[iaw]

Without having studied the equations, but having seen similar phenomenon occur in other work my guess would be that the equation to determine the total area of the magnetosphere (let's say A = M(V,r) where A is the area and V is applied voltage and r is the distance from the sun) includes a component based on the density of the solar wind at that position.

The component is likely in the denominator because as solar wind density goes down we'd expect the magnetosphere size to increase.

Now, the solar pressure is something like P=S(r) where P is the pressure and r is the distance from the sun and S contains some geometry and solar power terms.

If we look at M(V) we could probably then find an M'(V) s.t. M(V,r) = M'(V) / S(r). To get the force produced by the drive we'd take M(V,r) * S(r) which cancels out all radius terms.

Mathematically this would indicate that the thrust on the magnetosphere would be invariant for all r within the solar system (after that point other terms within S() and M() that are based on solar output would likely start to break down)