  The THz radiation emitted from rectangular mesas of single-crystal
$\mathrm{Bi_2Sr_2CaCu_2O_{8+\delta}}$ was studied using angular distribution
measurements and Fourier transform infrared spectroscopy. Unlike the recent
theoretical predictions, the results provide strong evidence for a dual-source
mechanism in which the uniform and non-uniform parts of the $ac$-Josephson
current act as electric and magnetic current sources, respectively. The latter
synchronizes with cavity modes of the mesa with integral harmonics of the
fundamental radiation.
