Abstract
Although the dynamical evolution of magnetic clouds (MCs) has been one of the foci of interplanetary physics for decades, only few studies focus on the internal properties of large-scale MCs. Recent work by Wang et al. (J. Geophys. Res.120, 1543, 2015) suggested the existence of the poloidal plasma motion in MCs. However, the main cause of this motion is not clear. In order to find it, we identify and reconstruct the MC observed by the Solar Terrestrial Relations Observatory (STEREO)-A, Wind, and STEREO-B spacecraft during 19 – 20 November 2007 with the aid of the velocity-modified cylindrical force-free flux-rope model. We analyze the plasma velocity in the plane perpendicular to the MC axis. It is found that there was evident poloidal motion at Wind and STEREO-B, but this was not clear at STEREO-A, which suggests a local cause rather than a global cause for the poloidal plasma motion inside the MC. The rotational directions of the solar wind and MC plasma at the two sides of the MC boundary are found to be consistent, and the values of the rotational speeds of the solar wind and MC plasma at the three spacecraft show a rough correlation. All of these results illustrate that the interaction with ambient solar wind through viscosity might be one of the local causes of the poloidal motion. Additionally, we propose another possible local cause: the existence of a pressure gradient in the MC. The significant difference in the total pressure at the three spacecraft suggests that this speculation is perhaps correct.
Highlights
Magnetic clouds (MCs) as a considerable subset of interplanetary manifestations of coronal mass ejections (CMEs) have been studied for many years, since the first identification by Burlaga et al (1981)
The authors raised three possible explanations: i) it might be locally generated through the interaction with the solar wind; ii) it might be internally generated by the expansion of magnetic clouds (MCs), during which magnetic energy is converted into kinetic energy, including the rotational component; and iii) it might be initially generated at the eruption of the corresponding CME, and carried all the way to 1 AU
We examined the MCs that occurred during February 2007 to June 2008, when the Solar Terrestrial Relations Observatory (STEREO) spacecraft were separated by about 1◦ to 58◦, and we found that the best case for our study is the MC on 19 November 2007, when the spacecraft were separated by 41◦
Summary
Magnetic clouds (MCs) as a considerable subset of interplanetary manifestations of coronal mass ejections (CMEs) have been studied for many years, since the first identification by Burlaga et al (1981). Abundant observational evidence has shown that MCs may experience expansion (Burlaga and Behannon, 1982; Klein and Burlaga, 1982; Farrugia et al, 1992, 1993; Lepping et al, 2002; Wang, Du, and Richardson, 2005; Gulisano et al, 2010), latitudinal and longitudinal deflection (Wang et al, 2002, 2004, 2006a, 2014; Kilpua et al, 2009; Rodriguez et al, 2011; Isavnin, Vourlidas, and Kilpua, 2013), and/or rotation (Wang et al, 2006b; Yurchyshyn, 2008; Yurchyshyn, Abramenko, and Tripathi, 2009; Vourlidas et al, 2011; Nieves-Chinchilla et al, 2012) when they propagate away from the Sun in the interplanetary medium In addition to these motions, the “poloidal plasma motion” here refers to the motion of a velocity component along the poloidal direction, i.e., around the MC axis, that has recently been found in the moving MC frame for many MCs by Wang et al (2015).
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