Goldreich–Julian density

Summary

Consider a rotating, magnetized, highly conducting star such as a pulsar. The Goldreich–Julian density is the charge density its magnetosphere must have so that the plasma there corotates with the star. For a star with angular velocity $\Omega$ and magnetic field $B$, it equals $-2\varepsilon_0 \, \Omega \cdot B$ in SI units, up to relativistic corrections. It sets the natural scale for the density of charged particles in pulsar magnetospheres, and comparisons with it decide where gaps and particle acceleration occur.