Under these approximations, the dispersion equation for electromagnetic waves includes two free-space modes that can escape the plasma as radiation (radio waves). These are called the ordinary ( o ) and extraordinary ( x ) modes. The ordinary mode is "ordinary" in the sense that the plasma response is the same as if there were no magnetic field, while the x -mode has a somewhat different refractive index. Importantly, each mode is polarized in opposite senses that depend on the angle with respect to the magnetic field. A quasi-circular approximation generally applies, in which case both modes are 100% circularly polarized with opposite senses.