= Radial solar-wind magnetic flux
{title2=$B_r r^2=\mathrm{constant}$}
<Magnetic flux freezing> in a highly conducting <solar wind> can carry open <magnetic field lines> outward. A nearly radial <flux tube> occupying fixed solid angle has area proportional to $r^2$, so constant <magnetic flux> gives $B_r\propto r^{-2}$. If a field is approximately dipolar inside an opening radius $r_o$ and radial beyond it, its radial component at $r\gg r_o$ is enhanced by order $r/r_o$ compared with continued dipolar $r^{-3}$ decay. Opposite polarities preserve zero net flux through a closed surface; this approximation does not introduce a magnetic monopole.
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