A new resistive, low-frequency (ω≪ωba∼ν a, where ω is the frequency of the mode and ωba and νa are the bounce frequency and collision frequency for species a) instability is found in a tokamak geometry. The mode is driven unstable by the parallel viscous (bootstrap) current induced by the radial pressure gradient. It has a growth rate γ∼(ω *e2νi)1/3 for νi≫ω*e, where ω*e is the electron diamagnetic drift frequency and νi is the ion-ion collision frequency. The frequency dependence of the parallel viscosity is also derived and utilized to show that drift wave branches of these modes are stable, but that this new mode remains unstable when the diamagnetic drift effects are included.
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