[论文解读] The Relationship Between the Expansion Speed and Radial Speed of CMEs Confirmed Using Quadrature Observations of the 2011 February 15 CME

N. Gopalswamy, P. Mäkelä+2 more|arXiv (Cornell University)|May 3, 2012

Solar and Space Plasma Dynamics被引用 4

一句话总结

本研究利用2011年2月15日STEREO与SOHO的四眼观测数据，验证了日冕物质抛射（CME）的膨胀速度（Vexp）与径向速度（Vrad）之间的经验关系。通过STEREO-B测量到CME的半角宽度（w = 38°），推导出Vrad = 1.14 × Vexp，得到的径向速度为1023 km/s，与STEREO-A（945 km/s）和STEREO-B（1058 km/s）的直接测量值高度一致，几何模型的误差为3.4%–7.6%，验证了其有效性。

ABSTRACT

It is difficult to measure the true speed of Earth-directed CMEs from a coronagraph located along the Sun-Earth line because of the occulting disk. However, the expansion speed (the speed with which the CME appears to spread in the sky plane) can be measured by such a coronagraph. In order to convert the expansion speed to radial speed (which is important for space weather applications) one can use an empirical relationship between the two that assumes an average width for all CMEs. If we have the width information from quadrature observations, we can confirm the relationship between expansion and radial speeds derived by Gopalswamy et al. (2009a). The STEREO spacecraft were in qudrature with SOHO (STEREO-A ahead of Earth by 87o and STEREO-B 94o behind Earth) on 2011 February 15, when a fast Earth-directed CME occurred. The CME was observed as a halo by the Large-Angle and Spectrometric Coronagraph (LASCO) on board SOHO. The sky-plane speed was measured by SOHO/LASCO as the expansion speed, while the radial speed was measured by STEREO-A and STEREO-B. In addition, STEREO-A and STEREO-B images provided the width of the CME, which is unknown from Earth view. From the SOHO and STEREO measurements, we confirm the relationship between the expansion speed (Vexp) and radial speed (Vrad) derived previously from geometrical considerations (Gopalswamy et al. 2009a): Vrad = 1/2 (1 + cot w)Vexp, where w is the half width of the CME. STEREO-B images of the CME, we found that CME had a full width of 76o, so w = 38o. This gives the relation as Vrad = 1.14 Vexp. From LASCO observations, we measured Vexp = 897 km/s, so we get the radial speed as 1023 km/s. Direct measurement of radial speed yields 945 km/s (STEREO-A) and 1058 km/s (STEREO-B). These numbers are different only by 7.6% and 3.4% (for STEREO-A and STEREO-B, respectively) from the computed value.

研究动机与目标

• 验证基于几何考虑推导出的CME膨胀速度与径向速度之间的经验关系。
• 检验是否可利用依赖宽度的公式，从膨胀速度准确推断径向速度。
• 通过改进单点地球基观测中的CME速度估计，降低空间天气预测的不确定性。
• 利用STEREO与SOHO的四眼几何关系，独立测量CME的宽度与径向速度。

提出的方法

• 利用SOHO/LASCO日冕仪，从地球视角测量CME在天球平面的膨胀速度（Vexp）。
• 采用STEREO-A与STEREO-B通过立体成像技术直接测量CME的径向速度（Vrad）。
• 从STEREO-B图像中测量CME的全宽，确定其半角宽度（w）为38°。
• 应用几何公式Vrad = ½(1 + cot w)Vexp，将Vexp转换为预测的Vrad。
• 将预测的Vrad与STEREO-A和STEREO-B的直接测量值进行比较，评估预测精度。
• 利用四眼几何关系（STEREO-A领先87°，STEREO-B滞后94°）确保观测的独立性与互补性。

实验结果

研究问题

• RQ1 经验关系式Vrad = ½(1 + cot w)Vexp是否能准确预测地球方向CME的径向速度？
• RQ2 能否通过四眼观测获得的CME半角宽度（w）提升从膨胀速度推算径向速度的精度？
• RQ3 与直接立体测量相比，利用几何模型预测的径向速度准确度如何？
• RQ4 依赖宽度的公式在多大程度上减少了空间天气预测中CME速度预报的不确定性？

主要发现

• 从STEREO-B图像测得CME的半角宽度（w）为38°，使Vrad = 1.14 × Vexp关系中的宽度因子为1.14。
• SOHO/LASCO测得的膨胀速度（Vexp）为897 km/s，由此预测的径向速度为1023 km/s。
• STEREO-A与STEREO-B的直接径向速度测量值分别为945 km/s与1058 km/s，与预测值的偏差分别为7.6%与3.4%。
• 3.4%–7.6%以内的吻合度证实了该几何模型在径向速度估计中的有效性。
• 本研究表明，四眼观测显著提升了从膨胀速度预测CME径向速度的准确性。
• 结果支持将Vrad = ½(1 + cot w)Vexp公式作为空间天气预报中可靠的工具使用。