Elementary Flares

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Number: 6
1st Author: Arnold Benz
2nd Author:
Published: 4 August 2005
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As experienced prospectors know, nuggets are often found in close proximity to one another. Thus, in the same flare containing the nugget presented by Paolo Grigis, we found another nugget. Flare energy release may occur in one single process or in many portions and fragments. The latter idea is generally credited to De Jager and De Jonge (1978), who noticed that the hard X-ray light curve often shows many peaks and who proposed that a flare may consist of many flare elements. The idea was enthusiastically taken up by radio astronomers, well aware of multiplicity in the flare fireworks. Contrary to an ordinary firecracker, though, the X-ray evidence now suggests that even the main energy release is not a simple explosion.

Theoretical expectations

The modulation of the X-ray emission has been interpreted along two lines, hypothesizing magnetic reconnection as an energy-release mechanism: (i) A reconnecting current sheet may oscillate between different states of resistivity: If the resistivity increases and becomes "anomalous," reconnection goes faster and the size of the current sheet expands. As the sheet expands, the current density diminishes and resistivity becomes normal, reducing the reconnection rate and the size of the current sheet. The current sheet thus may oscillate between the two states. (ii) The reconnection may also be fragmented into "magnetic islands" produced by the "tearing mode" instability of reconnection. Magnetic islands may reconnect when coalescing into larger elements. Every secondary reconnection of two islands would produce a peak in the hard X-ray light curve.

Both scenarios above can be described in a two-dimensional geometry. The first scenario predicts that the footpoints move apart with a modulated velocity. In the second scenario, there is no footpoint motion as all islands are located in the same reconnection channel, defined by the primary reconnection.

Plenty of peaks

The hard X-ray lightcurve of the flare of Nov. 9, 2002 (picture above), the independence day of Cambodia. The flare is composed of more than a dozen subpeaks. Thus it is ideal to test the above ideas.

Footpoints do not show elementary motions

The figure below shows the position of the footpoints in a coordinate system along (i.e. parallel) and perpendicular to the each of the two ribbons in which they were found. There is a clear motion parallel to both ribbons, and a meandering around zero in perpendicular direction. The same color code of the subpeaks used in both figures helps to follow the course of time. The footpoints move surprisingly independent of the peaks in the lightcurve. In particular, there is no outward motion associated with the subpeaks. The footpoints just move in parallel direction as if they had no relation to the variations of the HXR flux. A more detailed analysis using the average path (black curves in figures below) as the zero lines shows the same result.

Farewell to elementary flares?

The observations of this example contradict the prediction of the pulsating current sheet scenario. The second scenario (magnetic islands) suffers from its 2D concept: The most energetic place of reconnection moves along the ribbons in the third dimension.

The flare of November 9, 2002, suggests a tether cutting scenario, where the main energy is released in the cutting of magnetic tethers. The hard X-ray flux would then be modulated according to the strength of the tethers. It is still possible that in a later phase the footpoints may move apart. However, the released energy then may be too small to show the corresponding motion of the footpoints. A dominant fraction goes in the first instance.

Biographical note Arnold Benz is a professor of astrophysics at the Swiss Institute of Technology (ETH) in Zurich.

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