Two-stage SEE Shows Reconnection

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|name = Nugget
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|title =  
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|number = 157
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|number = 158
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|first_author = Brian Dennis
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|first_author = Yang Su
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|second_author = Yang Su
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|second_author = Brian Dennis
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|publish_date = 7 September 2011
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== Introduction: A Two-Stage SEE ==
== Introduction: A Two-Stage SEE ==
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Using SDO EVE light curves and AIA images, Woods et al. (2011) have shown that in some solar eruptive events (SEEs) a second stage of heating can occur an hour or more after the first. Aschwanden et al. (2009) attributed similar late peaks seen in the EUV light curves from STEREO/EUVI to the cooling of the plasma heated to higher temperatures during the initial energy release. In one well-observed event seen with both STEREO A and B, they estimated a conductive and radiative cooling time of ~40 minutes that they argued was consistent with the observed delay of ~1 hour between the soft X-ray and EUV peaks. Woods et al. (2011), however, claim that this could not possibly be the explanation in at least one case. The hot plasma at ~2 MK that produced the second-stage peak in the EVE light curves was revealed by the AIA images to be from a different location than the plasma in the first stage and hence could not have resulted from the cooling of the initially heated plasma.
+
[http://sdo.gsfc.nasa.gov/ SDO] light curves and images in the extreme ultraviolet ([http://en.wikipedia.org/wiki/Extreme_ultraviolet EUV]) suggest [ref. 1] that in some "solar eruptive events" (SEEs), a second stage of heating can occur an hour or more after the first.  
 +
Earlier work [ref. 2] had attributed similar late peaks seen in the EUV light curves from STEREO/EUVI to the cooling of the plasma heated to higher temperatures during the initial energy release.  
 +
In one well-observed event seen with both STEREO A and B, this earlier work had estimated a conductive and radiative cooling time of ~40 minutes,
 +
which the authors argued was consistent with the observed delay of ~1 hour between the soft X-ray and EUV peaks.  
 +
The new interpretation throws this into doubt; Ref. [1] asserts that this could not possibly be the explanation in at least one case.  
 +
The hot plasma at ~2 MK that produced the second-stage peak in the SDO/[http://lasp.colorado.edu/eve/ EVE] light curves was revealed by the SDO/[http://aia.lmsal.com/ AIA] images to be from a different location than the plasma in the first stage, and hence could not have resulted from the cooling of the initially heated plasma.
-
[[File:LightCurves 08March2011.jpg|200px|thumb|left|Fig. 1. AIA, GOES, and RHESSI light curves of the SEE on 8 March 2011 with temperature and emission measure estimates in the bottom panel.]]
+
[[File:LightCurves 08March2011.jpg|200px|thumb|left|Fig. 1. RHESSI, GOES, AIA, and EVE light curves of the 2-stage SEE on 8 March 2011. Also shown in the bottom panel are the temperatures and emission measures obtained as a function of time from RHESSI and GOES spectral data.]]
-
 
+
-
We report here on an event on 8 March 2011 with similar strong evidence for a second stage of energy release high in the corona. In this event shown in Figure 1, the second-stage energy release was seen as a second peak in the GOES X-ray light curves showing that the plasma was heated to some 15 Mk, almost the same temperature as the plasma heated during the first stage nearly two hour earlier. RHESSI spectral analysis confirms the similarity of the temperatures with ~20 MK for the first peak and 15 MK for the second (shown in Fig. 1, bottom panel). AIA images throughout this event show that the second-stage heating occurred at a much higher altitude in the corona than the first, again confirming that it was not the result of cooling from the first stage.
+
 +
We report here on an event on 8 March 2011 with similar strong evidence for a second stage of energy release high in the corona. In this event shown in Figure 1, the second-stage energy release was seen as a second peak in the GOES X-ray light curves, showing that the plasma was heated to some 15 Mk at that time, almost the same temperature as the plasma heated during the first stage nearly two hours earlier.
 +
RHESSI spectral analysis confirms the similarity of the temperatures with ~20 MK for the first peak and 15 MK for the second (shown in Fig. 1, bottom panel).
 +
The EVE curves of the change in irradiance above the preflare level shown in the third panel from the top in Fig. 1 also show the second peak at the two highest temperatures of 8 and 12.6 MK. AIA images throughout this event show that the second-stage heating occurred at a much higher altitude in the corona than the first, again confirming that it was not the result of cooling from the first stage.
== Magnetic Reconnection ==
== Magnetic Reconnection ==
-
[[File:AIA 131 200622 bw.jpg|200px|thumb|left|Fig. 2. Still from AIA movie in the 131 Angstrom passband at 20:06:22 UT showing the remarkable similarity to the classical picture of X-point magnetic reconnection.]]
+
[[File:AIA 131 200622 bw.jpg|200px|thumb|right|Fig. 2. Still from AIA movie in the 131 Angstrom passband at 20:06:22 UT showing the remarkable similarity to the classical picture of X-point magnetic reconnection.]]
 +
 
