The reactive energy of transient EM fields

We give a physically compelling definition of the instantaneous reactive energy density associated with an arbitrary time-domain electromagnetic field in vacuum. In Heaviside-Lorentz units, where ε0 = μ0 = 1, it is given in terms of the energy density U(r, t) and the Poynting vector S(r, t). The fact that the energy of an EM field in vacuum generally flows at speeds less than c was noted almost a century ago by Bateman [3, page 6]. To the best of my knowledge, this important insight has remained undeveloped and largely unappreciated. I believe this phenomenon, and its relation to reactive energy as detailed, are fundamental features of electromagnetic fields which ought to be studied both theoretically and experimentally.