Presentor: Kuniko Hori
Tether-cutting action in two sigmoidal filaments
Hori, K. (Mullard Space Science Laboratory), Alexi, G. (Mullard Space Science Laboratory), Akioka, M. (Communications Research Laboratory), Ueno, S. (Hida Observatory)
The impressive S (or inverse-S) mark appearing on the lower corona,
called sigmoid, is known as the manifestation of highly sheared
magnetic structures (Rust and Kumar 1996).
Recent studies using Yohkoh Soft X-ray Telescope (SXT; Tsuneta et al.
1991) has indicated that soft X-ray sigmoids, i.e., hot (\ge 2MK)
S-shaped features, are strongly linked with eruptive phenomena, such
as filament eruptions, flares, and coronal mass ejections (CMEs)
(Sterling and Hudson 1997; Hudson et al. 1998; Canfield et al. 1999).
However, previous papers have focused on the magnetic topology
(e.g., helicity, shear buildup) of the sigmoid and connection
with the resultant CME (e.g., missing mass deduced from dimming),
and thus the physical process involved in the sigmoid formation and
eruption is still not well addressed (e.g., van Driel-Gesztelyi et al.
2000).
In this paper, we present observations of two sigmoidal filaments,
in which development of the cool (\sim 10^4 K) sigmoids was well
resolved with the high-cadence H \alpha and microwave images (\le 30 sec).
One is an active region filament and the other is a quiet region filament,
whose long term evolution was reported by Glover et al. (2001).
Both filaments were overlying the magnetic neutral line and their
activities were associated with GOES B class flares.
Comparison of SXT images and H \alpha images illustrates that each
filament almost coincides with a soft X-ray long structure that is
apparently consisted of two short loops. The key features commonly
observed in these filaments can be summarized as follows:
i) The initial activity (small brightening) started at the middle of
the sigmoidal filament.
ii) The activity propagated upward crossing the filament at the junction
of the two soft X-ray short loops, suggesting the so-called
"tether-cutting" action (Sturrock 1989; Moore and Roumelios 1992).
After the flare, the active region filament extended northward and formed
an inverse-S structure, which is not visible in soft X-ray, whilst
in the quiet region filament, the curved southern half of the filament
disappeared but the other half remained.
Microwave observation provides information on the heated plasma
propagating along the magnetic features. We examine thermal evolution
within the sigmoidal filaments and compare the results with the
tether-cutting scenario which was proposed for flare/filament eruptions.
Possible magnetic reconnection that took place below the filament
is discussed.