Study Of The Formation And Evolution Of Coronal Plumes With Multi-wavelength Imaging Observations Taken By SDO

Coronal plumes are long bright ray-like structures,which extend from the base of solar coronal holes into the high corona and interplanetary space.Plumes were suggested to be an important source of the solar wind,therefore a further investigation about the formation and evolution of plumes will help us to understand the formation mechanism of the solar wind.Studying of the activities occuring at plume footpoints in the photosphere,chromosphere and corona are important to understand the physics of the formation and evolution of plumes.In the photosphere,previous observations show that magnetic reconnnection between unipolar flux concentrations and nearby small-scale bipoles are involved in the formation of plumes.In the chromosphere and corona,small-scale jet-like and transient brightening events are found near plume footpoints that could create and maintain the plumes.Moreover,the relationship between coronal bright points and plumes is another intriguing problem.It is found that not every plume corresponds to a coronal bright point,and vice versa.Previous studies focused on the existence of relationship between plumes and specific activities of the solar atmosphere.However,no firm conclusion has been given yet.Our study will emphasize on the dynamic evolution process of plumes and the association between plumes and the dynamic plasma phenomenon in the solar atmosphere.Using data observed by AIA and HMI onboard SDO in the chromospheric and coronal passbands,we analyse 15 plumes to study the formation and evolution of plumes and activities occuring near plume footpoints in the photosphere,chromosphere and corona.Firstly,we find that plumes overlie areas of mixed magnetic flux within the dominant unipolar magnetic fields with various brightness,which differs from EIT observation.This could be resulted from higher resolution and better sensitivity of the AIA instrument,Furthermore,by tracing the dynamic evolutionary process of plumes and the corresponding magnetic field,we find that plumes form when unipolar network elements converge toward each other.We also get that the peak of intensity of plumes is followed by the peak of the magnetic flux with a time-delay for a few hours,which indicates that the plume intensity peaks at the moment of the magnetic cancellation.Then,observations by 304A show the link between plumes and nearby chromospheric acitivities,i.e.,spicules and macrospicules.Plumes and spicules appear in the same locations,and the emergence macrospicules may give rise to bright plumes,which result from cancellation of mixed-polarity flux near the base.Macrospicules can support bright plumes for hours despite they erupt only for a few minutes,which conforms with the radiative cooling timescale expected in the corona.Based on the above,we suggest that the formation of plumes is associated with open magnetic field,and small-scale magnetic cancellation is one of the reason which can raise and maintain bright plumes.Finally,we find plumes are closely associated with the bright points.A polar plume and the BP at its base will give rise to the striking bright plumes.However,not every plume corresponds to bright points at its footpoint.In conclusion,our results show that the convergence of open magnetic fields is basic physics of the formation of plumes,plasma activities impacting on the evolution of plumes are associated with cancellation between dominant unipolar flux and opposite polarity flux,these activities may be the macrospicules in the chromosphere or coronal jets resulted from bright points in the corona.