1. Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 305-701, South Korea
2. National Fusion Research Institute, Daejeon 305-333, South Korea
3. NASA Goddard Space Flight Center, Code 674, Greenbelt, Maryland 20770, USA
4. Korea Astronomy and Space Science Institute, Daejeon 305-348, South Korea
5. Department of Astronomy and Space Science, Chungbuk National University, Cheongju 361-763, South Korea