It is shown that the presence of ionospheric O+ ions in the inner central plasma sheet (ICPS) during growth phase of the substorms can excite nonresonant low-frequency, long wavelength RH mode instability under certain conditions. For the ICPS parameters during enhanced convection events, the excited modes have real frequencies of (0.1--5.0) mHz, and the typical maximum growth rates of ~(0.1--4.0) mHz with corresponding wavelengths of ~(1--30) RE (RE being the Earth's radius). The excited low-frequency modes are expected to saturate to the maximum level of ΔB2/B20≈(2.5¿10-3-0.25), where ΔB and ΔB0 are the fluctuating magnetic field associated with the unstable modes, and the equilibrium magnetic field respectively. The large amplitude magnetic field fluctuations due to the nonresonant low-frequency instability driven by O+ ions could twist the ICPS magnetic field into flux ropes, and also produce localized minima, in the magnetotail magnetic field near the neutral axis, which can be the potential sites for the excitation of the tearing modes leading to substorm onset. ¿ American Geophysical Union 1995