arXiv subjects

Yi Bi

Publications and source records attributed to Yi Bi.

23 records · Page 2Linked to original sources

arXiv2018

The Formation of a Small-scale Filament after Flux Emergence on the Quiet Sun

We present observations of the formation process of a small-scale filament on the quiet Sun during 5-6 February 2016 and investigate its formation cause. Initially, a small dipole emerged and its associated arch filament system was found to reconnect with overlying coronal fields accompanied by numerous EUV bright points. When bright points faded out, many elongated dark threads formed bridging the positive magnetic element of dipole and external negative network fields. Interestingly, an anti-clockwise photospheric rotational motion (PRM) set in within the positive endpoint region of newborn dark threads following the flux emergence and lasted for more than 10 hours. Under the drive of the PRM, these dispersive dark threads gradually aligned along the north-south direction and finally coalesced into an inverse S-shaped filament. Consistent with the dextral chirality of the filament, magnetic helicity calculations show that an amount of negative helicity was persistently injected from the rotational positive magnetic element and accumulated during the formation of the filament. These observations suggest that twisted emerging fields may lead to the formation of the filament via reconnection with pre-existing fields and release of its inner magnetic twist. The persistent PRM might trace a covert twist relaxation from below photosphere to the low corona.

astro-ph.SR↗

arXiv2015

Partial eruption of a filament with twisting nonuniform fields

The eruption of the filament with the kink fashion is often regarded as a signature of the kink instability. However, the kink instability threshold for the filament magnetic structure has been not widely understood. Using the H-alpha observation from the New Vacuum Solar Telescope (NVST), we present a partial eruptive filament. In the eruption, a filament thread appeared to split from the middle portion of the filament and to break out in a kinklike fashion. During this period, the left filament material remained below, which erupted without the kinking motion later on. The coronal magnetic field lines associated with the filament are obtained from the nonlinear force-free field (NLFFF) extrapolations using the 12 minutes cadence vector magnetograms of the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamic Observatory (SDO). We studied the extrapolated field lines passing through the magnetic dips that are in good agreement with the observed filament. The field lines are non-uniformly twisted and appear to be made up by two twisted flux ropes winding about each other. One of them has higher twist than the other, and the highly twisted one has its dips aligned with the kinking eruptive thread at the beginning of its eruption. Before the eruption, moreover, the highly twisted flux rope was found to expand with the approximately constant field twist. In addition, the helicity flux maps deduced from the HMI magnetograms show that some helicity is injected into the overlying magnetic arcade, but no significant helicity is injected into the flux ropes. Accordingly, we suggest that the highly twisted flux rope became kink unstable when the instability threshold declined with the expansion of the flux rope.

astro-ph.SR↗

arXiv2014

A Well-Founded Semantics for FOL-Programs

An FOL-program consists of a background theory in a decidable fragment of first-order logic and a collection of rules possibly containing first-order formulas. The formalism stems from recent approaches to tight integrations of ASP with description logics. In this paper, we define a well-founded semantics for FOL-programs based on a new notion of unfounded sets on consistent as well as inconsistent sets of literals, and study some of its properties. The semantics is defined for all FOL-programs, including those where it is necessary to represent inconsistencies explicitly. The semantics supports a form of combined reasoning by rules under closed world as well as open world assumptions, and it is a generalization of the standard well-founded semantics for normal logic programs. We also show that the well-founded semantics defined here approximates the well-supported answer set semantics for normal DL programs.

cs.PL↗

arXiv2011

The surge-like eruption of a miniature filament

We report on the rare eruption of a miniature H_alpha filament that took a surge form. The filament first underwent a full development within 46 minutes and then began to erupt 9 minutes later, followed by a compact, impulsive X-ray class M2.2 flare with a two ribbon nature only at the early eruption phase. During the eruption, its top rose, whereas the two legs remained rooted in the chromosphere and swelled little perpendicular to the rising direction. This led to a surge-like eruption with a narrow angular extent. Similar to the recent observations for standard and blowout X-ray jets by Moore et al., we thus define it as a "blowout H_alpha surge". Furthermore, our observations showed that the eruption was associated with (1) a coronal mass ejection guided by a preexisting streamer, (2) abrupt, significant, and persistent changes in the photospheric magnetic field around the filament, and (3) sudden disappearance of a small pore. These observations thus provide evidence that blowout surge is a small-scale version of large-scale filament eruption in many aspects. Our observations further suggest that at least part of H_alpha surges belong to blowout-type ones, and exact distinction between standard and blowout H_alpha surges is important in understanding their different origins and associated eruptive phenomena.

astro-ph.SR↗

arXiv2011

Observations of EUV and Soft X-ray Recurring Jets in an Active Region

We present simultaneous observations of three recurring jets in EUV and soft X-ray (SXR), which occurred in an active region on 2007 June 5. By comparing their morphological and kinematic characteristics in these two different wavelengths, we found that EUV and SXR jets had similar locations, directions, sizes and velocities. We also analyzed their spectral properties by using six spectral lines from the EUV Imaging Spectrometer (EIS) on board Hinode, and found that these jets had temperatures from 0.05 to 2.0 MK and maximum electron densities from 6.6$\times10^{9}$ to 3.4$\times10^{10}$ cm$^{-3}$. For each jet, an elongated blue-shifted component and a red-shifted component at the jet base were simultaneously observed in Fe{\sc xii} $λ$195 and He{\sc ii} $λ$256 lines. The three jets had maximum Doppler velocities from 25 to 121 km s$^{-1}$ in Fe{\sc xii} $λ$195 line and from 115 to 232 km s $^{-1}$ in He{\sc ii} $λ$256 line. They had maximum non-thermal velocities from 98 to 181 km s$^{-1}$ in Fe{\sc xii} $λ$195 line and from 196 to 399 km s$^{-1}$ in He{\sc ii} $λ$256 line. We also examined the relationship between averaged Doppler velocities and maximum ionization temperatures of these three jets, and found that averaged Doppler velocities decreased with the increase of maximum ionization temperatures. In the photosphere, magnetic flux emergences and cancellations continuously took place at the jet base. These observational results were consistent with the magnetic reconnection jet model that magnetic reconnection between emerging magnetic flux and ambient magnetic field occurred in the lower atmosphere.

astro-ph.SR↗