S-shaped active regions and the prediction of CMEs

Science Nugget: May 07, 1999


This week we're discussing something that happened last week, because it's still pretty exciting: a sigmoidal (S-shaped) active region produced an M-class flare, and a halo CME! Readers of previous Weekly Nuggets will be aware this is a topic about which we are enthusiastic.


In the present case, the shape of the active region in question was noticed by Ennio R. Sanchez, who wrote to the SXT Team and inquired about the likelihood of eruption. Here is a link to Dr. Sanchez's email, and below is an image showing the appearance of the region.



...and Observation

The real-time operators of Yohkoh, at Kagoshima Space Center, sent the following two emails about this region: they noticed the sigmoidal structure in the region, and within the next two hours they reported the flare eruption. The plot below shows the GOES light curve for this event.



The SXT images show impressive enhancement to the X-ray emission from the eastern (left) end of the region. However, no clear evidence is found for waves moving away from the region, for blobs ejected from the flare, or for the creation of cusped loops or arcade structures. We are reminded of previous nuggets discussing the link between eruptive activity and visible signatures thereof (more to the point, the lack of visible signatures thereof): 19-March-99, 26-March-99, 02-April-99, and 16-April-99. The X-ray light curve does not seem to be consistent with a "Long-Duration Event" (or alternatively, "long-decay event"), which is the usual X-ray signature of a coronal mass ejection. The lack of obvious X-ray signatures from an event clearly associated with a CME is a curiosity worth some serious thought.

Simon Plunkett, at NASA Goddard Space Flight Center, reports that the EIT and LASCO instruments on SOHO observed the event, and that LASCO imaged a halo CME. Linked here are Dr. Plunkett's email and a collection of movies from EIT and LASCO:

EIT movie in 195 Angstroms wavelength (72 kbyte)
 LASCO movie, C2 field of view (1.7 Mbyte)
LASCO movie, C3 field of view (428 kbyte)

For more discussion of various aspects of S-shaped active regions and their link to eruptive activity, check out the following entries of SXT Weekly Notes: 24-October-97, 10-April-98, 12-June-98, 19-June-98, 09-October-98, and 23-October-98. There's some further information on a set of webpages at Montana State University.

May 07, 1999: David McKenzie