SHINE 2009

Signatures of spatially extended reconnection in solar flares

Lyndsay Fletcher

University of Glasgow

SHINE 2009

Spatially extended reconnection?

‘Spatially extended’ could mean

(1)Extended along current sheet length (i.e. long, drawn out CS, like Sweet-Parker)

(1) Extended into 3rd dimension (e.g. an X-line)

(2) Both – i.e. the ‘monolithic current sheet’

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SHINE 2009

Vertically-extended current sheets?

Departed CME, plus several plasmoids

Extended post-CME current sheet?

Ciaravella & Raymond 2008

Fe XVII emission -> hot, turbulent, narrow, bright structure.

Closer to the Sun - HXR evidence for extended flare current sheet, but with multiple plasmoids – tearing instability?

Sui et al (2005) with field lines sketched on.

SHINE 2009

Flare Ribbons

Particularly the flare late phase shows this ordered behaviour. Early impulsive phase ribbons tend to look more irregular.

Bastille Day 2000 flare

Red = UV emission (C IV)Blue = EUV Fe IX/XGreen = EUV Fe XI/XII

Image made in flare late phase.

1.8 ×105 km

Evidence for (2), (3)? Spatially extended ribbons in UV and Hα, and the arcade of loops joining them demonstrate coronal reconnection ‘orchestrated’ over scales of 105km.

SHINE 2009

Flare hard X-ray footpoints

Energetically, UV and Hα ribbons are unimportant. The main sites of impulsive energy release are indicated by hard X-ray and ‘white light’.

HXR/WL sources are few in number and compact (i.e. not ribbon-like).

The point-like appearance of HXR sources could be due to HXR image dynamic range. Occasionally, impulsive ribbon-like HXR or white-light emission is seen (2 examples known).

Liu et al. 2007

Krucker et al. 2007

Compact HXR footpoints Extended HXR ribbons

SHINE 2009

HXR source motions

HXR and WL sources move systematically - this can be explained by one or two ‘primary’ sites of reconnection, which may be moving.

e.g. coloured crosses mark HXR footpoints progressing along the neutral line (Grigis & Benz 2004) (Need to understand how field anchored

in HXR sources maps into corona – difficult. See e.g. DesJardins et al 2009)

e.g. antiparallel source motions – reconnection in sheared arcade.

(Bogachev & Somov 2005)

SHINE 2009

Sub-structure of UV flare ribbons

Nishizuka et al 2009 – ribbon sub-structure has a distribution of size scales, down to ~ 2” and probably below.

UV/EUV and Hribbons may look continuous, but they are not. Individual sources can be isolated, by separating out their lightcurves.

SHINE 2009

Sub-structure of UV flare ribbons

The overall ordered ribbon motion is composed of individual motions of UV sources within ribbon characterised by drift (spreading) plus diffusion.

Above: Each dot and track shows start position and motion of a discrete UV footpoint (Fletcher et al. 04)

Left: all tracks plotted with start points at (0,0)

UV ribbons composed of many individual sources

SHINE 2009

Ribbon structuring => fragmented reconnection?

UV drift speeds (~50 km/s) consistent with overall ribbon spreading.

UV diffusion scales consistent with individual footpoints meandering along granular boundaries or along penumbral filaments.

Could there be smooth field and uniform reconnection in the corona, but tangled field mapping to chromosphere/photosphere and photospheric restrictions on footpoint trajectories?

Right now, I don’t think we can tell…

SHINE 2009

Light curves and correlations

Extract UV light curve for each tracked source

Filter, cross-correlate, identify pairs

SHINE 2009

Supra-arcade downflows

NB most SADs observed with Yokhoh/SXT occur during flare impulsive phase (Khan et al. 2007)

McKenzie & Savage (2009)

Supra-arcade downflows (‘tadpoles’) interpreted as retracting flux tubes. Very suggestive of a fragmentary coronal reconnection process.

After Klimchuk, McKenzie

SHINE 2009

Birn et al. 3D MHD simulations

Simulations of energy transport in reconnecting 3D magnetic field by Birn et al (2009).

Thermal energy flux, enthalpy flux and MHD Poynting flux tracked through corona.

High β and low β initial states used.

Low β state (β ~ 0.01) is appropriate for pre-flare conditions.

Low force-free High

Pre-flare

During eruption

SHINE 2009

Low case

Projection of energy flux onto x-y (photospheric) plane in low case

Initially smooth and almost straight separator splits into multiple reconnection sites (maybe interchange or ballooning instability?)

Vertical velocity component, initially low case.

Greyscale: Thermal energy flux/enthalpy flux

Orange: MHD Poynting flux

SHINE 2009

Conclusions

Flare ribbons provide evidence for extended reconnection volumes BUT with sub-structure on scales of a few arcsec and probably less.

HXR/WL footpoints on the other hand could involve a very small subset of all the reconnecting field. There is an overall magnetic ordering, but within this there is fragmentation.

Basic spatial scale? Probably not reached yet in observations

Does the fragmentation develop in an initially smooth structure, or is the underlying field already tangled/non-uniform before reconnection sets in?