Three is company

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Number: 24
1st Author: Marina Battaglia
2nd Author:
Published: 1 January 2006
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As J.R.R. Tolkien put it in his most famous book, three does not necessarily have to be a crowd, but it also means company. In this case, I am not talking about Hobbits , but about RHESSI flares featuring three hard X-ray sources.

We have done an extended "imaging spectroscopy" study on well-observed events, each having three hard X-ray sources. In all events, one source was visible at low energies (coronal source, in two events over the limb) and two sources were visible at high energies (footpoints). We analyzed the spectral time evolution of the individual sources. In this nugget I will focus on the coronal source and a most interesting finding: The coronal source shows SHS.


What stands behind this cryptic jargon? It is an abbreviation for "soft-hard-soft" and describes a feature in the time evolution of non-thermal flare spectra. It was first discovered by Parks & Winckler in 1969 and has been studied extensively since. In many flares, a hardening of the spectrum can be observed as the flux increases. A very nice picture illustrating this can be found in these earlier nuggets by Paolo Grigis and by Arnold Benz. In some cases, "soft-hard-harder" (SHH) is also observed; refer to this earlier nugget for a discussion of one such event. But back to SHS. The big question is now: Is this SHS phenomenon a feature of the particle acceleration mechanism, or is it instead caused by transport effects such as Coulomb collisions or an electric field? All previous studies have been made with full-sun spectra (no imaging), which are dominated by the footpoint emission at the relevant energies, so basically one has observed footpoint-SHS. RHESSI however provides the possibility of high-resolution imaging spectroscopy, so we can now analyze individual sources.


We analyzed five well-observed events, each showing three well separated sources, for the time interval when all sources where strong enough for reliable imaging spectroscopy. Lightcurves of one of them are shown in Fig. 1:

Figure 1: RHESSI lightcurves at low energies (3-12 keV, yellow) and high energies (25-50 keV, red) with GOES lightcurve (green). The blue lines give the analyzed time interval. An image of the event is given in Fig.2. It shows a source at low energies, well above the solar limb, and the contours of two footpoints which image only at higher energies:

Figure 2: "Clean" image of one event at energies 10-12 keV. The limb is given, as well as the contours of the footpoints at 34-38 keV.


For each of the sources we calculated a spectrum and fitted it with a thermal component at lower energies and a non-thermal power law at higher energies. The time evolution of the power-law index of the coronal source (the inverse of its spectral hardness, thus plotted upside-down) and the flux at 35 keV yields the following picture:

 Figure 3: Time evolution of spectral index (red) and non-thermal flux at 35 keV (blue) of the spectra of the coronal source. 

It is clearly notable that flux and inverse spectral index go up and down together. A very similar pattern is found for the coronal sources of the other four events. In one sentence: The coronal sources show SHS-behavior.

Physical relevance

What does this finding tell us? We believe that we have found strong evidence that SHS cannot be a transport effect. Coulomb collisions acting as a filter for low-energy electrons in the loop would not have a reactive effect on the coronal source. An electric field due to the return current, driving low-energy particles back up the loop, would even yield a softer spectrum in the coronal source. We conclude that SHS is a feature of the acceleration process itself. Biographical note: Marina Battaglia is a PhD student at ETH Zurich in the group of Arnold Benz.

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