  In this Letter, we theoretically demonstrate that a uniform static electric
field distribution can be partially converted to radiation fields when a
portion of the medium undergoes a temporal change of its permittivity. An
in-depth theoretical investigation of this phenomenon is developed for a
dielectric block with a step-like temporal change located inside a waveguide
charged with a DC voltage source. Closed analytical expressions are derived for
the radiated electric and magnetic fields. The exchange of energy between the
electrostatic and electromagnetic fields is discussed. The reconciliation
between the seemingly contradictory temporal and spatial boundary conditions
for the electric and magnetic fields at the interface of the time-varying
dielectric block is analyzed and elucidated. Our findings may provide an
alternative solution for generating electromagnetic radiation based on
time-varying media.
