Observational Study Of Photospherie Magnetic Activities And Small-scale Eruptions Associated With Coronal Bright Points

Coronal heating is One of the most famous problems in solar physics.In the last tens of years,people have been looking for the observational evidence of the coronal heating.With the improvements of the spatial and temporal resolution of the instruments,many small-scale phenomena are found in the solar transition region and corona in recent years.However,we still have very few knowledge on the physical nature of these small-scale phenomena.Motivated by finding direct signatures of coronal heating processes,more and more studies begin to focus on the small-scale phenomena.Coronal bright points(CBPs)are among the most typical small-scale phenomena in the solar corona.CBPs are small(on average 2O"-30")and short-lived(from a few minutes to a few tens of hours)bright structures.They are believed to be the signature of a direct energy deposition in the upper solar atmosphere.Furthermore,the eruptions from CBPs are the sign of a direct energy release as well,and these eruptions are the products of the evolution of CBPs.Many previous studies have shown that CBPs are asso-ciated with magnetic bipolar features(MBFs).Therefore,it is not difficult to understand that the eruptions from CBPs are also closely related to the evolu-tion of MBFs.In order to analyze the eruptions from CBPs better,we first start with the MBFs associated with CBPs,made a statistical research on two basic problems,that is,the formation of MBFs and the magnetic cancellation of MBF-s.Moreover,our study found that these physical processes occur in more than one way,and different physical processes correspond to different ways of occur-ring.After understanding these problems,we began to study the eruptions from the CBPs.Unlike their large-scale counterparts,small-scale CBPs are simple in structure,generally consist of loops connecting to MBFs.This provides us a rel-atively simple background environment for better analysis of the characteristics of the eruptions.In addition,small-scale CBPs and associated MBFs have short lifetime.We could study CBPs,MBFs and eruptions in the evolution of their lifetime,which allows us to obtain complete evolutionary information of these physical processes in time.Generally speaking,as one of the candidate observa-tional evidences for coronal heating,in order to understand its physical nature,we start with its related magnetic field evolution and gradually to its main way of energy release,i.e.,eruptions.Through the research on the formation and evo-lution of magnetic field associated with CBPs,we have a better understanding of the evolution and heating of CBPs.We know that the formation of CBPs is actually from the MBFs with different formation ways,and the disappearance of CBPs is also due to different occurring ways of magnetic cancellation.CBPs have simple magnetic field structures and background environments.As a small-scale counterpart of the large-scale eruption,the study of the CBPs provides us a unique opportunity to understand the eruptive phenomena in the solar atmo-sphere.Our results also show that the small-scale eruptions from CBPs are very similar to the large-scale eruptions in the solar atmosphere,so it can be speculat-ed that they may have the same physical mechanism.Furthermore,the study of the eruptions from CBPs brings us a step closer to understanding the evolution of the CBPs and their physical nature.CBPs are associated with MBFs and magnetic cancellation.First,we in-vestigate how BP-associated MBFs form and how the consequent magnetic can-cellation occurs.We analyze longitudinal magnetograms from the Helioseismic and Magnetic Imager(HMI)to investigate the photospheric magnetic flux evolution of 70 BPs.From images taken in the 193 Apassband of the Atmospheric Imaging Assembly(AIA)we determine that the BPs’ lifetimes vary from 2.7 to 58.8 hr.The formation of the BP MBFs is found to involve three processes,namely,emergence,convergence,and local coalescence of the magnetic fluxes.The formation of an MBF can involve more than one of these processes.Out of the 70 cases,flux emergence is the main process of an MBF buildup of 52 BPs,mainly convergence is seen in 28,and 14 cases are associated with local coales-cence.For MBFs formed by bipolar emergence,the time difference between the flux emergence and the BP appearance in the AIA 193 Apassband varies from 0.1 to 3.2 hr with an average of 1.3 hr.While magnetic cancellation is found in all 70 BPs,it can occur in three different ways:(I)between an MBF and small weak magnetic features(in 33 BPs);(II)within an MBF with the two polarities moving toward each other from a large distance(34 BPs);(III)within an MBF whose two main polarities emerge in the same place simultaneously(3 BPs).While an MBF builds up the skeleton of a BP,we find that the magnetic activities responsible for the BP heating may involve small weak fields.Second,eruptions from CBPs are investigated in detail.The study aim-s to explore in full detail the morphological and dynamical evolution of these eruptions in the context of the full lifetime evolution of CBPs.Observations of the full lifetime of CBPs in data taken with the Atmospheric Imaging Assembly(AIA)on board the Solar Dynamics Observatory(SDO)in four passbands,HeⅡ 304 A,Fe ix/x 171 A,Fe Ⅻ 193 A,and Fe ⅩⅧ 94 A are investigated for the occurrence of plasma ejections,micro-flaring,mini-filament eruptions,and mini coronal-mass ejections(mini-CMEs).Data from the Helioseismic and Magnetic Imager are analysed to study the longitudinal photospheric magnetic field evo-lution associated with the CBPs and related eruptions.First and foremost,our study shows that the majority(76%)of quiet Sun CBPs(31 out of 42 CBPs)pro-duce at least one eruption during their lifetime.From 21 eruptions in 11 CBPs,18 of them occur,on average,～17 hr after the CBP formation.The average lifetime of the CBPs in AIA 193 A is～21 hr.The time delay in the eruption occurrence coincides in each CBP with the convergence and cancellation phase of the CBP bipole evolution during which the CBPs become smaller until they fully disappear.The remaining three eruptions happen 4--6 hr after the CBP formation.In 16 out of the 21 eruptions,the magnetic convergence and can-cellation involve the CBP main bipoles,while in three eruptions,one of the BP magnetic fragments and a pre-existing fragment of opposite polarity converge and cancel.In one BP with two eruptions,cancellation was not observed.The CBP eruptions involve in most cases the expulsion of chromospheric material either as an elongated filamentary structure(mini-filament,MF)or a volume of cool material(cool plasma cloud,CPC),together with the CBP or higher overlying hot loops.Coronal waves were identified during three eruptions.A micro-flaring is observed beneath all erupting MFs/CPCs.Whether the destabilised MF caus-es the micro-flaring or the destabilisation and eruption of the MF is triggered by reconnection beneath the filament remains uncertain.In most eruptions,the cool erupting plasma either partially or fully obscures the micro-flare until the erupting material moves away from the CBP.From 21 eruptions,11 are found to produce mini-CMEs.The dimming regions associated with the CMEs are found to be occupied by both the ’dark’ cool plasma and areas of weakened coronal emission caused by the depleted plasma density.This study demonstrates that the small-scale loop structures in the quiet Sun,the evolution of which is deter-mined by their magnetic footpoint motions and/or ambient field topology,evolve into an eruptive phase that triggers the ejection of cool and hot plasma in the corona.