Observations of the field-aligned residual flow inside magnetic cloud structure

In this paper, we report two MC events observed by WIND spacecraft with good examples of field-aligned residual flow inside the MC structure. For both events, the co-moving frames are determined through the deHoffman-Teller (HT) analysis and the axial orientations are inferred by the newly developed minimal residue (MR) method. The nature coordinate system for both events are constructed with velocity of the HT frame and the inferred MC axis, the field and flow remaining in the HT frame are analyzed at this coordinate system. As a result, we find that the residual flows in the co-moving HT frame of the two MC events are almost anti-parallel to the helical magnetic field. We speculate that the field-aligned residual flows are large scale coherent hydrodynamic vortices co-moving with the MCs at the supersonic speed near 1 AU. Data analyses show that the event in slow ambient solar wind is expanding at 1 AU and another one in fast solar wind does not show apparent expansion. Proton behaviors for both events are quasi-isothermal. Accelerated HT analysis shows that both events have no suitable HT frame with constant accelerations, which suggests that both events may be moving at the constant speed near 1 AU under the assumptions of the HT analysis. For both events, the ratio of the dynamic pressure to the magnetic pressure is larger than that of the thermal pressure to magnetic pressure, which suggests that the dynamic effects due to the plasma flows remaining in the co-moving HT frame are more important than the thermal effects in the study of MC evolution and propagation. Supported by the National Natural Science Foundation of China (Grant Nos. 40621003, 40804046, 40536029, 40674084 and 40523006), the National Basic Research Program of China (“973” Project) (Grant No. 4902006CB806304), the Specialized Research Fund for State Key Laboratories, and the Open Research Program of the State Key Laboratory of Space Weather, Chinese Academy of Sciences