1. Institute for Geoscience, University of Tübingen, Sigwartstrasse 10, 72076 Tübingen, Germany, Indian Institute of Geomagnetism, New Panvel, Navi Mumbai 410218, India, National Institute of Oceanography, Dona Paula, Goa 403004, India, and Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India