I do not understand how a particular parasitic effect is model. I have read that on a coaxial cable there is a problem due to the fact that the external conductor's magnetic field crosses the air around it. This can be represented by a parallel inductance, as shown in the following figure.
But I do not understand the physical phenomenon (and so why we put an inductance in parallel). A part of magnetic field lines go in the air, and so? What does it happen physically?