Why are halo coronal mass ejections faster

• Published in: Research in astronomy and astrophysics 2010-05, Vol.10 (5), p.461-472
• Main Authors: Zhang, Qing-Min, Guo, Yang, Chen, Peng-Fei, Ding, Ming-De, Fang, Cheng
• Format: Article
• Language: English
• Subjects: CME事件; 光强度; 平均流速; 平均速度; 日冕物质抛射; 汤姆逊散射; 蒙特卡罗模拟; 观测特性
• ISSN: 1674-4527; 2397-6209
• DOI: 10.1088/1674-4527/10/5/006

Halo coronal mass ejections （CMEs） have been to be significantly faster than normal CMEs, which is a long-standing puzzle. In order to solve the puzzle, we first investigate the observed properties of 31 limb CMEs that clearly display loopshaped frontal loops. The observational results show a strong tendency that slower CMEs are weaker in white-light intensity. Then, we perform a Monte Carlo simulation of 20000 artificial limb CMEs that have an average velocity of ～523km s -1. The Thomson scattering of these events is calculated when they are assumed to be observed as limb and halo events, respectively. It is found that the white-light inten-sity of many slow CMEs becomes remarkably reduced when they turn from being viewed as a limb event to being viewed as a halo event. When the intensity is below the background solar wind fluctuation, it is assumed that they would be missed by coronagraphs. The average velocity of ＂detectable＂ halo CMEs is ～922km s -1, very close to the observed value. This also indicates that wider events are more likely to be recorded. The results soundly suggest that the higher average velocity of halo CMEs is due to that a majority of slow events and some of narrow fast events carrying less material are so faint that they are blended with the solar wind fluctuations, and therefore are not observed.