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Fig. 3

image

Snapshot from simulation 1 after reaching the stationary regime. On the left panel, four field lines are shown and colour-coded according to the field-aligned Alfvén speed travel time from the apex of the line divided by the rotation period. Field lines are increasingly twisted as field-aligned Alfvén travel time increases. As the system is symmetric with respect to the equatorial rotation plane, only the upper hemisphere is shown. Neither magnetic field lines nor streamlines cross the equator. Colour contours of the fluid velocity in the equatorial plane illustrate the modulation of the plasma flow by the rotating dipole as well as the field lines lagging effect at large radial distances where the co-rotation velocity exceeds the Alfvén velocity. On the right panel, the flow velocity vx, the sound speed and the fast-mode propagation velocity along the x-axis (the spin axis) are plotted as a function of x. In the simulation the flow thus reaches a terminal speed of the order of the fast speed.

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