The impressive S (or inverse-S) mark appearing in the lower corona called sigmoid, is understood as the manifestation of highly sheared magnetic structures (Rust & Kumar 1996). Recent studies using the Yohkoh Soft X-ray Telescope (SXT) have indicated that soft X-ray sigmoids, i.e., hot (≥ 2MK) S-shaped features, are strongly linked with eruptive phenomena, such as filament eruptions and coronal mass ejections (CMEs) (Sterling & Hudson 1997, Hudson et al. 1998, Canfield et al. 1999, Glover et al. 2000). However, previous studies have focused on the magnetic topology of the sigmoid (e.g. helicity and shear buildup) and/or connection with the resultant CME (e.g. missing mass deduced from coronal dimming), and thus the physical process involved in sigmoid formation and eruption is still not well addressed (Pevtsov et al. 1996, van Driel-Gesztelyi et al 2000). We present observations of two sigmoidal filaments, in which the development of cool (～ 10~4 K) sigmoids was well resolved in high-cadence Ha and microwave images ( ≤30 s). One is an active region filament and the other is a non-active region filament. Both filaments delineated the magnetic neutral line and partially erupted in association with a GOES B class flare. Ha observations indicate that each filament consists of two curved segments that became active in a different manner. Here we focus on the role of the junction at which these two filament segments come together (i.e. the midpoint of the filament). Prior to or during the filament eruption we found a topological change at the junction that seems to suggest a linkage of the two filament segments. Throughout the events, magnetic flux cancellation was observed at the junction. Considering these features, we propose that there is a strong "tether" at the junction that is crucial for the bodily eruption of sigmoidal filaments. Possible magnetic reconnection that took place at the junction in the low atmosphere as the origin of the "tether-cutting action" is discussed.
展开▼
