Controlled Explosions

Science Nugget: March 17, 2000

Introduction The target of this week's observations, and of Max Millenium Flare Campaign #6, was NOAA active region 8906. Towards the end of the week AR 8906 rewarded our patience with an M1.4 flare, and was also the site of many C class events. SXT observations suggest that the activity  involved a small number of linked or interacting loops. The flares which occurred were by and large, not eruptive, and seemed  to involve only one or two small loops.

The flaring region

SXT observations of AR 8906 show a tangle of small loops which brightened intermittently and in small numbers - one or two at a time. Some of these brightenings qualified as flares. The image below shows the AR at a number of different times through the week.  (Note, the images are made by summing one hundred AlMg filter exposures for each panel. Each panel is centered on the same heliographic position, showing how different loops are bright at different times, but always in the AR core.)

(click on this image for a full-size version)

What is causing the loops to flicker on and off? Is it the same process which drives the flares?

Two Flares

We have high cadence observations of two flares, which occurred on March 13th and 15th. In both these events, all of the observing wavelengths we have examined so far (soft and hard X-rays from Yohkoh, and 1600 A UV continuum from TRACE) have shown that activity is very localized. The flares are confined one or two loops, and didn't noticably affect the rest of the active region.  There was little evidence of eruptivity (possibly a faint cusp in one event), loop filling, or any of the other large-scale stuff we often associate with flares.  So though the flares are fairly energetic, something is keeping the process firmly under control.

For example, the comparison on the left of the HXT Lo Channel and the SXT Be channel in the flare on March 13th (C6.8)  below show how these wavelengths line up, while the mosaic on the right shows how the HXT emission in the higher channels lines up (each row is a single time interval, and the 4 columns are Lo, M1, M2 and Hi). It's all happening in those two bright blobs (note, the whole field of view is about 70,000km x 70,000km),


A TRACE 1600 A movie of the event is available here

The flare on March 15th was larger (M1.4), but still occurred entirely within a small SXT loop. However, within the loop there is a lot of activity. The brighest pixels in HXT Lo channel images move around, as can be seen in this movie (which shows HXT Lo channel on the left and SXT Be on the right).  The TRACE 1600 movie also shows bright blobs moving around a loop, though of course TRACE sees a  loop which looks very much more complicated than that seen by SXT. Perhaps this is the clue - there is enough structure and complexity in the magnetic field at a small scale to cause a flare, or loop brightenings, and this all happens in isolation from the overall field of the active region.

The image below on the left shows the TRACE 1600A emission at several times overlaid with contours of  HXT Lo channel emission, coincident in time to within half a second  (apologies for the quality of the TRACE images - couldn't quite work out how to deal with the saturated pixels!).  On the right is the light curve of the 1600A emission, compared with the HXT emission in the three lower energy channels.

Hmmm. Perhaps one could convince oneself that the brightest TRACE 1600A emission comes from near the brightest HXT emission, and that a couple of the spikes in the HXT channels line up with small bumps in the 1600A channels? It is hard to say. Previous studies of UV and X-ray emission in flares had indicated that they might both be a result of the same process of bombardment of the atmosphere by high energy particle beams, but this set of observations may caution against hanging onto such preconceptions.

An Explanation?

The lack of large scale response to these flares is kind of interesting. Perhaps this is because they occur right in the core of the active region, explosive activity is being confined by overlying magnetic fields. Or perhaps they are caused by relatively simple loop-loop interactions   (see the Great Flare Cartoon Archive)  which result in a transfer of current, local heating and reconfiguration of the field, but no filament eruption and no involvement of field outside the two interacting loops.

March 18, 2000
L. Fletcher (
H.Hudson (