Subject: SOHO/LASCO Partial HALO CME 110921 BACKSIDED UCMEO 93001 10921 1630/ 10921 62212 80454 2112/ 050// 323// 31019 ///// ///// ///// ///// ///// ///// 99999 PLAIN SOHO/LASCO observed a Partial HALO CME on September 21, 2011. Event is first seen in C2 at 22:12 UT as a loop over the NW. The front expands quickly. Material spans PA 211-50 degrees for a total coverage of 199 degrees. The event continues into the C3 field beginning 23:30 UT (first frame 8.2 Rsun) to 04:54 UT. Time/height measurements were marked to 5 points in C2 with speeds of 876 Km/sec at position angle 303 degrees. The event is marked to 9 points in C3 where it accelerates. Speeds average through both fields to 1019.7 Km/sec at PA 306 degrees. Acceleration of 3.96 m/sec^ STEREO Becon data has data gaps: COR2 B gap 17:55-23:25 UT. Shows a CME in progress, full halo. COR2 A gap 18:54-01:54 UT. Shows a cme in progress, full halo. EUVI 195 B gap 18:06-23:16 UT. No evidence. EUVI 195 A gap 19:05-01:54 UT. No evidence. SDO/AIA does not show any evidence of the event on disk. These events are most probably the cause of the HALO event seen in LASCO. The observation of this event is considered 'FAIR'. This CME is determined a BACKSIDED event. Images and movies and time/height information for this event is available on the web at: http://umbra.nascom.nasa.gov/lasco/observations/halo/2011/110921/ Best regards, Kevin Schenk Solar Heliospheric Observatory EIT and LASCO Operations NASA-Goddard Space Flight Center, USA BT