Solution

Let $ab=L$.By applying the law of electromagnetic induction to the countour $baa'b'$ we get:

\begin{equation} E L=\frac{d\Phi}{dt} \end{equation}

In time $dt$ the wave moves a distance $cdt$.So the change of the magnetic flux is:

\begin{equation} d\Phi=BLcdt \end{equation}

By plugging eq(2) into the first we find that(in SI units):

\begin{equation} B=\frac{E}{c} \end{equation}

In Gaussian units Faraday's law for the same countour looks like:

\begin{equation} EL=\frac{1}{c}\frac{d\Phi}{dt} \end{equation}

So the answer in CGS units is:

\begin{equation} B=E \end{equation}