AGU Dec 2000

The Structure and Evolution of a Sigmoid Observed During Whole Sun Month 3

Lyndsay FletcherSarah Gibson,

and the WSM3 sigmoid structureworking group.

AGU Dec 2000

The WSM 3 working group is

S. Gibson, L. Fletcher, D. Alexander, D. Biesecker,

J. Burkepile, G. Del Zanna, P. Demoulin, C. Mandrini,

H. Mason, Y. Liu, N. Nitta, C. D. Pike, J. Qiu, K. Ko,

B. Schmieder, B. Thompson

The sigmoid study uses data from:

Yohkoh SXT, TRACE, SoHO CDS, EIT and UVCS, BBSO,MLSO CHIP and PICS

AGU Dec 2000

The importance of sigmoids

Canfield, Hudson and McKenzie (1999) found that large,sigmoidal active regions have a high correlation with eruptivesignatures (seen in SXT).

AGU Dec 2000

From

Canfield H

udson and McK

enzie 1999

Bundle of field lines, twisted beyond ~ p becomes kink-unstable?

AGU Dec 2000

Sigmoids as Predictors of Eruptive Activity

Glover et al. (2000) find:

Of the ~100 sigmoidal regions studied by Canfield et al, only ~40% are single loop sigmoids Other sigmoidal regions are formed by two or more sets of loops arranged in an S configuration. 35% of single loop sigmoids were found to have eruptive signatures in LASCO - a slightly higher percentage than for multiple loop sigmoids. Sigmoids may form prior to or following an eruption

So the situation is somewhat more complex than the initial thoughtsregarding the loss of stability of a twisted flux tube.

AGU Dec 2000

The sigmoid region, AR 8668 was tracked across the disk. It underwent several eruptions, visible in SXTThe Ha filament formed in stages, partially disappeared and thenreformed in a slightly different orientation.

Small flares on Aug 16 - 17 are being analysed by Canfield et al.Detailed magnetic field modelling on Aug 19th by Liu et al.The analysis of a jet seen on Aug 26 is being led by Ko

Overall evolution of WSM sigmoid region

Aug-16-2000 14 UT Aug-17-2000 23 UT

Eruption in southern SXT loops Eruption in northern SXT loopsHa filament not seen Southern part of Ha filament visible

Aug-19-2000 01 UT Aug-20-2000 03 UT

SXT sigmoidal appearance begins? Southern SXT loops brightenS-shaped Ha filament fully formed Southern part of Ha filament thickens

21-Aug-2000 00 UT 22-Aug-2000 00 UT

SXT sigmoid well-formed Ha filament starts to break up Ha filament disappears

Ha filament reforms in south - longer and less bent

23-Aug-2000 05UT

Periods of eruption in SXT loops followed by Ha filament formation.

Ha sigmoidal filament present without clear SXT sigmoid

SXT sigmoid well-formed as Ha filament starts to break up.

AGU Dec 2000

H alpha filament evolution movie

Filament (southern part) first visible on 16thGrows between 17th (17:30) and 18th (14:30)Stable until eruption on 21st; 22nd onwards S part reforms

movie

AGU Dec 2000

August 19 - well-formed filament. Liu et al. have performed various field extrapolations

Potential

Linear FF Non-linearFF

Non-linear FF fields show twisted, dipped field lines which couldprovide filament support.

On August 21st the coronalsigmoid was well-formed.

Events include a small EUV loop brightening and (probablyassociated) filament disappearance

CDS Observations showa backwards s pattern visible inmany wavelengths, from chromospheric to high coronal.

This is clearly (at low heightsat least) not a consequence ofa single twisted field structure.

Observations of August 21

He I Mg IX

Si XII Fe XIX

Comparisons with TRACE identify the CDS sigmoid at <1MK withmoss - the transition-region of high-pressure loops

AGU Dec 2000

The low-lying sigmoidshape is determined by the distribution ofmagnetic plage.

Overlaying with contours of SXT emission show also longer sigmoidal loops connecting to plage further south.

AGU Dec 2000

Comparison of SXT andMg X emission suggeststwo sets of sigmoidal loops. A short set, connects in the neighbourhood of the sunspot.Above this lies a longer set.

A linear force-free fieldextrapolation also showsthis.

AGU Dec 2000

CDS rasters suggest a shear between short loops and their footpoints

Fe XIX

He I O V

Si XII

AGU Dec 2000

A small brightening (not a GOES flare) on Aug 21 at 18UT reveals short 1.5MK loops in an apparently sheared structure.

AGU Dec 2000CDS density, temperature, abundance ratios before the brightening

AGU Dec 2000 CDS density, temperature, abundance ratios after the brightening

Following the brightening, CDS Si XII lineshifts show upflows of ~ 30-40 km/s at the ends of the short sigmoidal loops

This may suggest that the brightening leads to evaporation of chromospheric material into the loops

-Could be used to assist in the identification of active field lines in magnetic modelling?

AGU Dec 2000

TRACE movie of regionfrom Aug 22-26

Evolution beyond Aug 21

shows long, low-lying TRACE loopsFlux emerging into centre of sigmoidsmall northern filament apparently overlying unipolar plagesouthern part of filament becomes visible in absorption at limb.

CDS spectroheliogramson the limb show theremaining southern partof the filament, and both cool and hot loops in the north.

The filament is embedded in diffuse, hot emission.

From Si X diagnostic:Coronal density ~ 3.109cm-3

footpoint density ~1010cm-3

But no strong evidence for density variations around filament.

SummaryAll wavelengths show evidence of sigmoidal shape

Complex relationship between presence of Ha s-shaped filament and coronal sigmoid - eruptions can both (a)result in field-lines necessary to support filament material (b)lead to destabilisation and ejection of filament

Two (or more) sets of loops? Interfaces may provide locations for reconnection and loss of stability ( e.g. brightening on 21st followed by filament eruption)

Helicity or just complexity? Short TRACE/SXT loops must be fit with non-potential fields; long ones with potential fields (interface with rest of corona?)

Excellent set of sigmoid data for comprehensive analysis