A Study Of The Early Dynamics And Thermal Properties Of Coronal Mass Ejections

Coronal mass ejections(CMEs)are one of the most spectacular eruptive phenom-ena in our solar system,and have the potential impact on the space environment safety of the earth.In order to forecast the space weather caused by CMEs,it's nessesary to understand their dynamics,especially their origins and early evolution in the low corona.In this paper,we study the formation and early evolution of a limb CME and its associated shock wave that occurred on 2014 January 8.The extreme ultraviolet(EUV)images provided by the Atmospheric Imaging Assembly(AIA)on board Solar Dynamics Observatory disclose that the CME first appears as a bubble-like structure.Subsequently,its expansion forms the CME and causes a quasi-circular EUV wave.Interestingly,both the CME and the wave front are clearly visible at all of the AIA EUV passbands.Through a detailed kinematical analysis,it is found that the expan-sion of the CME undergoes two phases:a first phase with a strong but transient lateral over-expansion followed by a second phase with a self-similar expansion.The temporal evolution of the expansion velocity coincides very well with the variation of the 25-50 keV hard X-ray flux of the associated flare,which indicates that magnetic reconnection most likely plays an important role in driving the expansion.Moreover,we find that,when the velocity of the CME reaches?600 km s-1,the EUV wave starts to evolve into a shock wave,which is evidenced by the appearance of a type ? radio burst.The shock's formation height is estimated to be?0.2Rsun,which is much lower than the height derived previously.Finally,we also study the thermal properties of the CME and the EUV wave.We find that the plasma in the CME leading front and the wave front has a temperature of?2 MK,while that in the CME core region and the flare region has a much higher temperature of ?8 MK.In addition,we find some unusual solar radio signals at?4000 MHz,which are similar to the pattern of a type ? radio burst but drifts to higher frequencies at a rate of?0.3 MHz per second during 7 minutes.Its derived density is?5 × 1010 cm-3 and in-creases slowly with time.Joint imaging observations of hard X-ray(HXR)and extreme ultraviolet(EUV)help to locate the particle accelerating source region and calculate its thermal proprieties,including slowly increasing densities(?5×1010 cm-3)and tem-peratures(?14 MK).The results obtained from joint observations of HXR and EUV are similar to those derived directly from the radio spectrum,implying the possibility of this scenario:plasma blobs that are ejected along the current sheet via magnetic re-connection collide with underlying density fluctuations where particles are accelerated.High frequency observations at several GHz could be able to provide another way for us to deeply understand the local conditions and the physical mechanisms of particle acceleration.