\item $\sigma_{\perp} = \alpha \rho_0$, $\alpha \ll 1$\\
$\sqrt{\rho_0^2+\sigma_{\perp}^2}=
\sqrt{\rho_0^2+\alpha^2\rho_0^2}=
- \rho_0\sqrt{1+\alpha^2}=
+ \rho_0\sqrt{1+\alpha^2}\stackrel{Taylor}{=}
\rho_0(1+\frac{\alpha^2}{2}-\frac{\alpha^4}{8}+\ldots)$\\
$\Rightarrow \Phi-\Phi_0=
\frac{D}{2}\left[\rho_0^2\left(2+\alpha^2-