We relate measurements made during substorm growth and expansion to available plasma compositional observations and propose that asymmetries in the distribution of enhanced densities of O+ (of ionospheric origin) may define regions of the plasma sheet in which tearing mode growth rates are increased and the instability threshold is lowered. We make qualitative estimates of the growth rates of the linear ion tearing mode using statistical models of the dusk-dawn and earthward-tailward distributions of O+. The theoretical results predict maximum ion tearing growth rate in the range XGSM=-10 to -15 RE and YGSM~5 RE. These values are in reasonable accord with substorm observations since many asymmetries in particle and field phenomena associated with substorms favor the interpretation that substorm onset occurs in the dusk sector of the near-earth plasma sheet. Present results therefore suggest that substorm initiation and development may be closely related to distribution patterns of ionospheric ions.