HARD X-RAY EMISSIONS along H-alpha RIBBONS

HARD X-RAY EMISSIONS ALONG Ha RIBBONS
by Jeongwoo Lee and Ju Jing

Introduction

Comparisons of hard X-ray (HXR) sources with H-alpha ribbons has become a popular topic thanks to the work of Forbes & Priest in 1984 that suggests a simple, but quantitative relation between the ribbon motion and the coronal magnetic reconnection rate. These two observables work as quite different utilities: H-alpha ribbons as mapping of the coronal magnetic reconnection geometry onto solar surface and the HXRs as particle signature of the magnetic energy release. The rationale is, therefore, that any similarity found between them may be not a mere coincidence but significant evidence for the Forbes & Priest's theory (hereafter Standard 2D model) that gives the framework to relate them. It is then ideal to make such comparisons both in time and in space. Nonetheless, studies on this topic have thus far focused on the time correlation. The main reason is that HXRs mostly appear in the form of foot-point sources (FP-HXR) and are spatially distinct from H-alpha ribbons. Meanwhile, unusual ribbon-like HXR (R-HXR) sources were found during the RHESSI observation of the 2005 May 13 flare as reported in a previous nugget (#40). In this nugget we present a result of our comparison of the HXR intensity along the ribbons with the local energy release rate predicted by the Standard 2D model for the interest of checking the applicability of the 2D model to a more realistic 3D situation.

R-HXR and H-alpha Ribbons

Figure 1 shows the RHESSI images (contours) on top of inverse H-alpha images from the Optical Solar Patrol Network (OSPAN). The RHESSI images are created at 25-50 keV using CLEAN for six 1-min time intervals from 16:39:34 - 16:45:34 UT. Two of them are shown here, that correspond to before and after the flare maximum, respectively. Until the time of flare maximum, we typically see the discrete FP-HXR sources (left panel) and, after the maximum phase, the R-HXR source (right panel). The symbol connected lines are our manual read-outs of the H-alpha ribbon axis. The curve consists of 201 points and we mark every 20th point with the cross symbol. The distance indices j selectively denoted here are used for the display in Figure 2.

Fig.1. The RHESSI 25-50 keV Hard X-ray sources (contours) overlaid on OSPAN inverse H-alpha images (grayscale). The left panel shows the FP-HXR sources before the flare maximum and the right, the R-HXR source after the maximum phase. The symbol connected lines trace out the ribbon axis and are used to read out the local HXR intensity and magnetic field strengths from a co-aligned MDI magnetogram.

R-HXR and Magnetic Fields

According to the Standard 2D model and later refining works, the local magnetic reconnection rate is given by uB and the energy release rate is proportional to uB^2 where u and B are local ribbon velocity and magnetic field, respectively. Since the ribbon in this event moves more or less uniform in all parts, it is unlikely that the local velocity is an important factor in the spatial correlations. We thus plot, in Figure 2, B and B^2 as proxies for the local reconnection rate and for the energy release rate (green and blue curves, respectively), against the HXR 25-50 keV intensity (red curve), as functions of the ribbon distance index j as defined in Figure 1. In the early phase (panels a-c) there are two FP-HXR sources located in regions with strong magnetic field. After the flare maximum phase (panel c), the elongated R-HXR sources show up, lacking spatial correlation with the magnetic field (panels d-f). The spatial correlation between the FP-HXR intensity and B or B^2 shown in the left panels can be explained by the Standard 2D model. But the broad R-HXR intensity distributions in the right panels lack of such correlation, and apparently evidence against the model prediction.

Fig. 2. The spatial distribution of HXR intensity (red) in comparison with that of magnetic field (green) and square of the magnetic field (blue), shown as functions of the ribbon distance index defined in Fig. 1. Panels (a) to (f) correspond to the six 1-min time intervals from 16:39:34 to16:45:34 UT in which the 25-50 keV RHESSI images were created.

What does the R-HXR source tell us about?

In an attempt to judge whether the R-HXR source implies a shortcoming of the Standard 2D model or still reconciles to it, we played with several scenarios shown in Figure 3. Scenario A depicts the Standard 2D model in which the 3D magnetic arcade is a stack of 2D structures which are independent of each other. The ribbon sections with stronger magnetic field are connected to the coronal parts with higher energy release rate to become brighter HXR sources that are locally confined. This is why FP-HXR sources are expected under the Standard 2D model.

B and C are alternative scenarios brought up here to explain the R-HXR. In Scenario B we advocate the 2D model at cost of introducing another mechanism for particle redistribution along the arcade axis. Scenario C denies the 2D model by allowing an intrinsic change of the magnetic field in the third dimension that is missing from the 2D model. Scenario B may result as, for instance, the electrons are diffusively transported to spread out over the skirts of the arcade via the process of fanning out of the electrons along the separatrix or any such actions caused by acceleration mechanisms. Scenario C will be realized if the coronal magnetic field evolves from a distribution B(s) to another B'(s) as a result of magnetic reconnection (e.g. sigmoid to arcade transformation proposed by Sterling et al. 2000) so that the local energy release rate in the corona redistributes along the X-line. The renewed coronal energy release distribution may not exactly reproduce itself in the photosphere because the field lines are still converging to the strong fields down there, but can smooth the FP-HXR out to some extent to produce the R-HXR.

We'd like to vote for scenario C, because it requires an uncommon type of magnetic reconnection such as the sigmoid to arcade transformation seen in this event, whereas scenario B can occur in general, leaving it a puzzle why the R-HXR is so rare.

Fig. 3. Scenario A is the Standard 2D model that explains FP-HXR. Scenario B advocates the 2D model at cost of introducing another mechanism for particle redistribution along the arcade axis to explain the R-HXR. Scenario C denies the 2D model by regarding the R-HXR as indicating the intrinsic magnetic field change in the corona along the dimension unspecified by the 2D model.

Conclusion

The R-HXR phenomenon found from the RHESSI imaging of the 2005 May 13 flare could have naively been welcomed as a supporting feature of the standard 2D model. A quantitative consideration on the local energy release rate, however, suggests that the R-HXR, in fact, hardly reconciles to the 2D model even at a moderate degree of magnetic field variation within the ribbon area. Any difficulties in explaining the R-HXR can, of course, be useful in understanding why FP-HXRs are so common in nature. On the other hand, the R-HXR observation made us re-think what is missing in the Standard 2D model. We often regard the magnetic energy release as the most important consequence of magnetic reconnection, and this is what the 2D model is designed to do. However, the role of magnetic reconnection is not only in energy release but in magnetic flux exchange leading to a magnetic field re-distribution. The energy release is obvious in observation but the latter is not. Maybe, the R-HXR is telling us about the re-organization of the coronal magnetic fields along the third dimension that is absent in the Standard 2D model.

Jeongwoo Lee and Ju Jing work as research scientists in New Jersey Institute of Technology.