[1] UNIV COLORADO,DEPT ASTROPHYS PLANETARY & ATMOSPHER SCI,BOULDER,CO 80309
[2] UNIV CAMBRIDGE,INST ASTRON,CAMBRIDGE CB3 9EW,ENGLAND
[3] UNIV CAMBRIDGE,DEPT APPL MATH & THEORET PHYS,CAMBRIDGE CB3 9EW,ENGLAND
[4] COLUMBIA UNIV,DEPT ASTRON,NEW YORK,NY 10027
[5] NASA,GODDARD INST SPACE STUDIES,NEW YORK,NY 10025
[6] NATL CTR ATMOSPHER RES,BOULDER,CO 80307