  Intense electric currents called electrojets occur in weakly ionized
magnetized plasmas. An example occurs in the Earth's ionosphere near the
magnetic equator where neutral winds drive the plasma across the geomagnetic
field. Similar processes take place in the Solar chromosphere and MHD
generators. This letter argues that not all convective neutral flows generate
electrojets and it introduces the corresponding universal criterion for
electrojet formation, $\nabla\times
(\vec{U}\times\vec{B})\neq\partial\vec{B}/\partial t$, where $\vec{U}$ is the
neutral flow velocity, $\vec{B}$ is the magnetic field, and $t$ is time. This
criterion does not depend on the conductivity tensor, $\hat{\sigma}$. For many
systems, the displacement current, $\partial\vec{B}/\partial t$, is negligible,
making the criterion even simpler. This theory also shows that the
neutral-dynamo driver that generates electrojets plays the same role as the DC
electric current plays for the generation of the magnetic field in the
Biot-Savart law.
