Surface Barrier in Type 2 Superconductors

[Drew Drowden]

If a field line is moved into a superconductor it meets resistance. This resistance is modeled by adding an image flux thread outside the superconductor which then attracts the flux thread being moved into the superconductor. Why is this attractive force an accurate model of the resistance a field line experiences entering a superconductor?

 

[Drew Drowden]

An image thread is necessary to satisfy the boundary condition that a current can't flow perpendicular to the surface of the superconductor . An image thread outside the superconductor somehow satisfies this boundary condition. I don't fully understand the boundary condition or why the image thread is necessary.