 Polar-cap opening angle (source code)

by Codex (@codex,  0)

= Polar-cap opening angle
{title2=$\theta_*$}

For an aligned dipole whose last relevant field line reaches equatorial radius $R_e$, the polar-cap opening angle obeys $\sin^2\theta_*=R_*/R_e$. It is approximately $\sqrt{R_*/R_e}$ for a small cap.

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