MORPHOLOGICAL FEATURES OF SPOTLESS FLARES AND A POSSIBLE THEORETICAL MODEL

From comprehensive analysis of observational data of 20 spotless flares observed at Yunnan Observatory during the peak period of 21 solar cycle and of two major spotless two-ribbon flares observed at Purple Mountain Observatory on April 23,1981 and Beijing Observatory on Oct.15,1980,the morphological features of spotless flares are summarized as follows: 1.All spotless flares occured on one or both sides of the quiescent filament extending along the line of zero longitudinal magnetic field H=0.2.The chromospheric fibrils surrounding the filament in a spotless active region underwent a great change prior to the flare onset.The fibrils crossing perpendicular to the long axis of the filament could turn to be parallel or nearly parallel to it during one or two days before the flare occured.3.All spotless flares were associated with the activation of filament pre-existing in this region.In general,the activation preceded the flare by be tween one or two days and a few minutes.It frequently takes the form of expansion,enhanced internal motions and a slow rise into the corona.It is coincident with the result of Dodson and Hedeman.4.The combined analysis of the data of the event on April 23,1981 observed at Purple Mountain Observatory with that observed at Big Bear Obs.indicate that the sudden disappearance of the filament is about two minutes earlier than the flare onset.5.Afew data of the spotless two-ribbon flare that the seeing was very good make known that the ribbons appear to be a chain structure and its expansion velocity is about 5 km/s-30 km/s.On the basis of the Kippenhahn-Schliiter model of filament,we have explained qualitatively these features.From the calculated results of magnetic energy,we think the free energy stored in this sheared field surrounding a quiescent filament can be as much as the released energy in a middle flare which occurs in active region.In accordance with W.Van Tend and M.Kuperus's equilibrium of coronal current,when filament current exceeds the local limit of current the force caused by the non-equilibrium can make the filament current rise and reach a new equilibrium state.This is a good theoretical explanation for the activation of the filament.It is well known that the magnetic configuration of the filament is constantly adjusted in the activation.So it might become unstable under a certain specific physical condition because dense filament matter overlies coronal matter.If g_eff>0,the magnetic configuration will be unstable and the Kayleigh-Taylor instability would occur.After the instability breaks out,magnetic field lines will reconnect quickly according to Petschek's mechanism.If once the current density inside the sheet exceeds the critical value for developing of micro-turbulence,the flare will occur explosively.