Magnetic Field Topology Associated With Solar Eruptive Events And Coronal Heating

It is now realized that the plasma in the solar corona is highly coupled with the magnetic field,which plays a key role in the heating and dynamic processes of the solar corona.The present observations and theories also reveal that the eruptive phenomena in the solar corona,which may seriously affect the solar-terrestrial environment,mostly originate from the magnetic field.Therefore,studies on the magnetic field of the solar corona is crucial for understanding the nature of solar activities and forecasting the space weather.With the developments of the modern instruments,both in space and on ground,the vector magnetic field in the solar photosphere can be measured with high preci-sion and high temporal and spatial resolutions.By employing the force-free model,the magnetic field in the solar corona can be reconstructed with the observed magnetogram as the bottom boundary.This makes it possible to study the topological structure and the magnetohydrodynamic processes of the corona based on the observations.In this thesis,we preform topological analyses of the three-dimensional magnetic field in the solar corona with the multi-wavelength observations from Solar Dynamics Observa-tory,Hinode,and Ramaty High Energy Solar Spectroscopic Imager.We focus on the role of magnetic field in the solar eruptive events(e.g.,solar flares and corona mess ejections)and the heating of the coronal plasma.Based on the magnetic structure,the solar corona can be divided into different topological domains.The boundaries(e.g.,separatrices)between different magnetic domains are places where non-ideal processes(e.g.,magnetic reconnection)favor to take place.These topological boundaries would also trace the morphology of the emis-sion features in observations,such as the flare ribbons.To study the magnetic topology and its association with solar eruptive events,we analyze an X-class circular-ribbon flare on 2012 October 23.The flare showed three ribbons in Ca II H emission,with two highly elongated ones inside and outside a quasi-circular one.A hot channel was found in the extreme-ultraviolet(EUV)emissions that infers the existence of a magnetic flux rope(MFR).Two hard X-ray(HXR)sources in the 12-25 keV energy band were located at the footpoints of this hot channel.With the optimization method,we recon-struct the non-linear force-free field of the active region and identify three topological structures:a 3D null-point,a flux rope below the fan of the null-point,and a large-scale quasi-separatrix layers(QSL)induced by the quadrupolar-like magnetic field of the active region.We find that the null-point is embedded within the large-scale QSL.In this case,all three identified topological structures must be considered to explain all the emission features associated with the observed flare.Besides,the HXR sources are regarded as the consequence of the reconnection within or near the border of the flux rope.Since many eruptive events originate from the highly sheared magnetic arcades or MFRs,it is necessary to perform a quantitative assessment of the topology and evolu-tion of an MFR and its relationship with the associated activities.We reconstruct the magnetic field of active region 12017 from 2014 March 28 to 29,where 12 flares were triggered by the intermittent eruptions of a filament(either successful or confined).From the coronal magnetic field,we find an MFR that is co-spatial with the filament.We determine the boundary of the MFR by a closed quasi-separatrix layer(QSL)en-veloping it.Then,the twist number and the magnetic helicity are calculated for the field lines composing the MFR.The results show that the closed QSL structure(envelope of the MFR)gets smaller as a consequence of the flare occurrence.We also find that the flares in our sample are mainly triggered by kink instability.Moreover,the variation of the twist number is more sensitive than that of other parameters to the occurrence of flares.Besides these eruptive events,a long-lasting problem in solar physics is how the plasma is heated to several millions kelvin in the solar corona.Several different mech-anisms have been proposed,including Alfven wave dissipation and magnetic recon-nection(nano-flares).Both of them are capable of providing the required power,in generic circumstances,neither has yet been used in a quantitative model of observa-tions fed by measured inputs.We show that nano-flare is capable of producing an active region corona comparable both quantitatively and qualitatively with extreme-ultraviolet(EUV)observations.In an ideal plasma without magnetic reconnection,field line footpoints should move at the same velocity as the plasma they find them-selves in.In reality,however,there is a discrepancy observed between the footpoint motion and that of the local plasma due to reconnection,which we name as non-ideal motion.Based on this picture,we come up with a new expression for the heating power proportional to the non-ideal velocity,which can be calculated by using a time series of the observed vector magnetograms.Our model is free from the anomalous resistivity assumption and only depends on the length scale of flux elements reconnected in the corona,which could be constrained from observations and found to be around 160 km in our case.The modeled is free from the anomalous resistivity assumption and only column differential emission measure agrees to a reasonable extent with that derived using EUV images from multiple wavelengths.Synthesized EUV images resemble ob-servations both in their loop-dominated appearance and their intensity histograms.In a conclusion,we provide compelling evidence that nano-flares are a viable mechanism for heating the corona.