A Knotty Flare

Science Nugget: March 10, 2000


In the absence of anything particularly spectacular in this week's observations with Yohkoh, we will do a little preparation for next week's big event. Next week Yohkoh will participate in Max Millenium Observing Campaign number 6, the prime goal of which is to secure observations of flare signatures coming from low down in the solar atmosphere. While SXT most often shows us the hot coronal loops formed during and after solar flares, other instruments observing at different wavelengths, are particularly good at illustrating where and how the solar chromosphere and transition region responds to the onslaught of heat and particles generated by a flare. This in turn may help us to identify what parts of the overall flaring structure are the first to be disrupted, and where the major energy release takes place.

A Good Example from the Archives: Jun 22 1999

On Jun 22 1999, Yohkoh SXT and TRACE were both observing a flaring active region, and by great good fortune, TRACE was making high-cadence images in the Lyman alpha line (1216A). Lyman alpha emission, from neutral hydrogen, comes from the chromosphere, where the temperature is 10,000 - 20,000K.

The flare was a complicated-looking beast. The Lyman alpha observations at the maximum of the flare impulsive phase (18:21 - 18:23 UT) show two bright knots of emission and a flare ribbon

These same features, notably the two bright knots, show up in the SXT images at the same time, which is a bit weird perhaps, since we are used to SXT showing us large fuzzy loops. (K)Not in this case! What SXT might be showing here is the location of strong heating of the lower atmosphere, maybe by particle beams. Or perhaps the knots are in fact two very tiny hot loops?


Relationship to Hard X-ray emission

We think that we understand that hard X-ray (HXR) emission, as detected by the Hard X-ray Telescope (HXT) on Yohkoh, is generated by high energy electrons, accelerated in the flare, when they hit the the `brick wall' of the chromosphere. So the location of HXR emission tells us where these electron beams arrive, and we can possibly also deduce where they came from.

(click on image for a full-size version)

We find that the HXR emission is again concentrated into two bright knots, and overlaying contours of HXR emission on top of a Lyman alpha image from the same time, we find that these line up with the two bright Lyman knots. This suggests very strongly that the Lyman alpha emission has something to do with electron beams - maybe electron beams heat up a cooler part of the atmosphere so that it radiates in Lyman alpha, or maybe they collisionally excite hydrogen atoms.

So is flare electron acceleration taking place somewhere in the corona in between these two knots? Are these the only places that accelerated electrons are reaching the chromosphere? Could it be that more of the Lyman alpha emission is caused by electron beams, in which case the TRACE images might tell us more about flare particle acceleration than we can see from HXT images alone? Questions, questions...

The following image, courtesy of Harry Warren (who alerted the author to this event), shows how the HXR light curve and the Lyman alpha light curves compare.

(click on the thumbnail for a full-size image.)

Although the time resolution of the Lyman alpha curve is not very good, we can see some interesing things. The Lyman alpha and HXR emission peaks at roughly the same time, and the little bumps in the Lyman alpha light curve and in the HXR light curves line up fairly well. But the Lyman alpha emission has started to rise before the HXRs really get going. So it's possible that the Lyman alpha emission is caused by something other than electron beam bombardment, or maybe early in the flare the bombardment just isn't strong enough to generate visible hard X-rays.

What will next week's observations add to this?

Next we will be running TRACE sequences designed to look at the chromospheric signatures of flares at a much higher cadence than the observations shown here - we estimate we can make TRACE images once every 4 seconds. This will enable us to make a better comparison to the time evolution of the HXR and soft X-ray signatures. Furthermore, we will be searching for another type of chromospheric signal - H alpha linear polarization, which is thought to be generated by low energy protons accelerated in flares. The role of protons, in flares is little understood, because the low energy ones have few detectable signatures (H alpha linear polarization is thought to be one of them) and the radiation from the high energy ones has not yet been imaged. After next week we hope we will be able to say a little more about protons, electrons, where they are accelerated and how many of each there are, in at least one flare.


March 11, 2000
L. Fletcher (fletcher@sag.lmsal.com)