It's simple rotary wing aerodynamics, at work.
Rotary wing aerodynamics shows us that when blades rotate, it creates induced flow through the rotor disc, forcing air downward. To fixed wing guys... it's akin to "prop wash", but we call it "rotor wash."
Now lets look at the photo. The rotor blades are rotating counter-clockwise, and if you look... you'll see that the vortices being created by the rotating blades, are also rotating in the same counter-clockwise motion. If you look on the right side of the aircraft (starboard side), the blades are pushing that vortex up against the main fuselage... which serves to "contain" that vortex. In comparison, the left side (port side) has no fuselage to push the vortex against, so that vortex is larger and more spread out. Also... notice that the left vortex is more forward of the aircraft, whereas the right vortex is more aft of the aircraft. This is simply because of the direction of the rotating blades, which "pushes" those vortices in their direction.
If this aircraft was out of ground-effect, and maintaining a higher hover, then the two vortices will spill out, becoming larger, and eventually blending together.
What's interesting, is that at a hover, and especially with high power applications, you can feel the vortices buffeting against the side of the aircraft, and making a slight rocking motion.
Hope that helped.
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