Two-stage SEE Shows Reconnection

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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.

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, 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. 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.

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Another remarkable aspect of this event is the sequence of events 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 a minute. This was followed by perhaps the clearest evidence yet for magnetic reconnection high in the corona. A fine bright line appeared in the AIA 131 Angstrom images at the start of the second peak followed by apparently a classic X-point reconnection with initially great 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. 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.

Authors

Brian Dennis, Goddard Yang Su, Goddard and The Catholic University of America, soon to be at the University of Graz, Austria.

References

Aschwanden et al. (2009)

Woods et al. (2011)

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