1. The Kyoto University Museum Kyoto University Yoshida‐honmachi Sakyo Kyoto 606‐8501 Japan
2. National Institute of Polar Research 10‐3 Midori‐cho Tachikawa Tokyo 190‐8518 Japan
3. General Systems Studies Graduate School of Arts and Science The University of Tokyo 3‐8‐1 Komaba Meguro‐ku Tokyo 153‐8902 Japan
4. Department of Physics Graduate School of Science The University of Tokyo 7‐3‐1 Hongo Bunkyo‐ku Tokyo 113‐8654 Japan