1. Lockheed Martin Solar and Astrophysics Laboratory, 3251 Hanover Street, Organization ADBS, Building 252, Palo Alto, CA 94304, USA.
2. High Altitude Observatory, National Center for Atmospheric Research, Post Office Box 3000, Boulder, CO 80307, USA.
3. Institute of Theoretical Astrophysics, University of Oslo, Post Office Box 1029 Blindern, 0315 Oslo, Norway.