A plane electromagnetic wave \(\mathbf{E}=\mathbf{E}_ {m} \cos (\omega t-\mathbf{k} \mathbf{r})\) propagates in vacuum. Assuming the vectors \(\mathrm{E}_ {m}\) and \(\mathbf{k}\) to be known, find the vector \(\mathrm{H}\) as a function of time \(t\) at the point with radius vector \(r=0\)