Magnetic wave is directed along the negative z-direction.
Thus,
$B_{z}=B_{0} \cos (k y+\omega t)^{2} k=\left{(10.3 \mathrm{nT}) \cos \left[(1.8 \mathrm{rad} / \mathrm{m}) \mathrm{y}+\left(5.4 \times 10^{6} \mathrm{rad} / \mathrm{s}\right) \mathrm{t}\right]\right} \mathrm{k}^{\wedge}$