AGU Dec 2000 The Structure and Evolution of a Sigmoid Observed During Whole Sun Month 3 Lyndsay Fletcher Sarah Gibson, and the WSM3 sigmoid structure working 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 eruptive signatures (seen in SXT).

AGU Dec 2000 From Canfield Hudson and McKenzie 1999 Bundle of field lines, twisted beyond ~  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 thoughts regarding 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 SXT  The H  filament formed in stages, partially disappeared and then reformed in a slightly different orientation. •Small flares on Aug 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 UTAug UT Eruption in southern SXT loops Eruption in northern SXT loops H  filament not seen Southern part of H  filament visible

Aug UTAug UT SXT sigmoidal appearance begins? Southern SXT loops brighten S-shaped H  filament fully formed Southern part of H  filament thickens

21-Aug UT22-Aug UT SXT sigmoid well-formed H  filament starts to break up H  filament disappears

H  filament reforms in south - longer and less ‘ bent ’ 23-Aug UT •Periods of eruption in SXT loops followed by H  filament formation.  H  sigmoidal filament present without clear SXT sigmoid •SXT sigmoid well-formed as H  filament starts to break up.

AGU Dec 2000 H alpha filament evolution movie •Filament (southern part) first visible on 16th •Grows 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-linear FF Non-linear FF fields show twisted, dipped field lines which could provide filament support.

On August 21st the coronal sigmoid was well-formed. Events include a small EUV loop brightening and (probably associated) filament disappearance CDS Observations show a backwards ‘ s ’ pattern visible in many wavelengths, from chromospheric to high coronal. This is clearly (at low heights at least) not a consequence of a single twisted field structure. Observations of August 21

He IMg IX Si XIIFe XIX Comparisons with TRACE identify the CDS sigmoid at <1MK with ‘ moss ’ - the transition-region of high-pressure loops

AGU Dec 2000 The low-lying sigmoid shape is determined by the distribution of magnetic plage.

Overlaying with contours of SXT emission show also longer sigmoidal loops connecting to plage further south. AGU Dec 2000

Comparison of SXT and Mg X emission suggests two sets of sigmoidal loops. A short set, connects in the neighbourhood of the sunspot. Above this lies a longer set. A linear force-free field extrapolation also shows this.

AGU Dec 2000 CDS rasters suggest a shear between short loops and their footpoints Fe XIX He IO 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 2000 CDS 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 ~ 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 region from Aug Evolution beyond Aug 21 •shows long, low-lying TRACE loops •Flux emerging into centre of sigmoid •small northern filament apparently overlying unipolar plage •southern part of filament becomes visible in absorption at limb.

CDS spectroheliograms on the limb show the remaining southern part of 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 ~ cm -3 footpoint density ~10 10 cm -3 But no strong evidence for density variations around filament.

Summary •All wavelengths show evidence of sigmoidal shape  Complex relationship between presence of H  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