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Studying Magnetic Field Reorganisation in Flares Using Hard X-ray and UV Data. Lyndsay Fletcher University of Glasgow. Footpoints – tracers of magnetic field evolution Ribbon structure and footpoint motions Footpoint intensity variations. Flare Footpoints.
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Using Hard X-ray and UV Data
University of Glasgow
RHESSI Hard X-rays provide a very direct signature of fast electrons when they reach the chromosphere
HXR sources separate, moving along the ribbons
UV ribbons much more extended than HXR sources – observational limitation?
Location of main source centroids
14 March 2002, 8s integrations
Magnetic topology and accelerated particles
sources may assist in constraining field evolution.
Time evolution of HXR
High resolution (and unsaturated) TRACE UV/EUV observations from the
impulsive phase show emission ‘fragmented’ along length of ribbon on
scales as small as 2”. Observation cadence as low as 2s.
Pixels are ~ 0.”5 x 0.”5
What can be learned by tracking these footpoints?
UV footpoint source intensity variations
UV footpoints flicker on and off; time correlations exist often between
distant pairs (but often not between near neighbours).
Peaks in the product v BLOS for individual footpoints fairly well correlated
in time with peaks in the UV brightness
(~50% of fp brightenings occur within ± 10s of vBLOS peak; Monte Carlo
simulations with random vBLOS peak locations give about 25% association)
By correlating footpoint intensity variations, can find pairs of locations
(See also work on correlations in H alpha ribbons by Asai)
Why do we typically only see a couple of HXR sources but more extended
(and complicated) UV separatrices? What is special about these locations?
Can we relate spatial fragmentation of the ribbons to fragmentation of
the reconnection region (albeit within a larger-scale organising structure)?
Can we usefully provide limits to geometry in which acceleration
mechanisms must operate (e.g. loop lengths – cf Miller review)