Coherence constraints on physical parameters at bright radio sources and FRB emission mechanism

We discuss physical constrains that observations of high brightness temperature coherent radio emission, with brightness temperatures as high as $T_b \sim 10^{35}$ K, impose on the plasma parameters at relativistically moving astrophysical sources. High brightness temperatures imply a minimal plasma energy density at the source. Additional important constraints come from the fact that resonantly emitting particles lose most of their energy to non-resonant inverse Compton and synchrotron processes. We also interpret recent observations of high-to-low frequency drifting features in the spectra of repeating FRBs as analogues of type-III Solar radio bursts produced by reconnection plasma beams within magnetospheres of highly magnetized neutron stars.