Three-phase life leads to corpulent X-ray loops

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Studying the spatial and temporal properties of X-ray loops

The RHESSI era has undoubtedly allowed us to resolve X-ray sources to a high degree. RHESSI cannot resolve individual loops but it does allow us to infer what the overall emitting X-ray region does, whether it consists of one or many loops. However, even during the RHESSI era, the spatial properties of coronal X-ray loops have not yet been explored extensively. The first real study of loop top spatial properties occurred in 2008 with Xu et al. (2008) which showed the lengths (the dimension parallel to the guiding magnetic field) of X-ray emitting loops increasing with energy. This is opposite to the trend observed for hard X-ray footpoints, where the sizes appear to decrease with energy. Follow up studies were performed by both Kontar et al. (2011) and Guo et al. (2012). Kontar et al. (2011) were able to show that the widths (the dimension perpendicular to the guiding field) of X-ray loops also increased with energy, an interesting discovery since electrons should not be able to move perpendicular to the guiding field. [Magnetic turbulence] was suggested as one possible explanation for this.

Figure 1:

The studies of Xu et al. (2008) and Kontar et al. (2011) concentrated on the energetic changes of loop spatial properties and were obtained through the use of Visibility Forward Fitting (VIS FWDFIT) ([1]). Unlike other imaging algorithms, VIS FWDFIT allows us to find relatively accurate position and size measurements from the moments of the X-ray visibilities by comparing them with a simple distribution such as an elliptical Gaussian or in our case a curved elliptical Gaussian, which is basically a loop. Therefore, X-ray sources in the corona that appear as a singular, simple loop shape can be fitted in this way, giving us the loop length Full Width at Half Maximum (FWHM), loop width FWHM and the loop top position (with a little bit of work! - see Jeffrey & Kontar (2013)).

Using the VIS FWDFIT method, we decided to extend the investigation of loop spatial properties with a very simple study; observe for the first time how these X-ray loops changed with time during the X-ray rise, peak and decay phases of the flare. For this we managed to find three suitable events with a simple loop shape: 23rd August 2005 from 14:22, 14/15th April 2002 from 23:58 and 21st May 2004 from 23:42.

Figure 2: Loop spatial changes for the 23-Aug-2005 (left column), 14/15-Apr-2002 (middle column) and 21-May-2004 (right column) at each observation time through the impulsive and decay stages of X-ray emission. The bottom graphs show quantitatively how both the loop lengths and widths change over these periods for each flare.

Three phase X-ray lifetime

We found that all three X-ray loops exhibited similar and interesting temporal trends in both their spatial properties and other properties such as temperature, number density and thermal pressure. Peaks in X-ray emission denoted periods where there were changes in the loop spatial dynamics and interestingly of all, a study of temperature, volume, number density and thermal pressure showed how the X-ray loops went through three phases denoted by three clear peaks: the first in temperature, the second in X-ray emission and the third in thermal pressure. Each of these phases, for each flare can be seen clearly in Figure 2. Before the peak in X-ray emission, both the loop length and corpulence fell and after the peak the loop corpulence rose again while the loop length seemed to remain approximately constant - at least for two flares that only have one clear X-ray peak. The changing shapes (corpulence and length) for each event can be seen in Figure 1. Loops start relatively long and thin and then become smaller before growing in corpulence at later stages. This figure also shows the quantitative changes in lengths and widths of each loop.

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.

Suggested explanation for our observations

Although we see the the volume of the loop decreasing before the peak in X-ray emission, the relationship between temperature and volume does not support simple compressive heating as in a collapsing magnetic trap model. The shrinking loop widths over time are much harder to explain than the decreasing loop lengths since electrons should be unable to move across the guiding field lines threading the corona. This leads to the suggestion of the field lines themselves being squeezed together during this time.

Whatever the process causing the initial temperature increase and shrinking corpulence, we do believe that thermal conduction towards the lower corona and chromosphere are mainly responsible for phases two and three of the loops life. Thermal conduction causes chromospheric evaporation and this leads to increasing the number density and thermal pressure in the loop. This would produce the decreasing loop lengths as electrons interact at shorter and shorter distances and increasing thermal pressure in the region could probably balance the shrinking process. Increasing thermal pressure would eventually cause the increasing loop corpulence.

A far more detailed study of the three events can be found in Jeffrey & Kontar (2013).

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