Substorm onset consists of the brightening of the onset arc followed by dipolarization onset and expanding auroral bulge. Plasma pressure in the near-Earth plasma sheet enhances with enhanced dayside reconnection E-field. Precipitations of plasma sheet particles enhance the oval conductance and the auroral electrojet during the growth phase. Auroral electrojet intensifies to Cowling electrojet when the divergence of the northward Hall current is blocked from closure in the equatorial plasma sheet to induce the southward polarization E-field observed in the midnight sector.
Substorm onset arc brightens when the upward field-aligned current density exceeds a few mA/m2 due to divergence of intense Cowling electrojet. Closure of field-aligned currents along Alfvén wavefront completes the azimuthal Cowling electrojet current loop (CECL).
Alfvén wavefront of the CECL incident on the near-Earth plasma sheet disrupts the cross-tail current to form the substorm current wedge - an embodiment of magnetosphere-ionosphere coupling during southward IMF.