Halo CME Mail

Date: Tue, 25 Sep 2001 11:07

From: Gareth Lawrence

Subject: Full Halo CME on 2001/09/23, most probably backsided

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LASCO observed a full halo CME on 2001/09/23. The event was first
observed in C2 at 20:30 UT as a bright ragged front over the W limb
with fainter extensions over the N and S. The CME is arguably a full
halo event at this time but it is difficult to discern its front
in the E and SE due to other activity in those areas. Certainly
by 21:30 UT this front completely surrounded the C2 occulting disk.
The front first appeared in C3 at 21:18 UT with full coverage of
the C3 occultor by 21:42 UT.

Around this time both EIT and GOES show minimal activity. There
is small transient brightening in AR 9622, located N09W43, and
GOES records a H-alpha flare but not an X-ray event. The EIT
event is visible at 20:48 and the GOES event peaks at 20:47,
but the leading edge of the CME is well into the C2 field of
view by 20:30 plus there is evidence of a small slow front in
C2 data that is consistent with a small transient. We thus
identify this CME as backsided. In addition, if this event
were in fact frontsided, it would be overtaken by the much
faster full halo CME associated with the X2.6 X-ray flare
and subsequent proton event, which left the sun some 14 hours
later.

For reference, images and movies of this event are available at:

ftp://ares.nrl.navy.mil/pub/lasco/halo/20010923

Apologies for the delay in announcing this alert, which is
due in part to staff sickness and also to problems with data
retrieval over the weekend.

Best wishes,
        Gareth Lawrence.

++
| Gareth Lawrence,                   E-mail:grl@kreutz.nascom.nasa.gov |
| SOHO-LASCO Operations Scientist,                                     |
| CUA, Mail Code 682.3,                                                |
| NASA Goddard Space Flight Center,      Phone:  +1-301-286-2941       |
| Building 26, Room G-1,                         +1-301-286-3447       |
| Greenbelt, MD 20771.                   Fax:    +1-301-286-0264       |
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