Thursday, July 11, 2013

1307.2874 (Andrew R. Inglis et al.)

Hard X-ray and ultraviolet emission during the 2011 June 7 solar flare    [PDF]

Andrew R. Inglis, Holly R. Gilbert
The relationship between X-ray and UV emission during flares, particularly in the context of quasi-periodic pulsations, remains unclear. To address this, we study the X-ray and UV emission during the eruptive flare of 2011 June 7 utilising X-ray imaging from RHESSI and UV 1700A imaging from SDO/AIA. This event is associated with synchronous quasi-periodic pulsations in both the X-ray and UV emission, as well as substantial motion of the hard X-ray footpoints. The motion of the footpoint associated with the left-hand flare ribbon is shown to reverse direction along the flare ribbons on at least two occasions. Over the same time interval, the footpoints also gradually move apart at v ~ 12 km/s. This is consistent with the measured plane-of-sky thermal X-ray source outward velocity of ~ 14 km/s, and matches the gradual outward expansion of the UV ribbons. However, there is no associated short-timescale motion of the UV bright regions. We find that the locations of the brightest X-ray and UV regions are different, particularly during the early portion of the flare impulsive phase. Correlation analysis of the measured flare arcade parameters reveals that the 25 - 50 keV hard X-ray flux is only weakly correlated with the other arcade properties, despite begin strongly correlated with the integrated UV emission. We characterise this event in terms of long-term behaviour, where the X-ray nonthermal, thermal, and UV emission sources appear consistent, and short-term behaviour, where the emission sources are inconsistent. We suggest that the short timescale behaviour of hard X-ray footpoints, and the nature of the observed quasi-periodic pulsations, is determined by fundamental, as-yet unobserved properties of the reconnection region and particle acceleration sites. This presents a challenge for current three-dimensional flare reconnection models.
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