冕流电流片的白光和紫外观测研究

Study of the Streamer Current Sheet with White-light and UV Observations

日冕电流片是日冕磁重联发生的主要区域,这一过程将磁能转化为等离子体的热能和动能.通过选取大角度光谱日冕仪(Large Angle and Spectrometric Coronagraph,LASCO)的白光与远紫外日冕成像光谱仪(Ultraviolet Coronagraph Spectrometer,UVCS)的紫外观测,研究了2003年1月3日观测到的冕流电流片.LASCO C2白光数据显示电流片中的等离子体团在视场中可从60 km·s^-1加速至340 km·s^-1,加速度为60 m·s^-2;假设视向深度为0.3–1.5 R⊙,得到所研究电流片在UVCS狭缝高度处的平均电子数密度约为(1.52–7.60)×10^7cm^-3.对沿UVCS视场狭缝分布的[Fexviii]974A和Lyα谱线强度进行研究,发现电流片处的[Fexviii]谱线强度比周围明显增大,计算得到所研究时段内电流片的电子温度范围为(2.94–4.04)×10^6K;而在电流片处的Lyα谱线强度相对周围变化不大,在电流片内部两侧强度比中心略高,可能的主要原因是电流片内部中心处等离子体的运动速度要比两侧快,这使得中心比两侧有更强的多普勒暗化作用.以UVCS观测的Lyα和[Fexviii]谱线的辐射强度比和计算的电子温度为约束条件,发现当狭缝电流片处等离子体运动速度约为237–254 km·s^-1时,通过理论计算的Lyα和[Fexviii]谱线的辐射发射率比值和观测谱线强度比值相当.在该速度范围内,电流片内部Lyα辐射的碰撞项约为辐射项的42%–57%.此事件中的冕流电流片比通常情形下的冕流电流片中等离子体温度更高、运动速度更大,可能的原因在于其南侧爆发的两个日冕物质抛射促进了电流片中的磁重联过程,更多的磁能释放用于等离子体的加热和加速.所得研究结果可以为我国将要发射的先进天基太阳天文台(Advanced Space-based Solar Observatory,ASO-S)未来的资料处理提供重要参考.

Current Sheets(CSs)are the main region where magnetic reconnections can convert magnetic energy into plasma thermal and kinetic energies.We have studied a CS in the streamer observed on 3 January 2003 by combining the White-Light(WL)images observed by the Large Angle and Spectrometric Coronagraph(LASCO)and the UV spectra detected by the Ultraviolet Coronagraph Spectrometer(UVCS).LASCO C2 WL data showed that the velocity of a blob in the CS increased from 60 km·s^-1 to 340 km·s^-1 with an acceleration of 60 m·s^-2 in its field of view.Assuming the light-of-sight(LOS)depth of 0.3–1.5 R⊙,the average electron number density of the CS was(1.52–7.60)×10^7 cm^-3 at the height of the UVCS slit.We investigated the intensity distributions of the[Fe xviii]974A and Lyαlines along the UVCS slit.It is shown that the intensity of[Fe xviii]line in the CS was significantly higher than those of the surroundings,and the deduced electron temperature range of the CS was(2.94–4.04)×10^6 K during the studied period.However,the intensity of the Lyαline in the CS did not change much when compared with those of the surroundings,and within the CS the intensity on both sides were slightly higher than that in the center.It is possible that the plasma moved faster in the center,and resulted in stronger Doppler dimming effect.Using the observed intensity ratio of Lyαand[Fe xviii]lines observed by UVCS and the calculated electron temperature as constraints,we found that the theoretically calculated emissivity ratio of Lyαand[Fe xviii]lines was close to their observed intensity ratio when the plasma velocity range was 237–254 km·s^-1 at the position of the CS.The collisional component of the Lyαline was about 42%–57%of the radiative component in the CS within the speed range above.The streamer CS we studied had a higher plasma temperature and a faster blob speed than the typical values in normal situations.The possible reason is that the two CMEs at the southern side enhanced the magnetic reconnection process in the CS,and more magnetic energy was released to heat and accelerate plasma.Our results on the CS can be regarded as pre-studies of the data analyses for the future mission of the Advanced Space-based Solar Observatory(ASO-S).