Modulation Of Solar Eruptions By The Background Magnetic Field Of The Sun

Solar flares and coronal mass ejections(CMEs)are energetic eruptive activities.When they occur,space environment of the earth may be disturbed and human life can be affected.These two solar activities have strong relationship with each other and they are deemed to be different manifestations of the same physical process.Usually,solar prominence/filament eruption is also accompanied by a CME,so the three solar activities are associated with each other.What is the factor that modulates the process of the solar eruptions?Combining statistical and case study,we try to identify the factor and understand the eruption mechanism.1.Comparative study between the active region magnetic fields of eruptive and confined flaresAlthough solar flare and CME are closely related,observations show that they do not always occur together and we often find that flares occur without CMEs.Based on whether a flare is accompanied by a CME or not,flares can be categorized into eruptive flares and confined flares.So what is the essential factor that distinguishes their explosive behavior?By a comparative study,we find out the key factor that determines whether a two-ribbon flare is associated with a CME.(1)Statistical analysis of critical height for eruptive flares and confined flaresWe select 60 two-ribbon flares above M class in 2011-2015 and categorize them into two groups including 35 eruptive flares and 25 confined flares,based on whether a flare is accompanied by a CME or not.We calculate the decay indexes n=-hd In Bex/dh of magnetic field at different height above the polarity inversion line(PIL)of the active regions where flares originate.Then we get the critical height based on the threshold n=1.5 of torus instability(TI for short)by linear interpolation.In general,the critical height for confined flares is larger than that of eruptive flares.The distribution of critical height for eruptive flares and confined flares peaks at 20-30Mm and 60-70Mm respec-tively.We suggest that the key factor that determines whether flares lead up to CMEs is how rapidly the overlying magnetic filed decays.(2)Statistical analysis of n(h)profileAs a result,we find that the profile of n(h)have two kinds:(1)n increases mono-tonically with height for more than 80%of the flares.(2)n has a saddle-like profile and there are only nine saddle-like cases among which five are eruptive and four are confined.The value of n at saddle bottom is generally larger for eruptive flares than confined flares(3)The study of empirical relationship between critical height and centroid dis-tance of active region and analysis of active region complexityWe find that there exists a strong correlation between d and hccrit:hcrit is approxi-mate half of d.According to our criteria of complexity classification for active regions,70%of the flares originate from multipolar field active regions and the rest from dipolar field.For the nine flares which have saddle-like n(h)profile,they are all from multipolar field active regions.However,we can not conclude that multipolar field active regions always produce saddle-like n(h)profile cases,because some flares with monotonously n(h)profile also originate from multipolar field.Based on the above study,we conclude that the key factor that determines whether flares lead up to CMEs is how rapidly the overlying magnetic filed decays,however the decay law of magnetic filed overlying the eruption structure may be affected by the magnetic reconnection nearby,which will affect its eruptive behavior.2.Identify the causal links among the sympathetic eruptionsSometimes,we can observe several solar activities at different places in a relatively short time interval and this phenomenon is referred to sympathetic eruptions.However,it is difficult to determine the causal links among them.We study the sympathetic erup-tions in and outside of NOAA AR 12242 on 2014 December 20.By observation and analysis we suggest that slipping-like magnetic reconnection should play a key role in connecting the eruptions.However,the observational effects and signatures of large scale-field magnetic reconnection on sympathetic eruptions in the literature have sel-dom been identified.The magnetic reconnection transfers magnetic flux from one fila-ment to the other filament and consequently adjust how fast the magnetic filed decays with the increasing height.As a result magnetic confinement becomes stronger above the latter and weaker above the former which becomes unstable to the torus instability and erupts finally.Generally,based on the above two studies,we conclude that background magnetic field can modulate the solar eruption effectively.