An infinitely long, thin, field‐reversing ion layer immersed in a much denser, cooler background plasma is examined for instabilities driven by the velocity difference between the layer ions and the background plasma, taking into account the radial mode structure, as well as Landau damping by the background plasma. The only instability is the low frequency (&ohgr;≪&OHgr;i) magnetosonic mode; the worst mode has growth rate &ggr;∼&ohgr;∼vA/a, andk‐‐∼k⊥∼a−1, whereais the width of the layer. This mode might be stabilized by electron Landau damping and transit‐time magnetic damping, when the temperature of the layer ions is comparable to their energy.