  The dynamic properties of flare ribbons and the often associated filament
eruptions can provide crucial information on the flaring coronal magnetic
field. This Letter analyzes the GOES-class X1.0 flare on 2014 March 29
(SOL2014-03-29T17:48), in which we found an asymmetric eruption of a sigmoidal
filament and an ensuing circular flare ribbon. Initially both EUV images and a
preflare nonlinear force-free field model show that the filament is embedded in
magnetic fields with a fan-spine-like structure. In the first phase, which is
defined by a weak but still increasing X-ray emission, the western portion of
the sigmoidal filament arches upward and then remains quasi-static for about
five minutes. The western fan-like and the outer spine-like fields display an
ascending motion, and several associated ribbons begin to brighten. Also found
is a bright EUV flow that streams down along the eastern fan-like field. In the
second phase that includes the main peak of hard X-ray (HXR) emission, the
filament erupts, leaving behind two major HXR sources formed around its central
dip portion and a circular ribbon brightened sequentially. The expanding
western fan-like field interacts intensively with the outer spine-like field,
as clearly seen in running difference EUV images. We discuss these observations
in favor of a scenario where the asymmetric eruption of the sigmoidal filament
is initiated due to an MHD instability and further facilitated by reconnection
at a quasi-null in corona; the latter is in turn enhanced by the filament
eruption and subsequently produces the circular flare ribbon.
