Numerical Study Of Filament Channel Formation

The sun is the closest star to our earth.Observations have revealed that there are many complicated activities in the sun's atmosphere,such as sunspots,prominences,and flares.There are still many unclear physical mechanisms behind these phenomena.The research object of this paper is the solar prominence,which is a bright band-like structure protruding from the edge of the solar limb and the accumulation of low-temperature and high-density plasma suspended in the solar corona.When a prominence is observed on the solar disk,it presents a dark strip structure distributed along the magnetic polarity reversion line,so a prominence is also called a filament.The reason why prominences do not quickly fall to the surface of the sun is because they are supported by a special magnetic environment.The special magnetic environment where the prominences are located is also called filament channels,with structures of a sheared magnetic arcade or a helical magnetic flux rope,and the latter has more observational evidence.The formation mechanism of magnetic flux ropes in the quiescent regions relies on the largescale shearing and converging motion in the photosphere to accumulate the magnetic helicity and drive the magnetic flux cancellation.However,observations show that there is no such large-scale converging motion near filaments,only supergranular motions.And there is no significant difference between supergranulations in filament channels and the ones in other quiescent regions.The origin of magnetic helicity in filament channels can be explained by the helicity condensation theory,which believes that the rotation of granules or supergranules injects magnetic helicity in small scale and the magnetic helicity is reversely cascaded from small scales to the largest scale of a magnetic flux system through magnetic reconnection in the corona,leading to helicity condensation to the boundary of the magnetic flux system near the magnetic polarity reversion line.In order to study the relationship between filament channels and supergranulations,we carried out magnetic evolutionary numerical simulations in an adaptive-mesh-refined spherical grid covering two solar rotation periods with a time-varying supergranular velocity field at the boundary on the photosphere.The simulation started from a smooth dipole potential field and presented the formation process of a magnetic flux rope in a filament channel.We used a Poisson distribution to randomly place the center points of supergranules and generated Voronoi tessellation based on the points.Each piece of tessellation represents a supergranule.The horizontal velocity fields of the supergranules diverge from their centers to the edges with deflection under the Coriolis force and produce vortices around the converging centers between supergranules.We also found that if the Coriolis force is turned off left only with the differential rotation,the meridian circulation flows,and the undeflected supergranular velocity field,there was no magnetic flux rope forming after two solar rotations.Our numerical models show that the supergranulations under the Coriolis force dominates the helicity injection in the quiescent regions of the sun and support that supergranular helicity injection and helicity condensation generate the axial magnetic flux of filaments.Converging motions between supergranules drive the magnetic reconnection of sheared magnetic arcades to produce an uneven magnetic flux rope,which may explain the observed distribution of prominence feet.