  We found an exact analytical solution of the wave equation for a transient
electromagnetic field of synchrotron radiation in the frequency domain. The
exact solution represents the field which consists of the coherent and
incoherent components of synchrotron radiation and the space charge field of
the particle beam moving in a bending magnet. The field in the time domain is
gotten by numerically Fourier transforming the values of the field calculated
using the exact solution. The beam has an arbitrary charge density and current
density which satisfy the equation of continuity. The beam is moving in a
perfectly conducting rectangular pipe which is uniformly curved in a
semi-infinite bending magnet. The exact solution is not self-consistent, i.e.,
this is an exact expression of the field for a given beam current. We do not
solve the equation of motion of the beam in the present paper. On the basis of
the exact expression of the field found in the present study, we discuss the
applicability and accuracy of the paraxial approximation which is sometimes
used to calculate the field and spectrum of coherent or incoherent synchrotron
radiation.
