 Solution

Here $E_y^2+E_z^2=9A^2$, so the tip of the electric field traces a circle at constant magnitude: this is circular polarization. At fixed $x=0$, it starts at $+y$ when $t=0$ and initially rotates toward $+z$. This explicit rotation specifies the sense without relying on competing handedness conventions.

Using $\mathbf{B}=v_p^{-1}\hat{\mathbf{x}}\times \mathbf{E}$, with $v_p=\omega /k$, givesIn vacuum replace $v_p$ by $c$. The magnetic flux density also has constant magnitude and is perpendicular to $\mathbf{E}$ at every instant.