delta3,
I'm not totally convinced. The rotor downwash is blown downwards, and then deflects horizontally. Something has to act on the air to change its velocity. The ground doesn't do it...that would lead to upwards air. In the end, you need air pushing air away from center. I'd say a pressure gradient would be the culprit.
Just because I think there is a gradient, I'm not saying there is a 'ball of air' as discussed above. Hard numbers would be required to determine the magnitude and extent of this gradient.
Matthew.