 +
Another remarkable aspect of this SEE is the visual appearance of magnetic reconnection revealed by the the AIA movies during the second stage of energy release, particularly in the 131 Angstrom passband. First, a rapid ejection of hot plasma was seen starting high in the corona and moving out of the field of view in less than ten minutes. This was followed by perhaps the clearest evidence yet for magnetic reconnection in the corona. This is best seen in the movie in the AIA 131 Angstrom passband that you can download from the following location: [http://hesperia.gsfc.nasa.gov/~yangsu/20110308/SDO_AIA_131_only.mov AIA 131 Angstrom movie]. Fig. 2 shows an image when the reconnection was well developed. A fine bright line appeared at the start of the second peak followed by what certainly looks like a classic X-point reconnection with initially turbulence below, and later the appearance of rapidly falling new bright loops. Emission above the X-point was much fainter but suggested that hot plasma was also driven to higher altitudes at the same time. The whole sequence, lasting over an hour, had the same basic appearance as the classic model of magnetic connection at an X-point with newly connected field lines pulling rapidly away both above and below. There was minimal evidence for any particle acceleration at this time from the RHESSI light curves at >20 keV. However, any bright hard X-ray footpoints would most probably have been occulted since the flare ribbons were mostly over the limb. This is revealed by the 195 Angstrom image in Fig. 3 taken by STEREO-B from its perspective at ~90 degrees from the Earth-Sun line at nearly the same time as the AIA image in Fig. 2. Unfortunately, the reconnection, so clearly seen in the AIA movie, is not evident in the relatively low-cadence STEREO images.
-
Another remarkable aspect of this SEE is the visual appearance of magnetic reconnection revealed by the the AIA movies during the second stage of energy release, particularly in the 131 Angstrom passband. First, a rapid ejection of hot plasma was seen starting high in the corona and moving out of the field of view in less than ten minutes. This was followed by perhaps the clearest evidence yet for magnetic reconnection in the corona. This is best seen in the movie in the AIA 131 Angstrom passband that you can downnload from the following location: [http://hesperia.gsfc.nasa.gov/~yangsu/20110308/SDO_AIA_131_only.mov AIA 131 Angstrom movie]. Fig. 1 shows an image when the reconnection was well developed. A fine bright line appeared at the start of the second peak followed by what certainly looks like a classic X-point reconnection with initially turbulence below and the later appearance of rapidly falling new bright loops. Emission above the X-point was much fainter but suggested that hot plasma was also driven to higher altitudes at the same time. The whole sequence, lasting ~10 minutes???, had the same basic appearance as the classic model of magnetic connection at an X-point with newly connected field lines pulling rapidly away both above and below. There was minimal evidence for any particle acceleration at this time from the RHESSI light curves at >20 keV. However, any bright hard X-ray footpoints could have been occulted since the flare ribbons were mostly over the limb as revealed by images from STEREO-B, which saw the event at near Sun-center from its perspective at ~90 degrees from the Earth-Sun line.
+
[[File:stereo_195_2005UT.jpg|200px|thumb|left|Fig. 3. Image from STEREO_B in the 195 Angstrom passband with the disk visible from the Earth to the left of the line marked "RHESSI limb."]]
== Conclusion ==
== Conclusion ==
Line 33: Line 41:
== Authors ==
== Authors ==
-
Yang Su, currently at Goddard Space Flight Center as a postdoc with The Catholic University of America, will soon be at the University of Graz, Austria.
+
Yang Su, formerly at Goddard Space Flight Center as a postdoc with The Catholic University of America, now at the University of Graz, Austria.
Brian Dennis is the RHESSI Mission Scientist at Goddard.
Brian Dennis is the RHESSI Mission Scientist at Goddard.
Line 39: Line 47:
== References ==
== References ==
-
[http://adsabs.harvard.edu/abs/2009SoPh..256....3A Aschwanden, M. J., Wuelser, J. P., Nitta, N. V., & Lemen, J. R. 2009, Sol. Phys., 256, 3]
+
[1] [http://iopscience.iop.org/0004-637X/739/2/59 New Solar Extreme-Ultraviolet Irradiance Observations During Flares]
-
Woods, T. N., Eparvier, F. G., Hock, R., Jones, A. R., Woodraska, D., Judge, D., Didkovsky, L., Lean, J., Mariska, J., Warren, H., McMullin, D., Chamberlin, P., Berthiaume, G., Bailey, S., Fuller-Rowell, T., Sojka, J., Tobiska, W. K., & Viereck, R. 2011, Sol. Phys., in press.
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[2] [http://adsabs.harvard.edu/abs/2009SoPh..256....3A Solar Flare and CME Observations with STEREO/EUVI]

Latest revision as of 17:07, 22 August 2018


Nugget
Number: 158
1st Author: Yang Su
2nd Author: Brian Dennis
Published: 7 September 2011
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Contents

