 for flare occurrence. Third, the accuracies of these models vary greatly with changing parameters. We discuss several possibilities to improve the models.

• Journal of Astronomy and Space Sciences
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• v.34 no.2
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• pp.99-103
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• 2017

The sun is not equally bright over the whole sphere, but rather is darkened toward the limb. This effect is well-known as limb darkening. The limb darkening coefficient is defined by the ratio of the center intensity to limb intensity. In this study, we calculate the limb darkening coefficient using the photospheric intensity estimated from solar images taken by solar and helispheric observatory (SOHO) and solar dynamics observatory (SDO). The photospheric intensity data cover almost two solar cycles from May 1996 to December 2016. The limb darkening coefficient for a size of 0.9 diameter is about 0.69 and this value is consistent with solar limb darkening. The limb darkening coefficient estimated from SOHO shows a temporal increase at solar maximum and a gradual increase since the solar minimum of 2008. The limb darkening coefficient estimated from SDO shows a constant value of about 0.65 and a decreasing trend since 2014. The increase in the coefficient reflects the effect of weakened solar activity. However, the decrease since 2014 is caused by the aging effect.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.2
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• pp.67.2-67.2
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• 2016

The Sun has diverse variations in solar atmosphere's layers due to solar activity. This solar variations can be recognized easily by sunspots which appear on the solar photosphere. Thus the sunspot on the photosphere is utilized by direct index of the solar activity. The other variation of the photosphere is center-to-limb variation (CLV). In this study, we analyze the relative intensity observed by SOHO, SDO. The data of photospheric intensity are from full disk images of SOHO/MDI intensity ($6768{\AA}$, from May 1994 to March 2011) and of SDO/HMI intensity ($6173-6174{\AA}$, from May 2010 to June 2016). As the result, we found the latitudinal variation of the intensity. The daily photospheric intensity showed the solar cyclic variation with sunspot number. It has a little difference of phase with sunspot number.

• The Bulletin of The Korean Astronomical Society
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• v.26 no.1
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• pp.34.2-34.2
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• 2001

• The Bulletin of The Korean Astronomical Society
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• v.35 no.1
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• pp.27.2-27.2
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• 2010

We have studied the formation of coronal holes (CHs) associated with halo CMEs. For this study, we used multi-wavelength data from Yohkoh Soft X-ray Telescope (SXT), GOES Soft X-ray Imager (SXI), SOHO EIT 195 ${\AA}$, SOHO MDI magnetogram, MLSO He I 10830 ${\AA}$, and BBSO H-alpha. The CHs are characterized by open magentic field regions with low emission, density, and temperature and their open fields drive high speed solar winds which cause geomagnetic storms. So far, the formation and the evolution of CHs are not well understood. The formation of the dark region associated with the eruption of a CME is well known as "coronal dimming" which may be caused by the mass depletion near the CME footpoint. It is different from a typical CH since it persists for only one or two days. In this study, we present three cases that show the formation of coronal holes which are associated with three halo CMEs: 1) 2000 Jul 14, 2) 2003 Oct 28, 3) 2005 May 13. In the first case, hot plasma was ejected during a weak eruption and then filled out the pre-existing CH. After the halo CME occurred, the hot plasma region becomes a CH again. In the second and the third cases, we found newly formed CHs just after their associated CMEs. All three coronal holes are associated with strong flares and persist over 3 days until they disappeared by the solar rotation. Examining the MDI magnetograms, we found that the magnetic polarity of each CH region has one polarity. Based on these results, we suggest that the coronal holes can be formed by the CMEs and they should be distinguished from the coronal dimming.

• Journal of The Korean Astronomical Society
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• v.50 no.4
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• pp.105-109
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• 2017

The well-known solar cycle controls almost the entire appearance of the solar photosphere. We therefore presume that the continuous emission of visible light from the solar surface follows the solar cyclic variation. In this study, we examine the solar cyclic variation of photospheric brightness in the visible range using solar images taken by the Solar and Heliospheric Observatory (SOHO)/Michelson Doppler Imager (MDI). The photospheric brightness in the visible range is quantified via the relative intensity acquired from in the raw solar images. In contrast to total solar irradiance, the relative intensity is out of phase with the solar cycle. During the solar minimum of solar cycles 23-24, the relative intensity shows enhanced heliolatitudinal asymmetry due to a positive asymmetry of the sunspot number. This result can be explained by the strength of the solar magnetic field that controls the strength of convection, implying that the emission in the visible range is controlled by the strength of convection. This agrees with the photospheric brightness increasing during a period of long spotless days.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.114.2-114.2
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• 2012

태양 광구표면에서 시선방향 자기장 자료를 살펴보면 그 극성이 변하는 지점들이 선의 형태로 보이는데 이것을 Magnetic Polarity Inversion Line(MPIL) 혹은 Neutral Line이라 부른다. 기존의 연구에 의하면 태양활동영역에서 MPIL의 길이가 길수록 플레어 및 코로나물질방출(CME)과 같은 큰 규모의 분출현상들이 일어나는 빈도가 높다는 사실이 보고된 바 있다. 이런 점에서 볼 때 MPIL이 우주환경 예보의 측면에서 중요한 도구가 될 수 있을 것으로 기대된다. 하지만 여전히 MPIL의 기하학적, 물리적 특성 및 그 형성과 진화과정에 대한 이해가 부족한 상황이다. 우리는 본 연구에서 SOHO/MDI 시선방향 자기장 자료를 사용하여 태양활동 23주기에 나타난 308개의 태양활동영역에 대하여 MPIL의 길이, 곡률과 같은 기하학적인 특성을 연구하였고, 또한 MPIL주변의 자기장(평균 자기선속, 총 자기선속 등) 및 magnetic fragment들의 속도장(평균속력, 수렴 및 발산정도, vorticity 등)과 같은 물리적인 특성에 대한 통계적 조사를 수행하였다.