Observations Of Solar Filament Formation

Solar filaments are the cool and dense objects which are suspended above the solar-disk.On the solar-disk,the filaments seem like long dark features because they absorb the radiation from background.But when we see them above the solar limb,they seem like bright fires,which are so-called prominences.They have various shapes,such as arc-shaped,hedge,undergrowth,cloud,jet and so on.Their size and stable periods are different from each other.They always appear above the polarity inverse lines(PIL),thus they are used for tracing large-scale magnetic field.Filaments are 100 times cooler and 100 times denser than the coronal plasma.Basing on observations,we want to understand how do they form?why they appear near the PIL?where do the mass comes from?and what is the mechanism of mass replenishmentr?For that,we used the observations of Big Bear Solar Obervatory(BBSO),Solar Dynamics Observatory(SDO)and Interface Region Imaging Spectrograph(IRIS)to analyse two newly formed active region filments.For understanding the formation process,we studied the mass replenishment,filament magnetic configuration and endpoint spectra of the filaments.In our first work,we studied a newly formed filament,which was located in the Active Region(AR)NOAA 11726(S28W62)on 2013 June 5 from 16:40:19 UT to 17:97:58 UT.The observations of the New Solar Telescope(NST)in BBSO,Atmo-spheric Imaging Assembly(AIA)and Helioseismic and Magnetic Imager(HMI)both onboard SDO have been used.Basing on the analysis of this filament,we provided some results on the filament formation,counter-streaming and magntic configuration.First,We believe that this filament is formed via the cold mass injection caused by magnetic reconnection from lower atomosphere,it is a typical injection model process.The initial speed of injection is provided by the magnetic reconnection occuring in the lower atomosphere.And then,the pressure gradient force,which caused by the inequal heating between two footpoints of the magnetic tube,provides the force to maintain the speed of replenishing mass.Second,the counter-streaming phenomena are observed.Since they are all unidirectional flows,we think the counter-streamings may consisit of unidirectional flows with different direction.These flows are caused by the ther-mal inequality between two footpoints of the magnetic tube.At last,we determined the polarity of this filament using the method metioned by Chen et al.[33].And our non-linear force-free field(NLFFF)extrapolation also supports this result.In the second work,we analysed the observations of NST,IRIS and HMI/SDO about a newly formed active region filament located at AR NOAA12403(S27E24).Our observation period is from 17:00 UT to 19:00 UT on 2015 August 21.Basing on the observations,we get the results as follows:(1),the mass is injected with a speed about 30 km s-1 from the bright patches observed in the endpoints of the filament;(2),we can observe the bright ridges in both endpoints of the filament,and these bright ridges are all above the PIL;(3),the results of Q value distribution indicate that both the endpoints of filament are QSLs;(4),the bright ridges of the filament endpoints show triple-gaussian profiles which are typical magnetic reconnection indicators.Thus we confirm that the formation process is an injection process.Furthermore,the filament endpoints are bright ridges which consisit of many small bright patches with typical sizes of filament threads.It indicates that the QSL has fine structures.Moreover,we analyse the spectra of Si IV and Mg ? k lines of the bright pathes at the filament end-points.We found that these bright patches are indicators of the local heating between the high chromosphere and the transition region.This local heating indicates that magnetic reconnection occurs there.It is the first evidence of the exact magnetic reconnection lo-cation of filment formation.At last,we combine the observations of NST and IRIS,and confirm that the so-called counter-streaming phenomena in our observations consisit of the siphon flows with different directions.It is also the first observation evidence of the proposal of Chen et al.[33].