SOHO/SUMER Observations of Transition Region Explosive Events in Prominence

Explosive events (EEs) are small-scale dynamic phenomena often observed in the solar transition region (TR). EEs are characterized by non-Gaussian and broad profiles with enhancements in the blue/red wings with an average line-of-sight Doppler velocities of ～100 km·s-1. They have a small spatial scale of about 1 800 km and a short lifetime of about 60 s on average. EEs are often found to be associated with magnetic cancellation and reveal bi-directional flows with high velocities comparable to the local Alfvén velocity; they are generally regarded as the consequence of small-scale fast magnetic reconnections. Since the launch of SOHO spacecraft, the SUMER (solar ultraviolet measurements of emitted radiation) spectrograph has been widely used to study EEs. With high spatial and spectral resolution, and wide spectral coverage, SUMER was a powerful tool of ultraviolet spectroscopy and it has greatly increased our knowledge of EEs. Relationship between EEs and other small-scale events observed in the transition region, such as blinkers and EUV spicules have also been investigated during the SOHO era. However, the association between EEs and large-scale events such as prominence remains unclear. A sit-and-stare mode observation made by SUMER is selected for the study. We investigate the properties of EEs in a prominence. EEs are identified with analysis of the width of Si Ⅲ line (111.3 nm). The Si Ⅲ lines with a width greater than three standard deviations (3σ) were singled out for further visual inspection to finally determine the occurrence of EEs. It is found that the vast majority of explosive events concentrate in the bright knots of the prominence. EEs in the core of the prominence occur repetitively with a period of about 20 mins. It is proposed that the explosive events caused by small-scale fast magnetic reconnections are triggered by magnetic flux loops in the core of prominence. The blue shift of the explosive events is significant and possibly related to the initiation of a CME.