Study On The Initiation Mechanism Of Solar Eruption Based On Numerical Simulations

Severe space weather driven by solar eruptions can significantly impact high-tech industries,including aerospace,communication and navigation,and deep space exploration.An in-depth understanding of the mechanisms that initiate solar eruptions is crucial for accurate space weather forecasting.A recently proposed fundamental mechanism suggests that only a single bipolar field and the most common shear motion on the photosphere are needed in the solar eruptions.The mechanism shows that the shear-driven quasi-static evolution process within the magnetic arcades will spontaneously form a current sheet,and then trigger and drive the eruption through fast magnetic reconnection.Among all proposed initiation mechanisms,this mechanism stands out as the most concise,effective,and universal mechanism for solar eruptions,and we refer to it as the basic mechanism.Based on high-resolution numerical magnetohydrodynamics simulations,this thesis presents a novel systematic theory around this basic mechanism.This thesis conducts numerical simulation research on a series of magnetograms with different flux distributions and photospheric driving.We analyzed the initiation mechanism for producing solar eruptions in bipolar fields under different flux distributions,revealed a new initiation mechanism for homologous eruption,evaluated the effects of various photospheric driving on the coronal shear magnetic field,and explored the different roles of magnetic breakout reconnection and core current sheet reconnection in multi-polar magnetic field,as well as the key reasons for the eruption.To sum up,this work has promoted the overall understanding of the initiation mechanism of solar eruptions.Based on the observation of solar eruptive activities,it has been found that the magnetic fields in the source regions of large flares have long polarity inversion lines(PIL)with strong vertical magnetic field gradients and transverse magnetic field shears.However,the physical explanation behind this correlation remains elusive.To solve this question,we setted up a series of idealized bipolar magnetograms and calculated the ratio of the free magnetic energy to the potential energy of the fully open field in the space,the peak current density within the current sheet,and in their corresponding height.Analytical calculations found that the longer the PIL with strong gradient,the greater the explosive ability of the magnetograms and the stronger the open field current sheet.Meanwhile,we conducted a series of high-resolution numerical simulation experiments for the corresponding magnetic fields of these magnetograms.It is found that all of these simulations quasi-statically form a current sheet in the core field,and the magnetic reconnection at the current sheet will immediately trigger and drive the eruption.The strength of eruption is positively correlated with the strength of PIL.Thus,we have revealed that a bipolar field with a stronger PIL can have more non-potentialiality,and can form a stronger internal current sheet during the evolution process.As a result,the basic mechanism generates eruptions.Our simulation has confirmed the robustness of the basic mechanism in bipolar fields.Additionally,we have discovered a key predictive parameter in solar eruption,which is the strength of PIL.According to observations of the Sun,it is occasionally found that a specific active region repeatedly produces multiple coronal mass ejections with similar structures,which is referred to as homologous eruptions.The evolution of these eruptions represents a cyclic process of "storage,release,re-storage,and re-release" of coronal magentic energy.However,there is no simulation that can effectively reproduce this phenomenon at present.By continuously applying shear on the bottom PIL of a single bipolar magnetic field,we successfully replicated the homologous eruption phenomenon in the simulation,the energy evolution curve in the simulation exhibits a "saw-tooth" shape,with each spike representing an eruption.These eruptions are initiated by the dissipation of the slowly formed current sheet in the core field,which is consistent with the description of the basic mechanism.That is,the basic mechanism can explain the homologous eruptions phenomenon.Moreover,our results also show that for a given magnetic flux distribution,there is an upper threshold of magnetic field energy,and eruption occurs when the energy is close to this threshold.In addition to applying the usual shear driving in the simulations,we also investigated the effects of converging motion and photospheric dissipation on the evolution of coronal system.The application of these two types of driving causes the magnetic flux to move toward the PIL,leading to an increase in magnetic field gradient near the PIL and even the generation of a magnetic flux rope through magnetic cancellation.Previous work has found that the system triggers eruption due to the ideal instability of magnetic flux ropes.However,our results are different from the previous ones.Applying these two types of driving will make the sheared magnetic field continue to compress inward,thereby helping to form a current sheet in the core field.Magnetic reconnection at the current sheet will immediately initiate the eruption,and this is consistent with the basic mechanism.Moreover,the converging motion weaken the bondage of the overlying background field to the core field,facilitating eruption generation.During the triggering process,the ideal instability of the magnetic flux rope does not come into play.The magnetic field in the source region of a large solar eruption was often a complex multipolar magnetic field,in addition to a single bipolar field,we also conducted simulations of quadrupole magnetic field.According to the classical magnetic breakout mechanism,a magnetic null point exists above the core field.The breakout reconnection initiated from this point will establish a positive feedback loop between the breakout reconnection and the expansion of the core field,ultimately leading to an eruption.But there is no work to explain the role of magnetic breakout reconnnection.Through a series of full three-dimensional numerical simulations using quadrupolar fields with varying photospheric magnetic flux distributions,we demonstrated that the breakout reconnection accelerates the expansion of the core field,thereby facilitating the formation of core current sheets.The violent eruption can only be caused by magnetic reconnection occurring at the core current sheet.Further,we found that the key factor to eruption is neither the occurrence of magnetic breakout reconnection nor fast magnetic breakout reconnection,but rather the formation of a current sheet in the shear core field.If the bottom driving is stopped before the formation of the core current sheet,even though the fast reconnection has occurred in the breakout current sheet,the fast breakout reconnection alone cannot establish a positive feedback loop between the reconnection and the expansion of the core field.As a result,the system will gradually evolve to a steady state,and since the core current sheet is not formed,no eruption will occur.Through this set of simulation experiments,we have proved that the magnetic null point does not play a key role in the initiation of the eruption.Regardless of whether the magnetic field environment is bipolar or multipolar,the actual initial process of the eruption is the basic mechanism,thus achieving the theoretical unification of the initiation mechanism.Through a series of numerical simulations,we have systematically investigated the key factor to the eruption in the coronal magnetic field and the performance of the coronal magnetic field driven by different photospheric motions,and preliminarily clarified the role of reconnection and magnetic flux ropes in the initiation process of eruption,which has significantly improved our understanding of the initiation mechanism for solar eruptions.In addition,we confirmed the robustness of the basic mechanism and summarized the key conditions for its eruption,which can provide theoretical guidance for the development of physics-based solar eruption prediction methods.