Formation And Evolution Of Solar Magnetic Flux Ropes

A magnetic flux rope(MFR)is an important structure in many astrophysical,space,and laboratory contexts involving magnetized plasma.In the solar physics,MFRs are always related to solar eruption,such as filament eruptions,flares,coronal mass ejec-tions and so on.Solar eruptions are the dominant contributor to adverse Space Weather at Earth.Hence,understanding formation and evolution of solar magnetic flux ropes is significant for predicting space weather.In addition,the results from solar magnetic flux ropes may benefit other research in stellar physics.Although it is believed that an MFR can form on the Sun,it is not clear when and how it forms.The main idea of our studies is to determine the footpoints of MFRs that are anchored to the Sun.Then we can quantify the MFR's properties and discuss the formation and evolution of the MFR.In addition,we also use some advanced analytical methods,e.g.,Grad-Shafranov reconstruction,magnetic field extrapolation and data-driven numerical simulation.In this research,we investigate eruptive and non-eruptive evolution of pre-existing MFRs respectively.Then we study the dynamic formation of a magnetic flux rope during an eruption.In the end,we conduct a statistical study about MFRs' feet,to study the relationship between evolution of photosphere and formation of MFRs.The main studies are listed below:How an MFR evolves toward eruption remains unclear Here we investigate the continuous evolution of a pre-existing MFR,which is rooted in strong electric currents.The MFR evolution comprises a two-stage gradual expansion followed by another stage of rapid acceleration/eruption.Quantitative measurements indicate that magnetic twist of the MFR increases from 1.0±0.5 to 2.0±0.5 turns during the five-hour expansion,and further increases to about 4.0 turns per AU when detected as a magnetic cloud at 1 AU two days later.In addition,each stage is preceded by flare(s),implying reconnection is actively involved in the evolution and eruption of the MFR.The implications of these measurements on the CME initiation mechanisms are discussed.Recent tornado-like evolution in the sun attracted widespread attention.It is ac-cepted that this kind of evolution may explain as plasma moves along the helical field of cavity.However,we observed a whole rotation of a quiescent prominence on 2014 November 1,which may be one type of non-eruptive evolution of a pre-existing MFR.After detail analysis of SDO observations and PFSS method,we suggest that the tornado-like evolution of the prominence was governed by the helical kink instability.The kink-unstable prominence first rises and then interact with the overlying field,forming many bright thread-like structures.Then these structures rapid rotate and most material move along threads back to the sun outside the filament channel.Finally,it disintegrates and the remaining material then back to the filament channel.Although the presence of an MFR after solar eruptions has been verified by space-craft measurements near Earth,its formation on the Sun remains elusive.Here we study a new morphology of two-ribbon flare:the far end of two ribbons evolve from bright point to irregular ring.And these rings outline two conj ugate dimmings which are iden-tified as footpoints of an MFR.This kind of evolution indicate the formation of the MFR.Counting magnetic flux through the feet and the ribbon-swept area reveals that the rope's core is more twisted.It propagates to the Earth as a typical magnetic cloud possessing a similar twist profile obtained by the Grad-Shafranov reconstruction of its three dimensional structure.However,the studied MFR with non-uniform twist distri-bution post a major challenge of previous theoretical models.In addition,our analysis suggest that the non-uniform twist distribution may relate to the reconnection rate.By comparing the evolution of an MFR's feet and photospheric evolution.we found the reconnection between the MFR and the ambient field would also impact the photospheric field.During the eruption,the bright boundary of the MFR's foot has swept the overlapped region between the foot and the sunspot twice,suggesting two-stage reconnection.During the first reconnection,the sunspot wave disappears in the overlapped region.And the photospheric field decreases rapidly.In the second stage.the sunspot wave has resumed propagating along the overlapped region.However,the photospheric field in that region starts to increase fast than before.These observations confirm that once the MFR formed,it would expand and interact with overlying field.Is the photospheric evolution direct impact the formation of an MFR?Strong non-neutralized current are related to photopsheric evolution.In this study,we intend to test the relationship between MFRs' feet and strong non-neutralized current.We selected 10 active regions with strong non-neutralized current.From those active regions,we investigated 12 eruptions with obvious features that represent MFRs.The footpoints of MFRs are identified unambigously with conjuate dimmings.The results show that most MFRs' feet are co-spatial with strong non-neutralized current.The pre-existing MFRs are very likely related to the sunspot rotation.