It is shown by a computer simulation study of the magnetosphere-ionosphere (M-I) coupling that an enhanced magnetospheric convection can lead to auroral substorm onset (e.g., a sudden brightening of an auroral arc) in about 40 min after a southward turning of the IMF. The simulation results show that the enhanced M-I coupling can produce intense upward field-aligned currents in the midnight sector near the poleward boundary of a high-conductance belt associated with the diffuse auroral precipitation. Two necessary conditions for substorm onset are that (1) the polar cap potential must exceed a certain value (about 70 kV in our model) and (2) the convection reversal region must overlap with the poleward gradient of the diffuse auroral conductance in the ionosphere in the midnight sector. The matching ensures that the divergence of the Pedersen current colocates with the divergence of the Hall current to maximize the upward field-aligned current near the poleward boundary of the diffuse aurora in the midnight sector for substorm onset. Without the matching, the auroral brightening would not occur, even if the electrojets are appreciable. Substorm subsides when one of the two necessary conditions is no longer satisfied. This can occur owing either to a northward turning of the IMF or weakening of Alfven shielding due to substorm enhancement of the ionospheric conductance during a prolonged southward IMF. ¿ American Geophysical Union 1988