1. Terahertz Technology Research Center, National Institute of Communication and Technology, Tokyo, Japan
2. Department of Electrical, Electronics and Information and Communication Engineering, Osaka University, Osaka, Japan
3. Belgian Institute for Space Aeronomy, Brussels, Belgium
4. Japan Aerospace Exploration Agency, Tsukuba, Japan
5. Max Planck Institute for Solar System Research, Göttingen, Germany
6. Department of Geophysics, Tohoku University, Miyagi, Japan