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all active regions were classified “sigm oidal”. These sigmoidal regions accounted for 61% o f all eruptions observed in soft X-ray data.

In order to account for the apparent spatial inconsistency between the location o f the pre-flare sigm oid footpoints and those o f the post-flare arcade, Sterling et al. [2000] speculated that overlying unsheared loops m ay be disturbed by the twisted flux rope as it begins to rise and erupt. The resulting reconnection o f these loops w ould lead to heating, producing temperatures sufficient for these loops to be observed in SXT data. However, no evidence for this scenario was provided and these loops were not observed prior to eruption.

Several studies have shown that a magnetic flux tube may becom e unstable i f the twist from one end to the other exceeds a critical value [c.f. Priest, 1982]. Pevtsov, Canfield and Zirin [1996] proposed that the twist possessed by individual pre-flare loops would combine on reconnection, causing the level o f twist transferred to a larger post­ flare sigm oidal feature to exceed the maximum threshold for stability. This would lead to the sigm oid becom ing unstable and beginning to rise.

A number o f authors [Moore et al., 2001; van Driel-Gesztelyi et al., 2000; Titov & Dem oulin, 1999] have described a “sigm oid expansion” scenario by which an eruptive, transient sigmoidal feature is formed in association with flaring through reconnection o f two j-shaped loops.

According to the “sigm oid expansion” model o f van Driel-G esztelyi et al. [2000], this upward motion drives magnetic reconnection between highly sheared arcade field lines primarily located beneath the flux tube. A long S-shaped field line is formed at the border o f the flux tube. This joins the outer ends o f two anti-parallel j-shaped loops which existed separately as part o f the sheared arcade prior to activity comm encing. Short loops are formed below, connecting the internal parts o f the two pre-flare j-shaped loops. A s reconnection proceeds, the flux tube becom es extended, eventually becom ing insufficiently dense to be observed at SXT resolution. Consequently, the S-shaped loop w ill be observed to expand and disappear from view . Fast outward m otion o f the flux tube is thought to build a current sheet, o f the type described by Kopp and Pneuman [1976] (Figure 1.8), below the outward m oving feature. This im plies that the new ly

reconnected loops w ill adopt a cusp shaped appearance. This cusp shaped feature w ill gradually fade from view as the magnetic field relaxes [Forbes & Acton, 1996].

The above summary highlights some o f the questions regarding the nature o f sigm oidal features that have not yet been answered. Line-of-sight effects are important when observing disk features. It is therefore, often difficult to determine whether an apparently sigm oidal feature is comprised o f a single feature or many individual features, oriented such that they appear sigmoidal in projection. This raises the question o f whether a region should only be classified as sigmoidal i f it comprises a single structure or whether ‘projected sigm oids’ should also be included in the definition. Previous studies have inferred CME onset using soft-X-ray data alone. W hile flaring can be clearly observed in this data, CME onset can only be inferred by proxy and observations may be m isleading. Thus, the relationship between highly sheared structures and CME onset warrants further investigation.

This chapter begins with an extension o f the Canfield, Hudson and M cKenzie [1999] survey o f sigm oidal active regions, incorporating active regions previously classified as both sigm oidal and eruptive over an increased range in wavelength. This extended survey incorporates white light coronagraph data, EU V and H a disk observations together with increased resolution soft X-ray observations o f the same regions. The aim o f this extended survey is to clarify the relationship between the sigm oid-to-arcade development and CME onset.

F ollow ing the extended survey, two sample active regions are described in detail and comparison is made between the mechanisms involved in their eruption. These regions illustrate that S (or reverse-S) shaped features m ay appear in the corona as a result o f differing magnetic field configurations. Consequently, the probability o f their eruption is unlikely to be the same. The magnetic topology o f each region is discussed in the light o f current m odels. The implications o f these observations in terms o f CME onset prediction are then discussed.

Chapter 4. A ssociation o f CMEs with Sigmoidal A ctive Regions 77

4.2 Observations

It was decided to approach this investigation by extending the results o f Canfield, Hudson and M cK enzie [1999] to provide a more detailed study o f those active regions classified as sigm oidal and eruptive. In addition to the soft X-ray proxy o f a sigmoid-to-arcade development, the inclusion o f SOHO based instrumentation allow s confirmation o f CME onset through white light coronagraph data together with EIT extreme-ultraviolet observations o f on-disk onset signatures. A detailed description o f this technique can be found in the preceding chapter.

In order to improve statistics and m inim ise the number o f sigm oidal active regions interacting with adjacent structures, the Canfield, Hudson and M cKenzie study considered active regions that formed during the years 1993 and 1997. Both years exhibit an intermediate level o f solar activity. The SOHO m ission was launched in 1996 and so extension o f the Canfield, Hudson and M cKenzie results to include comparison with SOHO data is only possible using the latter half o f the initial list (active regions formed during 1997).

H a data from the B ig Bear Solar Observatory are also included in the extended survey. These data were available via the Yohkoh data archive at the rate o f a single image per day. A ctive region prominences however, are frequently seen to erupt on a tim escale o f 2 0-30 minutes, often reforming later [Martin, 1998]. A s a result, actual filament eruptions were rarely observed in the daily H a images. A ctive region filaments could also be observed in absorption in EIT Fe X ll (195 Â ) data. A ny changes to the filament observed using these data were recorded together with changes seen in H a data. Filaments were classed as “eruptive” i f their m orphology appeared to change or i f a full/partial disappearance could be observed follow ing activity in the region.

The Yohkoh SFD videodisk used by the Canfield, Hudson and M cKenzie study is com posed o f com posite data with an average rate o f approximately 50 im ages per day. The use o f com posite data compensates for the effects o f saturation in the SXT images but the inclusion o f only full disk data has the effect o f degrading the image resolution to between 5” and 10” per pixel as compared with the maximum 2 ”.45 per pixel resolution

[Rust & Kumar 1996; Tsuneta et al., 1991]. W hile this is undoubtedly advantageous when carrying out large surveys o f data, short duration events (<1 hr) may be missed and detailed observation o f sigmoidal active regions is inhibited. For the purposes o f this investigation it was decided to repeat the original Yohkoh/SXT survey to include the full range o f 9 ”.8, 4 ”.9 and 2 ”.45 per pixel Yohkoh/SXT data where available.

Appendix A contains the list o f 1997 “sigm oidal and eruptive” active regions identified by Canfield, Hudson and M cKenzie [1999]. The estimated eruption times, provided by Canfield [1999] are also included in this appendix.

For the purposes o f this study, the eruption and pre-eruption morphology o f each

active region previously classified as sigm oidal and eruptive were confirmed using SXT

data o f the highest available resolution. Comparison was then made between each o f the eruptive events previously attributed to eruptive sigm oids and SOHO/LASCO CME data for that period. This was achieved by means o f a LAS CO height-time extrapolation, an example o f which is illustrated in Figure 3.1. Each CME was traced back to eruptive activity on the disk, as described in Chapter 3. CMEs were only discounted i f :

a) an exact origin other than that o f the candidate region could be determined, or b) i f the time interval between observed event time and extrapolated CME onset time