1. Department of Physics, National Taiwan Normal University , No. 88, Section 4, Tingzhou Road, Taipei 116 , Taiwan, R. O. C.
2. Centre of Astronomy and Gravitation, National Taiwan Normal University , No. 88, Section 4, Tingzhou Road, Taipei 116 , Taiwan, R. O. C.
3. Mullard Space Science Laboratory, University College London ; Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK