Weekly Notes from the Yohkoh Soft X-Ray Telescope

(Week 36, 2002)

Science Nugget: September 6, 2002

Has RHESSI observed Yohkoh jets?


The soft X-ray jets became one of Yohkoh's most celebrated discoveries, as introduced in an early nugget and many subsequent ones. As Figure 1 shows, a jet is a slender column of hot gas shooting outward from a microflare (click on the thumbnail figure to get the parts labeled):

Figure 1. Splendid example of a soft X-ray jet observed by the Yohkoh soft X-ray telescope (SXT). Click on the image to see the components identified. The jets are sudden spurts of hot plasma emanating from the outside of a bright loop, not necessarily visible prior to the event.

The jets became even more interesting when they were identified with type III radio bursts by Aurass, Martens, Raulin, Kundu and others. This discovery may have resolved a long-standing question - namely, how could the type III burst radiation actually escape from the Sun? Without explaining in detail, the answer would be clear if a high-density duct existed, within which high-energy electron streams propagated outwards. The jets fitted that bill because of the correlations found with type III bursts.

RHESSI microflares

Now, the question naturally arises: we know that type III bursts require streams of high-energy electrons. If they then propagate in over-dense ducts (e.g., the soft X-ray jets) might we not observe them directly in hard X-rays? Here "hard" refers to photon energies above 10 keV, which would be bremsstrahlung directly emitted by the electrons themselves. We should see spikes of hard X-rays shooting outwards from the if so.

The plots below demonstrate that RHESSI may actually have done this.

Figure 2. Plots prepared by S. Krucker and S. Christe of UC Berkeley, showing tiny RHESSI hard X-ray events associated with type III radio bursts. From the top, the two GOES soft X-ray energy channels; a panel showing RHESSI hard X-ray counting rates at different energies (the vertical lines show the extent of solar viewing); a "dynamic spectrum" representation of RHESSI data, and a dynamic spectrum representation of WIND/WAVES radio data. The bottom two panels show a nice correlation between the radio-detected type III burst at 14:30 UT and a RHESSI hard X-ray burst.

This is all very complicated but the radio aficionadoes will recognize the classic "fast drift" structure of the radio bursts towards lower frequencies. This means outward motion from the Sun. At the same time the hard X-ray fans will see hard X-ray emission extending up to some 30 keV for a trivial soft X-ray event, evidence that this is not an ordinary flare or microflare.

While it is only a speculation as yet, it seems highly likely that a soft X-ray image of the source at 14:30 UT shown above would have been a soft X-ray jet, and that the hard X-rays would show a similar structure. Let us take this only as a prediction for the moment; it will soon be checked because (as one can see from the plot) such events may be numerous.


This science nugget tries to link Yohkoh observations (old) with RHESSI observations (new). We have already been doing this in earlier nuggets and will probably do more of it. RHESSI gives us unique new ways to see old things (such as the jets), and of course lets us see new things as well. For physical interpretation of the phenomena it is always best to have multiple kinds of observations (multiple wavelengths and/or particles) to work with. We will come back to the subject of type III bursts and hard X-rays in the context of direct particle observation at 1 A.U. in some future nugget, most likely.

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September 15, 2002

Hugh Hudson (hhudson@ssl.berkeley.edu), with thanks to S. Christe and S. Krucker.