It has been suggested that 'transient surface currents' caused by multi-MeV fast electrons can be responsible for the emission of low frequency radiation (e.g. in THz range) from ultra-intense laser–solid interactions. This mechanism has been analyzed, and analytic upper bounds on the intensity, electric field amplitude, and normalized vector potential have been developed and tested against 1D EM Particle-in-Cell simulations. The 'transient surface current' mechanism is effective and sufficiently efficient to fully account for all radiation that has been emitted in experiments so far.