Introduction: A Two-Stage SEE

SDO light curves and images in the extreme ultraviolet (EUV) suggest [ref. 1] that in some "solar eruptive events" (SEEs), a second stage of heating can occur an hour or more after the first. Earlier work [ref. 2] had attributed similar late peaks seen in the EUV light curves from STEREO/EUVI to the cooling of the plasma heated to higher temperatures during the initial energy release. In one well-observed event seen with both STEREO A and B, this earlier work had estimated a conductive and radiative cooling time of ~40 minutes, which the authors argued was consistent with the observed delay of ~1 hour between the soft X-ray and EUV peaks. The new interpretation throws this into doubt; Ref. [1] asserts that this could not possibly be the explanation in at least one case. The hot plasma at ~2 MK that produced the second-stage peak in the SDO/EVE light curves was revealed by the SDO/AIA images to be from a different location than the plasma in the first stage, and hence could not have resulted from the cooling of the initially heated plasma.

Fig. 1. RHESSI, GOES, AIA, and EVE light curves of the 2-stage SEE on 8 March 2011. Also shown in the bottom panel are the temperatures and emission measures obtained as a function of time from RHESSI and GOES spectral data.

We report here on an event on 8 March 2011 with similar strong evidence for a second stage of energy release high in the corona. In this event shown in Figure 1, the second-stage energy release was seen as a second peak in the GOES X-ray light curves, showing that the plasma was heated to some 15 Mk at that time, almost the same temperature as the plasma heated during the first stage nearly two hours earlier. RHESSI spectral analysis confirms the similarity of the temperatures with ~20 MK for the first peak and 15 MK for the second (shown in Fig. 1, bottom panel). The EVE curves of the change in irradiance above the preflare level shown in the third panel from the top in Fig. 1 also show the second peak at the two highest temperatures of 8 and 12.6 MK. AIA images throughout this event show that the second-stage heating occurred at a much higher altitude in the corona than the first, again confirming that it was not the result of cooling from the first stage.

Magnetic Reconnection

Fig. 2. Still from AIA movie in the 131 Angstrom passband at 20:06:22 UT showing the remarkable similarity to the classical picture of X-point magnetic reconnection.

Another remarkable aspect of this SEE is the visual appearance of magnetic reconnection revealed by the the AIA movies during the second stage of energy release, particularly in the 131 Angstrom passband. First, a rapid ejection of hot plasma was seen starting high in the corona and moving out of the field of view in less than ten minutes. This was followed by perhaps the clearest evidence yet for magnetic reconnection in the corona. This is best seen in the movie in the AIA 131 Angstrom passband that you can download from the following location: AIA 131 Angstrom movie. Fig. 2 shows an image when the reconnection was well developed. A fine bright line appeared at the start of the second peak followed by what certainly looks like a classic X-point reconnection with initially turbulence below, and later the appearance of rapidly falling new bright loops. Emission above the X-point was much fainter but suggested that hot plasma was also driven to higher altitudes at the same time. The whole sequence, lasting over an hour, had the same basic appearance as the classic model of magnetic connection at an X-point with newly connected field lines pulling rapidly away both above and below. There was minimal evidence for any particle acceleration at this time from the RHESSI light curves at >20 keV. However, any bright hard X-ray footpoints would most probably have been occulted since the flare ribbons were mostly over the limb. This is revealed by the 195 Angstrom image in Fig. 3 taken by STEREO-B from its perspective at ~90 degrees from the Earth-Sun line at nearly the same time as the AIA image in Fig. 2. Unfortunately, the reconnection, so clearly seen in the AIA movie, is not evident in the relatively low-cadence STEREO images.

Fig. 3. Image from STEREO_B in the 195 Angstrom passband with the disk visible from the Earth to the left of the line marked "RHESSI limb."

Conclusion

This well-observed SEE shows clear evidence for two stages of energy release separated by almost two hours. Both the high temperature and higher altitude of the second-stage plasma preclude the possibility that it was merely the same plasma heated during the first stage that had cooled sufficiently to emit in the lower temperature AIA passbands. There was little indication during the first stage that a second stage was imminent although the initial eruption did seem to stall as if it was either too weak to drive the ejection against the Sun's gravity or that there were overlying magnetic fields preventing the plasma from escaping.

This SEE also shows, during the second stage, strong visual evidence for magnetic topology changes, plasma turbulence, and newly formed retracting magnetic loops. The overall appearance during this phase is remarkably similar to the classic picture of X- or Y-point magnetic reconnection. Further analysis is required to establish if observations at other wavelengths support this interpretation.

Authors

Yang Su, formerly at Goddard Space Flight Center as a postdoc with The Catholic University of America, now at the University of Graz, Austria.

Brian Dennis is the RHESSI Mission Scientist at Goddard.

References

[1] New Solar Extreme-Ultraviolet Irradiance Observations During Flares

[2] Solar Flare and CME Observations with STEREO/EUVI

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