A sigmoid comes through!

Science Nugget: July 2, 1999


For this week's nugget we're going to explore the details of a sigmoid. Common knowledge now suggests these are loaded cannons, ready to go off at a moment's notice on the unsuspecting villagers.

Q: Did it?
A: This time, yes!

The two  image son the left show the sigmoid, better known as NOAA Active Region #8602, shortly before the energetic impulsive phase of the flare (which occurred during spacecraft night) and then again during the decay phase. The image at left is full resolution and the image at right is half-resolution. The images have been cropped approximately to the TRACE field of view.

In the GOES plot to the right, we can see (if we click the thumbnail image) the Yohkoh flare avoidance system (or Murphy's law), unfortunately, in operation -- the peak of the flare occurs during spacecraft night!  However, this doesn't prevent us from looking at the flare shortly before dusk and shortly after dawn to see what's up.

Specifically, let's see what TRACE and the MDI on the SOHO spacecraft were looking at in this same time frame. TRACE was in fact following this active region and its connections to the region to the west (to the right of these images), so the cadence is pretty slow. TRACE was able to continuously observe this region for the entire duration of the flare, but the results are underwhelming: TRACE didn't see anything.

Let us not discard TRACE so quickly, however. With nominally 25 full-resolution pixels to every one in SXT, we can certainly see a lot more of nothing with TRACE than we ever could with SXT. So, on the assumption that a lot of data is better than no data at all, may we present the following TRACE data dump on AR8602: white light, 1660 Å UV continuum, the Fe IX line at 171 Å, Fe XII at 195 Å and finally the Fe XV line at 284 Å. These images are ordered left to right in increasing temperature, from a modest 6000 Kelvin at the left to a ballpark of 2 million Kelvin at the far right.

Not to be left out is the measurement of the photospheric magnetic field with MDI, seen in the magnetic field image on the right, just below.  This "magnetogram" measures the line-of-sight strength of the magnetic field in the lower solar atmosphere, underneath this event. Black is one polarity, white the other, and gray is somewhere in between:

The sum is greater than the parts...

So, given all this data, what can we do? Knowing the location of the underlying magnetic field is extremely useful for understanding why parts of the image are bright, so a first cut is often to overlay contours of the magnetic field on the various images and see how they compare. For brevity, we've only done two of those here, but this is probably sufficient for our purposes:

(We've fiddled with the contour color here to get them to show up against the colored backgrounds.) It's clear that the sunspot to the west (at the right of the field of view) is a main player in this active region. The bright vertical structure (running north-south in the field of view) appears to be somehow related to the location of the inversion line, the fine line drawn between the two opposing polarities. This is nothing particularly new, but is pretty obvious in this flare. In the spirit of keeping this short, we'll only have a look at one more overlay: SXT brightness contours on the TRACE Fe IX image:

This overlay show quite clearly that TRACE and SXT aren't generally looking at the same plasma: we know that SXT is looking at much hotter, brighter material, while TRACE tends to look lower in the atmosphere at relatively cooler material. To really take this flare down -- for example, to determine why one part is so hot, while the other is relatively cool -- we'd need to spend some time looking at some movies of the various datasets. Doing this, we could look for events in one temperature/wavelenght regime, and try to find analogues in the others. A prime example might be to look at the footpoints of the hot SXT loops in the ultraviolet and white light seen by TRACE and MDI, respectively.   When we observe a flare in SXT, what happens at the footpoints, and when?   Good material for future research ...

July 2, 1999: Brian Handy