The Structure and Evolution of a Sigmoidal Active Region

Abstract

Solar coronal sigmoidal active regions have been shown to be precursors to some coronal mass ejections. Sigmoids, or S-shaped structures, may be indicators of twisted or helical magnetic structures, having an increased likelihood of eruption. We present here an analysis of a sigmoidal region's three-dimensional structure and how it evolves in relation to its eruptive dynamics. We use data taken during a recent study of a sigmoidal active region passing across the solar disk (an element of the third Whole Sun Month campaign). While S-shaped structures are generally observed in soft X-ray (SXR) emission, the observations that we present demonstrate their visibility at a range of wavelengths including those showing an associated sigmoidal filament. We examine the relationship between the S-shaped structures seen in SXR and those seen in cooler lines in order to probe the sigmoidal region's three-dimensional density and temperature structure. We also consider magnetic field observations and extrapolations in relation to these coronal structures. We present an interpretation of the disk passage of the sigmoidal region, in terms of a twisted magnetic flux rope that emerges into and equilibrates with overlying coronal magnetic field structures, which explains many of the key observed aspects of the region's structure and evolution. In particular, the evolving flux rope interpretation provides insight into why and how the region moves between active and quiescent phases, how the region's sigmoidicity is maintained during its evolution, and under what circumstances sigmoidal structures are apparent at a range of wavelengths.

Partially-erupting prominences: a comparison between observations and model-predicted observables

“…The Hα data recorded by BBSO showed a surviving filament about 2.5 h post-flare (see Fig. 10, 2nd row, 2nd column image in Gibson et al 2002).…”

Formation of current sheets and sigmoidal structure by the kink instability of a magnetic loop

“…(1) Such major energy release events, which we will refer to collectively as flares in the following, often show a distinctive soft X-ray (SXR) brightening of sigmoidal (S or reverse-S) shape shortly prior or during their early, impulsive stages of development (e.g., Rust & Kumar 1996;Manoharan et al 1996;Pevtsov et al 1996;Aurass et al 1999;Moore et al 2001;Gibson et al 2002). The sigmoidal shape is generally regarded as a signature of coronal currents in a twisted field and shows a hemispheric preference: forward (reverse) S shapes dominate in the southern (northern) hemisphere (Rust & Kumar 1996).…”

Magnetic Helicity and Large Scale Magnetic Fields: A Primer

“…This contrasts the situation of the α 2 dynamo in Figure 11 in which flux terms increase the saturation value but do not determine the difference between growth and decay. Gibson et al (2002). It is tempting to interpret the large scale right-handed writhe and small scale left handed striations along the sigmoid being consistent with what a magnetic helcity conserving dynamo would predict when the fluxes eject both small and large scale helicities from the interior.…”

“…Such structure might in fact be evident in the TRACE image of coronal loop shown in Fig. 15 from Gibson et al (2002). The figure shows left hand twist and right handed writhe.…”