Studying Magnetic Field Reorganisation in Flares
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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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Studying Magnetic Field Reorganisation in Flares Using Hard X-ray and UV Data

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Studying magnetic field reorganisation in flares using hard x ray and uv data

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


Studying magnetic field reorganisation in flares using hard x ray and uv data

Flare Footpoints

  • 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


Studying magnetic field reorganisation in flares using hard x ray and uv data

RHESSI 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 centroids

14 March 2002, 8s integrations


Studying magnetic field reorganisation in flares using hard x ray and uv data

Separatrix intersections

Magnetic topology and accelerated particles

  • Coronal field evidently more complicated than 2-D models. Footpoint

    sources may assist in constraining field evolution.

  • 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.

Time evolution of HXR

footpoint positions


Studying magnetic field reorganisation in flares using hard x ray and uv data

UV / 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?


Studying magnetic field reorganisation in flares using hard x ray and uv data

HXR locations


Studying magnetic field reorganisation in flares using hard x ray and uv data

I1600

I1600

v BLOS

v BLOS

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)


Studying magnetic field reorganisation in flares using hard x ray and uv data

  • By correlating footpoint intensity variations, can find pairs of locations

  • that 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)


Studying magnetic field reorganisation in flares using hard x ray and uv data

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 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)


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