A Splendid Limb Flare

Science Nugget: October 29, 1999


This week we obtained observations of a flare under a combination of circumstances which allowed a very nice view. The flare occurred in an active region which was passing over the west limb of the Sun. We were fortunate enough to have both Yohkoh SXT observations and excellent Yohkoh HXT (Hard X-ray Telescope) data from this event. This lucky combination of a strong flare, on the limb, is perfect for looking at loop evolution in profile, and watching the expulsion of material. It is also ideal for seeing faint hard X-ray coronal sources, if they are present. And in this event we observed an 'above-the-loop' hard X-ray source, or 'Masuda' source. The flare also showed many dynamic phenomena in SXR such as expanding loops and the ejection of material.

SXT Movies from 26-October-1999

The SXT movies of this event show some fascinating motions and loop expansions. In the quarter-resolution images (linked below), great large loops can be seen billowing outwards, finally extending beyond the field of view (i.e., beyond 640 thousand kilometers). Underneath, smaller loops can be seen roiling and moving, lighting up with hot gas. This inner region is better revealed by the half-resolution images, also linked below. Near the very center of the images, one can see the repeated expulsion of material, as one loop after another expands and appears to open up to space. Higher up, one sees the taller loops in violent motion. Is this in response to the ejection of material underneath these taller loops? Or is it a result of the changes in magnetic field connections/configurations which must be going on in the region?



Notice in the half-resolution movie the material which seems to be "re-directed" towards the south (downwards), so that it can escape towards the bottom-righthand corner. Also note the appearance of some loops to the righthand side (also in half-res, but barely discernible in quarter-res) which seem to initially retract downwards before subsequently expanding outwards. Are these merely illusions, due to limited temporal and spatial resolution? Or are these real physical motions? We suspect the latter, but it's too early to know for sure.

The 26-October-1999  `Masuda' source

First of all, what is a `Masuda' source? The `Masuda' source is named after Japanese solar physicist Satoshi Masuda who discovered the presence of blobs of hard X-ray emission formed high up in the corona, during some flares. This was a very exciting discovery for solar flare physics. To see the original Masuda source, from 1992, click here:


Hard and Soft X-ray emission

What is meant by `hard' X-ray emission? 'Hard' and 'soft' describe the energy of X-ray photons detected. The division is a bit fuzzy, but most solar physicists would describe a photon of 15keV or above as `hard', and a photon of 1keV or below as `soft' (however,  X-ray astronomers, studying accreting white dwarfs for example, happily describe a 2keV photon as `hard'). It's all a bit arbitrary, and the division in solar physics really refers to what we think is the generation process for these photons. Generally, we explain soft X-ray photons as bremsstrahlung emission produced by the rapid and continuous motions about one another of the charged particles in a hot plasma. The energy of the photon is related to the speed of the electron, which in turn is related to the temperature of the plasma. A 1 keV photon corresponds to a temperature of about 7.7 million Kelvin.

Click them image to link to NASA's solar flare theory pages, where bremsstrahlung radiation and many other aspect of flare theory are explained.

Yohkoh HXT observes photons between 14 and 93keV. Photons above 20keV, if generated by a hot plasma, would correspond to a temperature of hundreds of millions of degrees, and most of the time we don't expect these temperatures to be present. We think instead that this emission is also bremsstrahlung radiation but generated by electrons accelerated to close to the speed of light, which stream out of the flare acceleration site, through the solar plasma, emitting as they go.  So when we see hard X-ray emission, we are see where these accelerated particles are, and may be able to deduce where they came from.

Most hard X-rays are emitted from the lower regions of the atmosphere, where the density is high and the emission strong. But the Masuda sources are high up in the corona, above the location of the soft X-ray flare loops. This is just what is seen in our 26-October-99 event.

The two images above are from the early phase of the flare. The left-hand image is an overlay of the contours of the HXT Lo channel emission (14-23keV) on the SXT loops. The Lo channel emission is kind of amorphous, and mostly just maps the SXT emission. The right-hand image shows the contours of the HXT M1 channel (23-33keV) on the same SXT image and is resolved into 3 bright sources. One looks like a loop footpoint, as it is inside the limb of the Sun. Another is around the bright kernel of the SXT flare loop, but may also be another footpoint. But the furthest right source appears above the strongest SXT emission. This, we think, is a `Masuda' source. Notice that it appears near the site where blobs of material are observed to be ejected in the SXT movies. Could this be significant?

There have been many explanations for these above-the-loop HXR sources. Some people think that they are in fact caused by super-hot plasma at a temperature of 200 million Kelvin. Others think that they are generated by trapped, accelerated particles. Perhaps this flare will tell us more?

David McKenzie <mckenzie@physics.montana.edu>
Lyndsay Fletcher <fletcher@sag.lmsal.com>
October 29, 1999