Unipolar components travelling at the speed of light in vacuo
  We describe a physical situation where the time integral of the electric field of an electromagnetic wave packet is not zero. More specifically, the non-oscillating component of the Fourier spectrum, i.e. the component resulting from the Fourier transform of the electric field evaluated at $\omega=0$, can be associated with the area under a locally unipolar electromagnetic pulse that travels in free space at the speed of light. 
C. Varin and M. Pich\'e