1. Department of Astronomy and Space Science, Chungbuk National University, Cheongju 361-763, South Korea
2. National Fusion Research Institute, Daejeon 305-333, South Korea
3. Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 305-701, South Korea
4. Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742, USA
5. School of Space Research, Kyung Hee University, Yongin, South Korea
6. Korea Astronomy and Space Science Institute, Daejeon 305-348, South Korea
7. Korea University of Science and Technology, Daejeon, South Korea