  It is shown, by simulation and theory, that circularly or elliptically
polarized terahertz radiation can be generated when a static magnetic (B) field
is imposed on a gas target along the propagation direction of a two-color laser
driver. The radiation frequency is determined by
$\sqrt{\omega_p^2+{\omega_c^2}/{4}} + {\omega_c}/{2}$, where $\omega_p$ is the
plasma frequency and $\omega_c$ is the electron cyclotron frequency. With the
increase of the B field, the radiation changes from a single-cycle broadband
waveform to a continuous narrow-band emission. In high-B-field cases, the
radiation strength is proportional to $\omega_p^2/\omega_c$. The B field
provides a tunability in the radiation frequency, spectrum width, and field
strength.
