The presence of ionospheric-origin oxygen ions in the plasma sheet region results in only partial cancellation of the electron Hall current leading to the occurrence of the helicon mode rather than the Alfv¿n mode. It is shown that the presence of ionospheric-origin oxygen ion beams with anisotropic pressure can excite helicon mode instability in the near-Earth plasma sheet region provided their Alfv¿nic Mach numbers lie in a certain range. The helicon modes are easily excited under the conditions when the usual long wavelengths fire-hose modes are stable. The typical real frequencies of the excited helicon modes are between 1 to 10 mHz, and the typical e-folding time of the instability is about 3 to 15 minutes at wavelengths of 1 to 5 RE. Therefore these modes are likely to attain saturation during enhanced convection events lasting for a few hours. Large amplitude helicon modes would distort the ambient magnetic field and may be observable as flux ropes. Low-frequency turbulence produced by these modes could scatter electrons and help excitation of the ion tearing modes leading to substorm onset.¿ 1997 American Geophysical Union