A comprehensive theoretical analysis of direct Cerenkov excitation of the Earth ionosphere waveguide using ionospheric heating is presented. The model relies on transient ionospheric heating with a heater spot moving horizontally at the bottom of the waveguide with speed close to the speed of light. The cases of isotropic ionospheric conductivity, corresponding to heating altitudes below 70 km, and of anisotropic conductivity, corresponding to higher heating altitudes, are examined separately. It is found that enhanced radiation coupling requires that the speed of the heater approaches the speed of light. For the anisotropic case, such enhancement occurs independently of the direction of motion, while for the isotropic case, motion parallel to the ambient electric field is required.