Displacement current and Faraday's Law

An alternating current I$=$ Io cos$($wt$)$ flows down a long straight wire $($oriented along

the z$-$axis$),$ and returns along a coaxial conducting tube of radius a$.$ Assuming a

quasi$-$static case, the resulting magnetic field iJ is circumferential, so that the induced

electric field E is longitudinal. The magnetic field inside the tube shall be given by

fl$=$of e