Studying Magnetic Field Reorganisation in FlaresStudying Magnetic Field Reorganisation in FlaresUsing Hard X-ray and UV DataUsing Hard X-ray and UV Data

Lyndsay FletcherUniversity of Glasgow

Footpoints – tracers of magnetic field evolution

Ribbon structure and footpoint motions

Footpoint intensity variations

As reconnection progresses in the corona, the footpoints of ‘just reconnected’ fieldlines are illuminated (by particles / conduction)

This leads to the appearance of footpoints (HXR) or flare ribbons (H, UV/EUV), which ‘move’ through the magnetic field as the flare proceeds

Flare FootpointsFlare Footpoints

RHESSI ObservationsRHESSI Observations

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 centroids14 March 2002, 8s integrations

Magnetic topology and accelerated particles Magnetic topology and accelerated particles

Metcalf et al. (2003) show location of HXR and white-light sources with respect to calculated intersection of potential field magnetic separatrix surfaces with photosphere.

Separatrix intersections Time evolution of HXRfootpoint positions

Coronal field evidently more complicated than 2-D models. Footpoint sources may assist in constraining field evolution.

UV / EUV footpointsUV / EUV footpoints

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?

HXR locations

UV footpoint source intensity variations UV footpoint source intensity variations

Peaks in the product v BLOS for individual footpoints fairly well correlatedin time with peaks in the UV brightness

(~50% of fp brightenings occur within ± 10s of vBLOS peak; Monte Carlosimulations with random vBLOS peak locations give about 25% association)

UV footpoints flicker on and off; time correlations exist often between distant pairs (but often not between near neighbours).

v BLOS I1600v BLOS I1600

By correlating footpoint intensity variations, can find pairs of locationsthat are possibly linked in some way:

• direct magnetic connection (+ve correlated with –ve)• via a null? (+ve correlated with +ve)

(See also work on correlations in H alpha ribbons by Asai)

Questions:

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 ofthe reconnection region (albeit within a larger-scale organising structure)?

Can we usefully provide limits to geometry in which accelerationmechanisms must operate (e.g. loop lengths – cf Miller review)