This is associated with the sudden brightening of the aurora in the midnight
sector in what is known as the expansion phase of a substorm. The aurora are caused by intense
energetic particle precipitation which enhances the conductivity in a thin latitudinal strip
across midnight. (We can see this conductivity enhancement in empirical
and model conductivity distributions.) Here the electric field is directed
predominantly westward, associated with the global convection. The patch of enhanced conductivity
feeds back on the imposed electric field to produce an intense ionospheric electrojet current -
the Cowling effect.
This is the DP1 current system.
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