Implosion and Oscillation

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== The Flare ==
== The Flare ==
-
[[File:204f0.png|center|thumb|600px|Figure 1: Left - an EUV image of the flare, essentially from above, showing the photospheric magnetic field as colored contours (two polarities), plus the line of a slit used to make plots of position vs. time.
+
Figure 1 gives an overview of the flare as an image and as a "time-slice" plot of position vs time along a selected line.
 +
The latter shows the major features of the time development of the flare.
 +
The appearances are consistent with an early slow inflation, a sudden implosion, the coronal dimming resulting from the
 +
launch of the [http://en.wikipedia.org/wiki/Coronal_mass_ejection CME],
 +
the excitation of oscillations in the visible loop structures, and finally the damping-out of these oscillations.
 +
 
 +
[[File:204f0.png|center|800px|Figure 1: Left - an EUV image of the flare, essentially from above, showing the photospheric magnetic field as colored contours (two polarities), plus the line of a slit used to make plots of position vs. time.
These are in the middle and right plots on different time scales.
These are in the middle and right plots on different time scales.
The CME-related dimming is very obvious, as are the contracting and oscillating loops.  
The CME-related dimming is very obvious, as are the contracting and oscillating loops.  
Line 34: Line 40:
Note the different periods for the different loops.
Note the different periods for the different loops.
]]
]]
 +
 +
Note that the clearest oscillatory features occur on rather large spatial scales, and show parts of the corona that do not get blown away by the CME motions; this means that they occupy different volumes of the full 3D structure, suggesting that the motions detected are not uniformly radial as the
 +
[http://solarmuri.ssl.berkeley.edu/~hhudson/cartoons/ cartoons] often imply.
 +
Note also that the onsets of the oscillatory motions lag at greater distances.
 +
This is an important finding because it implies that the excitation arises in the core of the flare, and the wave of excitation propagates outwards with a (projected) speed estimated at about 300 km/s.
 +
Figure~2 shows the close relationship in time between the implosion and the dimming.
 +
 +
[[File:204f2.png|center|600px|Figure 2: Time series.
 +
]]
 +
 +
It would be natural to think of this as a rough estimate of the local
 +
[http://en.wikipedia.org/wiki/Alfvén_wave Alfv&eacute}n speed], hence bearing information about local physical properties.
 +
But we are interested here mainly in the global dynamics of the phenomena.
 +
One item of interest lies in the distribution of
 +
[http://en.wikipedia.org/wiki/Normal_mode eigenmodes (normal modes)] of the postflare structure.
 +
Because the periods of the oscillations vary with position, we might imagine that these eigenmodes of some complexity, a reasonable expectation.
 +
On the other hand the whole structure obviously needs to evolve in time as the corona tries to find its new equilibrium state following the event, and so the physical parameters (such as the plasma density) will have time variations.
 +
In this sense it may be inappropriate to think of the oscillations as eigenstates of a static structure.
== Pulsations seen by GOES ==
== Pulsations seen by GOES ==
 +
 +
The oscillating structures described above all lie far from the source of flare energy, which our analysis of exciter motion confirms as the strong-field regions of the low corona.
 +
We have also found pulsations in the GOES soft X-ray time series, though and these reflect the higher-temperature regions at the core of the event.
 +
Figure~2
== A new interpretation ==
== A new interpretation ==

Revision as of 15:39, 29 July 2013


Nugget
Number: 204
1st Author: Paulo Simões
2nd Author: Hugh Hudson
Published: July 1, 2013
Next Nugget: Acceleration region of a flare
Previous Nugget: Minoflares
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Contents

Introduction

We have many examples now of contracting motions at or before the impulsive phase of a solar flare (see, for example, Ref. [1], for one of the first reports, and an earlier Nugget.) Such an implosion is consistent with the idea of conservation of energy, since flare structures are basically volumes of almost pure magnetism (the plasma beta in the preflare corona is very low), and this implies that the energy storage can be represented by the volume integral of B2 as a first approximation. So, to release energy, the volume must shrink at the same time - hence a magnetic implosion. The magnetic structure of a flare also can support oscillatory motions, and these can be excited as a simple mechanical response to the implosion. Another earlier Nugget reports such pulsations detectable even at gamma-ray energies.

This Nugget reports a particularly clear example of implosion and oscillation in flare SOL2012-03-09 (M6.4), observed well by many observatories (including RHESSI, of course). This flare revealed new characteristics of the development of the implosion, and of the nature of the resulting oscillations. We take the opportunity below to speculate a little on the consequences of these newly found properties.

The Flare

Figure 1 gives an overview of the flare as an image and as a "time-slice" plot of position vs time along a selected line. The latter shows the major features of the time development of the flare. The appearances are consistent with an early slow inflation, a sudden implosion, the coronal dimming resulting from the launch of the CME, the excitation of oscillations in the visible loop structures, and finally the damping-out of these oscillations.

Figure 1: Left - an EUV image of the flare, essentially from above, showing the photospheric magnetic field as colored contours (two polarities), plus the line of a slit used to make plots of position vs. time. These are in the middle and right plots on different time scales. The CME-related dimming is very obvious, as are the contracting and oscillating loops.  The diagonal line shows a rough guess at the propagation sequence of excitation of the oscillations. Note the different periods for the different loops.

Note that the clearest oscillatory features occur on rather large spatial scales, and show parts of the corona that do not get blown away by the CME motions; this means that they occupy different volumes of the full 3D structure, suggesting that the motions detected are not uniformly radial as the cartoons often imply. Note also that the onsets of the oscillatory motions lag at greater distances. This is an important finding because it implies that the excitation arises in the core of the flare, and the wave of excitation propagates outwards with a (projected) speed estimated at about 300 km/s. Figure~2 shows the close relationship in time between the implosion and the dimming.

Figure 2: Time series.

It would be natural to think of this as a rough estimate of the local Alfv&eacute}n speed, hence bearing information about local physical properties. But we are interested here mainly in the global dynamics of the phenomena. One item of interest lies in the distribution of eigenmodes (normal modes) of the postflare structure. Because the periods of the oscillations vary with position, we might imagine that these eigenmodes of some complexity, a reasonable expectation. On the other hand the whole structure obviously needs to evolve in time as the corona tries to find its new equilibrium state following the event, and so the physical parameters (such as the plasma density) will have time variations. In this sense it may be inappropriate to think of the oscillations as eigenstates of a static structure.

Pulsations seen by GOES

The oscillating structures described above all lie far from the source of flare energy, which our analysis of exciter motion confirms as the strong-field regions of the low corona. We have also found pulsations in the GOES soft X-ray time series, though and these reflect the higher-temperature regions at the core of the event. Figure~2

A new interpretation

Conclusion

References

[1] "X-ray sources and magnetic reconnection in the X3.9 flare of 2003 November 3" (.pdf)

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