\documentclass[12pt]{article}
\pagestyle{empty}
\begin{document}

\begin{eqnarray*}
 E = & \frac{H}{\sigma_h\sqrt{2\pi}} \exp\left[-\frac{(r-r_{mh})^2}{2\sigma_h^2}\right]
\end{eqnarray*}
\end{document}
