1. Institut für Theoretische Physik, Lehrstuhl IV: Weltraum- & Astrophysik, Ruhr-Universität, Bochum, Germany
2. Institute for Physical Science & Technology, University of Maryland, College Park, Maryland 20742, USA
3. School of Space Research, Kyung Hee University, Yongin-Si, Gyeonggi-Do 446-701, South Korea