1. School of Mathematics and Physics, Aberystwyth University , Ceredigion SY23 3BZ, UK
2. School of Mathematics and Statistics, University of Glasgow , Glasgow G12 8QQ, UK
3. Institute for Space-Earth Environmental Research, Nagoya University , Nagoya 464-8601, Japan
4. School of Earth and Space Sciences, University of Science and Technology of China , Hefei 230026, China
5. Department of Mathematical Sciences, Durham University , Durham DH1 3LE, UK
6. High Altitude Observatory , NCAR, 3080 Center Green Drive, Boulder, CO 80301, USA