High Altitude Observatory (HAO), National Center for Atmospheric Research (NCAR), P.O. Box 3000, Boulder, CO, 80307-3000, USA

now at: Department of Communication Systems, Lancaster University, Lancaster, UK

High Altitude Observatory (HAO), National Center for Atmospheric Research (NCAR), P.O. Box 3000, Boulder, CO, 80307-3000, USA

University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada

Los Alamos National Laboratory, Los Alamos, New Mexico, USA

Department of Communication Systems, Lancaster University, Lancaster, UK

Sodankylä Geophysical Observatory, Tähteläntie 62, 99600 Sodankylä, Finland

University of California at Berkeley, Space Science Lab, Berkeley, CA 94720, USA