A long coaxial cable consists of a thin cylindrical inner conductor of radius \(a\) and a concentric thin cylindrical outer conductor of radius \(b\), where \(a < b\). The inner conductor carries a uniformly distributed current \(I\) in one direction, and the outer conductor carries an equal current \(I\) in the opposite direction. Which of the following integral expressions correctly gives the total magnetic energy \(U\) stored in the vacuum region between the conductors (\(a < r < b\)) for a section of length \(\ell\)?