tokens: changing from the inner to the outer an MC based on the fitting results. There is a roughly increasing trend of the tangential velocity away from the axis of the MC that continues in the ambient solar wind. However, this increase is not strictly monotonous, which may be attributed to fluctuations. 3. Possible Dependence of the Viscosity on Solar Wind Parameters 3.1. Data and Method The above analysis has shown some viscosity signature from a rough statistic. Here we study the role of viscosity further through detailed investigation of the change of the tangential velocity crossing the MC boundaries. By manually checking the change of the 1 minute averaged tangential velocity profile near the front/rear boundaries of the MCs, we further narrow down the sample by selecting the crossings during which the tangential velocity clearly increases from the inner to the outer the MC. Finally, nine crossings that we think represent the picture fairly well in Figure 1 are selected and shown in Figure 3 . A few data points indicated by the red symbols in Figures 3 (a), (b), and (d) are discarded in the following analysis because they deviate significantly away from the main trend of the crossings due to inevitable fluctuations. Figure 3. Change of the tangential velocities with time. Panels (a)–(d) show the change of the tangential velocities at the front boundary of the MC, and (e)–(i) at the rear boundary. The verticle solid line indicates the boundary of the MC. The ordinate axis is the tangential velocity, where the positive value means the anti-clockwise rotation and the negative value means the clockwise rotation. It is not clear how to obtain the viscous coefficient in the solar wind. Alternatively, we use the following formula to roughly assess the strength of the viscosity between the ambient solar wind and MC plasmain which is assumed to be a function of the viscous coefficient , and and are averaged tangential speeds in MC and solar wind, respectively. The formula follows the idea that if there is no viscosity should approach zero as well as , and if there is a strong viscosity should be equal to , leading to be infinitely large. Considering that there is more or less uncertainty in determining the boundary of an MC, we calculate the values of and based on the measurements within one and a half hours next to the boundary with the nearest half an hour ignored. The uncertainty in is propagated from the uncertainties in the averaged tangential velocities and . The averaged solar wind parameters, e.g., density, temperature, and turbulence intensity of the magnetic field, inside and outside of the boundary are also calculated within the same intervals to investigate the possible dependence of the strength of the viscosity between the ambient solar wind and MC plasma. 3.2. Results The scatter plots between and solar wind parameters inside and outside of the MC are shown in Figure 4 . The turbulence intensity of the magnetic field is estimated based on
ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-EntityOfFutureInterest I-EntityOfFutureInterest O O O O O O O O O O O O O O O O O O O O B-EntityOfFutureInterest I-EntityOfFutureInterest O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-EntityOfFutureInterest I-EntityOfFutureInterest O O O O O O O O O O O O O O O O O O O O O O O O B-EntityOfFutureInterest I-EntityOfFutureInterest O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-EntityOfFutureInterest I-EntityOfFutureInterest O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-EntityOfFutureInterest I-EntityOfFutureInterest O O O O O O O O O O B-EntityOfFutureInterest I-EntityOfFutureInterest O O O O O O O O O O O O O O O O O O O O O O O O