Mechanism Of Plasma Poloidal Motion In Interplanetary Magnetic Clouds

Coronal mass ejection(CME)is the most violent eruption activity in the solar atmosphere and the largest scale eruption phenomenon in the solar system,in which it can rapidly release a huge amount of mass carrying magnetic field into the interplanetary space.The magnetic clouds(MCs)as a considerable subset of interplanetary manifestations of the CMEs are testified that their large south magnetic field is the major cause of the large geomagnetic storms,hence,as a main driver for many disaster space weather events,the dynamical evolution of MCs in interplanetary space is an important research topic all the long.However,the people pay more attention to the dynamical evolution of MCs in interplanetary space and do little research on the plasma motion in the MC all the long.Because of the large scale structure of the MC,we only know about the local characteristic of the plasma motion in the MC with the help of local spacecraft.Furthermore,owing to the effect of the turbulence of the magnetic field in the MC,some motion may be covered by the large fluctuation.The poloidal plasma motion',here refers to that a velocity component along the poloidal direction,i.e.,around the MC's axis,was found in the moving MC frame for many MCs recently by Wang et al.(2015).However,what's the reason causing this motion?What's the relation between this reason and the environment?In this paper,we will talk about the reason causing the plasma poloidal motion and the relation between this cause and the solar wind parameters,and we hope to advance our further understanding of the dynamical evolution of MCs from the sun to the interplanetary space.1.The cause of the poloidal plasma motion inside a magnetic cloudWang et al.(2015)recently found the plasma poloidal motion in many M-Cs.For such motion,they raised three possible explanations as follows:(1)it might be generated locally through the interaction with solar wind;(2)it might be generated internally from the sun to the interplanetary space,during which magnetic energy is converted into kinetic energy including the rotational compo-nent;and(3)it might be generated initially at the eruption of the corresponding CME,and is carried all the way to 1 AU owing to the conservation of the angular momentum.In order to find out the cause of the poloidal plasma motion in MCs,we employ multi-spacecraft(STEREO-A,Wind and STEREO-B)observations to study the same MC.With the aid of the velocity-modified cylindrical force-free flux-rope model,we make the reconstruction of the MC at the three spacecraft and make a research on the plasma velocity component in the plane perpendicu-lar to the MC's axis.It is found that there was evident poloidal motion at Wind and STEREO-B but this was not clear at STEREO-A,and different poloidal mo-tion at three spacecraft suggest that there may be a local cause for the poloidal plasma motion inside the MC.We find that the rotational direction inside the MC is consistent with that of the ambient solar wind at both the front and rear boundaries.When the rotational directions of the tangential velocities in the so-lar wind are consistent,there is an evident poloidal motion in the MC,however,if the rotational directions of the tangential velocities in the solar wind are not consistent,there is no evident poloidal motion.These results supports the idea that the interaction between the solar wind and the MC is possibly a cause of the poloidal plasma motion inside the MC.At the same time,we find the tangential velocities at the two sides of the MC boundary are roughly correlated,and the rotational speeds of the ambient solar wind are overall larger than those of the MC plasma except for the STEREO-B.This is the characteristic of the viscosity.Hence,the interaction with the solar wind through viscosity might be only one of the local causes of the poloidal motion in the MC.2.The role of viscosity in causing the plasma poloidal motion in magnetic cloudsViscosity is an intrinsic property of the solar wind.The importance of viscos-ity in the solar wind has been demonstrated in the studies of CME's dynamical evolution,e.g.,the aero-dynamic drag force is a manifestation of the viscosity,which may dominant the dynamic evolution of ICMEs in interplanetary space in the prediction model about the CME arriving time.We try to study the effect of the viscosity causing the poloidal motion in the MC.If the viscosity is a cause of the plasma poloidal motion in MCs,the tangential velocities of plasma in the MC frame should gradually increase from inside to outside of the MC.We select 35 events satisfying the consistency between the direction of the poloidal motion in the MC and ambient solar wind and |vp| ? 10kms-1 from all the crossings(186),and get the average value of the plasma poloidal velocity every 30 minutes before and after the boundary of the MC.The statistical research shows that there are high pairwise correlations between the rotational velocity of the MC and solar wind plasma,furthermore,the fitting slope from the pairwise data indicates that the ambient solar wind rotational velocity is larger than the MC plasma poloidal velocity.There is a roughly non-monotonous increase tendency of the tangential velocity away from the axis of the MC that continues in the ambient solar wind,suggesting that the viscosity does play a role on the poloidal motion though the ratio of these events is only 19%.Hence,the viscosity played a minor role causing on the poloidal motion in the MC.Then,what is the relation between the viscosity and the solar wind param-eters,e.g.,density,temperature and turbulence intensity of the magnetic field?By checking the change of averaged tangential velocity profile near the front/rear boundaries of the MCs,9 crossings that we think fairly represent the picture that the tangential velocity clearly increase from inner to outer of the MC are choosed.It is found that there is a clear negative correlation between the viscosity and the density,a weak negative correlation between the viscosity and the turbulence strength and no evident correlation between the viscosity and the temperature.The physical mechanism behind this phenomenon is worthy of further studying.