The presence of a background magnetic field, as illustrated in Figure 2, can fundamentally alter the instability evolution. These simulations demonstrate this fact even when the initial magnetic field is very weak, so that, for example, the magnetic pressure is small compared to the gas pressure by several orders of magnitude, and the Alfven Mach number, MA  , is much greater than unity.

For example, the Figure 4 below illustrates the flow pattern that develops in a case in which the Alfven Mach number is 12.4; that is, in which the initial magnetic pressure is only 0.5% of the gas pressure. The flow has become almost laminar.

Figure 4.  Vorticity  in a case with MA = 12.4, showing the evolution towards sheet-like flow patterns. The time shown is only slightly later than the analogous time shown in Figure 3. Click on image for larger view. Click here for an animation. (1Mbytes)

 
The magnetic field structure that accompanies the above vorticity distribution is shown below.
 
 

Figure 5.  Magnetic field intensity for the flow illustrated above. Click  on image for larger view. Click here for an animation. (1Mbytes)

 

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