Acceleration-region Densities

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Number: 205
1st Author: Säm Krucker
2nd Author: Marina Battaglia
Published: August 5, 2013
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RHESSI has abundantly confirmed the importance of energetic electrons in solar flares. These electrons radiate via non-thermal bremsstrahlung, as detected and imaged in the hard X-ray (HXR) spectrum. Ultimately the energy comes from a reservoir created by the intense magnetic field in the corona, typically near a magnetic active region. Flare structures reflect the dynamics of the magnetic field as it restructures itself to create the flare (and, if powerful enough, to launch a coronal mass ejection). The first clear mapping of these processes via hard X-ray imaging came from the Yohkoh satellite, which detected the often-discussed "Masuda flare" [Ref. 1]. The interesting feature of this flare was the location of the hard X-ray emission: it was from a coronal region above the hot (soft X-ray) loops that virtually define a flare nowadays, rather than in the loops themselves. This region of the corona had no identifying features, other than the hard X-ray emission, but this was so powerful that the phenomenon immediately was taken to reflect the primary energy release of the flare, and its particle acceleration.

The Masuda flare was in 1992 (SOL1992-01-13, to give its official name), and since the launch of RHESSI in 2002 we have gradually been coming to understand more about the physics involved. Clean examples of "above-the-loop-top" emission, as in the singular case of the Masuda flare, have been rare. The event discussed here (SOL2012-07-19T05:58, M7.7) is one of the best in the RHESSI database, and it had copious supporting observation - specifically, from SDO/AIA with its wonderful images of flare structures as seen in thermal emissions.


The limb flare SOL2012-07-19T05:58 shows one of the most prominent above-the-loop-top HXR sources in the entire RHESSI data base (Figure 1). The coronal source is clearly visible in the 30-80 keV band for the whole duration of the main HXR peaks (05:20-05:27 UT). We illustrate the main features of this very nice event in Figure 1: time series, image, spectrum.

Figure 1:  Left, the hard X-ray and soft X-ray light curves of SOL2012-07-19T05:58, showing the Neupert effect as expected. Middle, images as indicated; right the RHESSI hard X-ray spectra.

From the right panel of Figure 1 one can see a clear distinction between the hard X-ray spectra of the footpoint sources and the coronal source. The latter is distinctly softer, though both have spectra strongly suggesting non-thermal power laws. Note that the above-the-loop-top HXR source is a remarkable 35 arc sec above the soft X-ray loops (as seen both by RHESSI and by the Fe XXIV response in the AIA 193Å filter. This large a separation is unprecedented, observationally. Furthermore, the coronal source clearly has a greater geometrical extent than the footpoint sources do (note: the relative weakness of the southern footpoint in this case is probably due to obscuration by the limb, i.e. the southern footpoint is mostly occulted by our Earth perspective). This greater spatial extent in the coronal source is noteworthy.

Imaging Spectroscopy

To understand the relationship between the coronal and loop sources, we have used RHESSI's capability for "imaging spectroscopy." In principle RHESSI can make a monochromatic image (a classical spectroheliogram, but in hard X-rays). We show some of these in Figure 2. The region separating the coronal source and the loop appears to be hotter than the loop source itself, a possible clue to the mechanism for energy release.

Figure 2:  Hard X-ray spectroheliograms, showing the tendency of the higher temperatures (higher energies) to link the loops with the coronal source.

Needless to say, although RHESSI resolution limitations do not permit a detailed analysis of the geometry of the coronal source, it's perfectly consistent with the original interpretation of the Masuda flare: large-scale magnetic reconnection somehow results in a huge bright hard X-ray source.

Differential Emission Measure

In astrophysical spectroscopy, the brightness of a source typically scales as nenif(T) in terms of the electron and ion densities, and the plasma temperature. The multiplier, often abbreviated as n2V, is called the emission measure, and its derivative is the differential emission measure. The AIA data allow us to estimate the source distribution in temperature by determining this temperature dependence, since there are several AIA passbands each with its own temperature sensitivity. We do not discuss the analysis here in detail, because it is highly technical, but refer to the paper we're describing (Ref. [3]). However in Figure 3 we do show the beautiful AIA images in three passbands, and their Neupert-like behavior as the flare develops.

Figure 3: Sets of images from AIA showing the region of interest, plus a time profile that shows the growth of the hot, dense plasma in the region of the above-the-looptop hard X-ray source.

Our analysis constrains the densities of the non-thermal electrons (ne) and the ions (ni) independently, and we find them to be each a few times 109 cm-3 in the coronal source region. This implies that the electron distribution function in this region cannot safely be assumed to be the usual configuration of a thermalized core plus a non-thermal tail. We appear to be seeing bulk energization of all of the electrons, and thus a radically different distribution function.


At last we have found a very clear example of the Masuda phenomenon, some 20 years after its original discovery (Ref. [1]), and one for which we have a great deal of modern data. This includes RHESSI and AIA, and the combination of these datasets has allowed us to understand the energetics of this new flare much better. We have come to the conclusion that the above-the-loop-top-source, well-observed here, contains a plasma that has a tail-dominated electron distribution function, ie that the acceleration process basically has worked on all of the electrons within its accessible volume: bulk acceleration.


[1] "A loop-top hard X-ray source in a compact solar flare as evidence for magnetic reconnection"

[2] "Measurements of the Coronal Acceleration Region of a Solar Flare"

[3] "Particle Densities within the Acceleration Region of a Solar Flare"

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