## Calculus (3rd Edition)

$\phi_0=0$, $\phi_0=\pi$
In spherical coordinates, we have \begin{aligned} &x=\rho \sin \phi \cos \theta\\ &y=\rho \sin \phi \sin \theta\\ &z=\rho \cos \phi \end{aligned} When $\phi_0=0$, we get $x=0,y=0, z=\rho$, which is a half-line. When $\phi_0=\pi$, we get $x=0,y=0, z=-\rho$, which is a half-line.