Do slow waves trigger pulsations in two-ribbon flares? An observational search
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This new theory suggested that slow waves, excited by an initial energy release, could propagate at an angle to the magnetic field linking flare ribbons, reflect in the chromosphere and recombine in the corona at a site further down the flaring arcade, causing another energy release. This process would then repeat all the way down the arcade (see Figure 1 for a cartoon example of this behaviour). These energy releases would then lead to the pulses we observe in flare lightcurves. | This new theory suggested that slow waves, excited by an initial energy release, could propagate at an angle to the magnetic field linking flare ribbons, reflect in the chromosphere and recombine in the corona at a site further down the flaring arcade, causing another energy release. This process would then repeat all the way down the arcade (see Figure 1 for a cartoon example of this behaviour). These energy releases would then lead to the pulses we observe in flare lightcurves. | ||
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Our approach was to consider what observational signatures this kind of regime may have, and then search for these signatures using RHESSI. For this case study we selected three flares, from 2002 November 9, 2005 January 19 and 2005 August 22. RHESSI allows us to measure the movement of the hard X-ray footpoints for these flares, as we can see in Figure 2. | Our approach was to consider what observational signatures this kind of regime may have, and then search for these signatures using RHESSI. For this case study we selected three flares, from 2002 November 9, 2005 January 19 and 2005 August 22. RHESSI allows us to measure the movement of the hard X-ray footpoints for these flares, as we can see in Figure 2. | ||
Revision as of 21:29, 13 February 2012
Introduction
A recent study by Nakariakov & Zimovets (2011), which was the subject of a recent RHESSI nugget, suggested that quasi-periodic pulsations (QPP) might be explained in two-ribbon flares via slow waves. QPP are periodic-appearing variations in the emission from solar flares, and can be seen in flaring lightcurves at a variety of wavelengths, from radio waves to soft and hard X-rays. The characteristic timescale of these variations can vary from just a few seconds up to several minutes, but their root cause remains the subject of some debate.
This new theory suggested that slow waves, excited by an initial energy release, could propagate at an angle to the magnetic field linking flare ribbons, reflect in the chromosphere and recombine in the corona at a site further down the flaring arcade, causing another energy release. This process would then repeat all the way down the arcade (see Figure 1 for a cartoon example of this behaviour). These energy releases would then lead to the pulses we observe in flare lightcurves.
Our approach was to consider what observational signatures this kind of regime may have, and then search for these signatures using RHESSI. For this case study we selected three flares, from 2002 November 9, 2005 January 19 and 2005 August 22. RHESSI allows us to measure the movement of the hard X-ray footpoints for these flares, as we can see in Figure 2.
Footpoint velocities and pulse timings
The first observable we consider is the relationship between hard X-ray pulse timings and the separation of hard X-ray footpoints. If the footpoints move closer together during a flare, the travel path of the slow waves also grows shorter, thus it may be that the interval between successive hard X-ray pulses would also grow shorter. We test this by comparing the pulse interval with the footpoint separation for each event (Figure 3). For these flares, the figure indicates that there is no clear correlation between the footpoint separation and the pulse timings.
Footpoint velocities at lightcurve peaks and valleys
Our second test involves measuring the velocity of footpoints as they move along the arcade ribbons. If the slow wave mechanism were to be true, we might expect the footpoints to jump from one location to another, rather than progressing smoothly along the ribbon. As a result the velocity of the footpoints would be roughly zero while the hard X-ray pulse was occuring, but large or discontinuous in the intervening time. Figure 4 shows our measurements of the 2002 November 9 flare.
