1. Department of Physics; Royal Military College of Canada; PO Box 17000; Station ‘Forces’; Kingston; Ontario K7K 7B4; Canada
2. Department of Astronomy and Space Physics; Uppsala University; 751 20; Uppsala; Sweden
3. Armagh Observatory; College Hill; Armagh; BT61 9DG; Northern Ireland
4. Department of Physics, Engineering Physics and Astronomy; Queen's University; Kingston; Ontario; K7L 3N6; Canada