Transverse rotation of the instantaneous field distribution and the orbital angular momentum of a light beam

The transverse beam pattern, usually observed in experiments, is a result of time averaging of the optical-frequency oscillations of the electromagnetic field distributed over the beam cross section. An analytical 'instant rotation criterion' (IRC) is derived to identify that these oscillations are coupled with a sort of rotation around the beam axis. For monochromatic beams, this criterion appears to be proportional to the usual expression of the beam orbital angular momentum (OAM). The relation between the IRC and the OAM also exists in the non-monochromatic cases but in a more complicated form.