We can find the energy flux density of the radiation field by computing its Poynting vector : S = c 4 π π E a × × B a , where the 'a' subscripts emphasize that we are taking only the acceleration field. Substituting in the relation between the magnetic and electric fields and simplifying gives, in the non-relativistic case, S = q 2 4 π π c | n × × ( n × × β β ˙ ˙ ) R | 2 n .