On the synchro-curvature radiation

It is generally believed that the radiation of relativistic particles in a curved magnetic field proceeds in either the synchrotron or the curvature radiation modes. We study an intermediate, the so-called synchro-curvature regime. Using the Hamiltonian formalism, we find the trajectory of relativistic particle in the curved magnetic field that has the form of a helix wound around the drift trajectory. We show that the transition to the limit of the synchrotron and curvature radiation regimes is determined by relation between curvatures of the drift trajectory and gyration motion around it. When both are approximately equal, the curvature of the trajectory start to noticeably oscillate with the frequency of particle gyration leading to the synchro-curvature radiation with averaged spectrum different from synchrotron or curvature one. The performed numerical calculations with energy losses and different initial pitch angles confirm the principal conclusions based on the simplified analytical treatment.