1. Birkeland Centre for Space Science, Department of Physics and Technology; University of Bergen; Bergen Norway
2. University Centre in Svalbard; Longyearbyen Norway
3. Birkeland Centre for Space Science; University Centre in Svalbard; Longyearbyen Norway
4. Department of Physics; University of Oslo; Oslo Norway
5. Istituto Nazionale di Geofisica e Vulcanologia; Rome Italy