1. GFZ German Research Centre for Geosciences, Section 2.3 Helmholtz Centre Potsdam Potsdam Germany
2. Sodankylä Geophysical Observatory University of Oulu Oulu Finland
3. Space Physics and Astronomy Research Unit University of Oulu Oulu Finland
4. Institute of Geology and Geophysics Chinese Academy of Sciences Beijing China
5. College of Earth and Planetary Sciences University of Chinese Academy of Sciences Beijing China
6. GFZ German Research Centre for Geosciences, Section 2.7 Helmholtz Centre Potsdam Potsdam Germany
7. Institute of Physics and Astronomy University of Potsdam Potsdam Germany
8. Department of Earth, Planetary, and Space Sciences University of California Los Angeles CA USA