  Dark photons may kinetically mix with our photon and modify Maxwell's
equations. We examine their impact on the Lorentz force acting on the plasma in
astrophysical magnetospheres. Solving the relevant magnetohydrodynamic
equations, we show that any dark photon effect is negligible. The existing
bound on dark photons with a mass below $10^{-14}$ eV, coming from Pioneer 10
observations of Jupiter's magnetosphere, relies on such an effect and is thus
unreliable. To rectify this, we use recent Juno analyses to isolate any dark
photon-induced modification of the Jovian magnetic field and hence place a
numerically similar limit.
