Hard X-rays in Descent

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Revision as of 11:12, 1 May 2013

Electron Beams in the Chromosphere

Figure 1: An illustration of thick target model-based HXR emission in a semicircular flare loop. From top to bottom, the injected spectral index drops from 5 to 4, which results in a descent of peak emission.

The most widely-used model used to interpret solar flare hard X-rays (HXRs) is the collisional thick target model (CTTM, Brown 71, Hudson 73). In this model, a population of electrons are accelerated in the corona, and propagate down to the chromosphere where they lose their energy to collisions and also emit bremsstrahlung as HXRs.

However, there remain a number of predictions based on this model which have yet to be observed. One example is that of HXR source motion in response to variation in the spectrum of injected electrons; as a beam 'hardens', the bulk of HXR emission should descend, especially at low photon energies [fig 1]. In this nugget, we outline an attempt to make an observation of this previously unseen phenomenon.

Detection of Hard X-rays

As this effect should be most easily observed at low photon energies – where thermal X-rays usually dominate the spectrum – we need to look at a specific type of flare known as an early impulsive event (Sui et al 2003). In these events, little plasma preheating takes place, and so early on, the spectrum is dominated at almost all energies by nonthermal bremsstrahlung. One such event, previously studied by Sui et al 2006, occurred on 28 November 2002.

As shown by RHESSI observations in Fig 2, this event did exhibit unique source motion. A source appears at the apparently looptop, splits into two, and descends down both legs of the loop until they reach the footpoints, around the time of the peak in HXRs. Spectral analysis of this event tells us that over this time interval, the index of the injection spectrum also drops from ~5 to ~4, constituting the commonly-observed spectral hardening seen in the early phase of a flare (e.g., Parks 1969). It is also indicated that the spectrum contains a very strong nonthermal component, and so the images are expected to reflect the behaviour of thick target emission.

Figure 2: Top: Lightcurve of soft (3-6 keV, red) and hard (12-25 keV, blue) X-rays. Also shown is the spectral index of the injected electron distribution (dashed line). Bottom: RHESSI images of 3-6 keV emission, taken at the time indicated by vertical bars in the lightcurve.

Explanation

Figure 3: Three phase loop lifetime shown by plots of plasma temperature, X-ray emission, loop width/corpulence and thermal pressure for each flare (left:23-Aug-2005, middle:14/15-Apr-2002 and right:21-May-2004). The pattern repeats for the 14/15-Apr-2002 flare due to the multiple X-ray peaks. The shaded orange bars denote each phase.

In order to understand this observed descent, a collisional model of electron beam propagation was used, very similar to that outlined in e.g. Brown et al 2002. Using this model, a comparsion was made between expected location of HXR peak emission, and to the observations made by RHESSI [Fig 3]. Because only collisions were taken into account, the only variable in this modelling process was the density model of the chromosphere. In order to explain the difference in descent rates between each photon energy, a variable scale height was required. This resulted in a density structure which was in agreement with previous studies.

Closer to an Answer

Analysis of this event has allowed us to finally verify a prediction of the CTTM which has, until now, not been observed. This observation is consistent with flare models that require a beam of electrons progpegating from the corona to the chromosphere. Alternative models exist, such as those which take into account transport of energy via Alfvén waves originating in the corona (e.g., Emslie & Sturrock 1982, Fletcher & Hudson 2008). The observations presented here can be used in future tests of these more recent theories. For a much more complete discussion, see O'Flannagain et al, A&A, accepted.

References and links in the article

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Acknowledgement.