Murphy's Law? No, Beginner's Luck

Science Nugget: October 22, 1999

Introduction

Most of this week, Yohkoh SXT has been carrying out special observations designed for a Yohkoh/TRACE/SoHO/GBO JOP.  The purpose of the JOP is to make high spatial resolution, high time-cadence observations of a solar flare, with emphasis on the flare impulsive phase. So far, in the operating lifetime of TRACE, any simultaneous flare observations by TRACE and SXT  have just been by chance, so this program is an attempt to observe flares in a co-ordinated program.

(jargon: TRACE = Transition Region and Coronal Explorer, SoHO = Solar and Heliospheric Observatory, GBO = Ground-Based Observatories, JOP = Joint Observing Program)

Murphy's Law

Now, it is not possible to predict reliably when and where a flare will occur, though there are some things which help. For example, flares are likely to happen where there are large and complex sunspot groups, and since flares are usually associated with the ejection of filament material it is also a good idea to look for big filaments. However, it really is a guessing-game. The best that we could do was to pick a suitable active region, sit on it a few days, keep our fingers crossed, and hope that Murphy's Law wouldn't mean that a flare would go off somewhere else entirely, or when the spacecraft was in night. Or maybe nothing at all would happen.....

This is a magnetogram of our chosen target AR complex. The region deemed `most likely to flare', called AR 8731, is the very large, strong bipole (black-white pair) in lower left of the complex

white = positive field (pointing away from the solar surface),
black = negative field (pointing towards the solar surface)

NOAA had predicted that AR 8731 might give an M-class flare. However, even though we are rapidly approaching solar maximum, and we expect that the flaring activity should be building to a peak, there have been remarkably few good flares this year (see the nugget on 6-Aug-99  for more on this). So we were a little anxious.

Delighted Solar Physicists Receive News of M-flare

'We're overjoyed' , 'Can't believe our luck'. Sagamihara, October 21st, early afternoon. SXT Chief Observers McKenzie and Fletcher were seen doing celebratory high-fives and talking animatedly  as they watched a movie of an M1.9 flare which had occured at 06:00 UT the same day, two days into the campaign.  Not only had the flare occurred in the predicted region, when both SXT and TRACE were looking at it, CO McKenzie had also decided to take advantage of an extra upload pass that day to tell SXT to go into PFI dominant mode, meaning that it got better coverage of the event than it might ordinarily have.  'In summary, we have excellent coverage of the pre-flare and impulsive phase' reported Fletcher. 'However' added McKenzie, `the gradual phase of the event was lost when the spacecraft went into night'.

A summary of the data so far can be found on the Max Millenium web site here. But to whet your appetite, by clicking on the thumbnails below you can see some SXT and TRACE  movies.

        

    Half-resolution SXT movie         TRACE movie in the 195A
    showing the formation of              filter, showing filament
    post flare arcade, plus                    lift-off, small brightenings
    some ejected material.                    - looks very different from the
    This images plasma at about          SXT movie. The bright material
    3-5 million Kelvin                        is at about 1.5 million Kelvin

So now what will happen?

Data from this event is still being collected from the different observatories, and the process of scientific analysis will soon commence. What do we hope to learn? Among other things, the various kinds of data will tell us about the triggering of flares, the ejection of material, and the distribution and evolution of temperatures during the flare. For an example of the latter, we present these last two images.

The left-hand image below is a Full-resolution SXT frame from 06:00:36 UT, during the rise phase of the flare. It faintly shows the arcade, primarily displaying the bright footpoints. In the right-hand image the intensity measured from the first image is overlaid onto a map of the temperatures in the region (actually the log of the temperature, linked below). The structures which are brightest in the SXT intensity image (contours in the overlay) appear to be cooler than some of their surroundings. Some but not all of this is due to instrumental effects (like scattering of photons within the telescope, which has not been corrected in the making of this temperature map); therefore the reader is cautioned not to believe any of the log-temperatures higher than 7.2. But notice in particular the interesting appearance of a temperature gradient in the small loop on the left-hand side of the frame (plotted at coordinates x=600, y=200 in the intensity image).


Intensity map
Temperature map,
with intensity contours

Now go back and look at the TRACE movie again -- that loop doesn't appear in the TRACE data until 15 minutes later. In fact, several of the structures which are visible in the SXT images can't be seen in the TRACE data, and vice versa. Is this a sign that the loop starts hot and then cools until it is visible in TRACE images? If so, then how quickly is it cooling? And is the loop heated impulsively just once at the beginning of the flare, or is it heated continuously for several minutes? These are only a few of the questions we hope to address. Stay tuned!

Lyndsay Fletcher <fletcher@sag.lmsal.com>
David McKenzie <mckenzie@physics.montana.edu>
October 22, 1999