Abstract Satellite observations during ionospheric modification experiments have reported trans‐ionospheric high‐frequency (HF) wave propagation even when the pump frequency lies below the peak plasma frequency. Such behavior is often attributed to large‐scale density irregularities that locally permit ordinary‐mode (O‐mode) propagation, but this does not fully explain propagation in magnetized, overdense conditions where O‐mode is forbidden. In these conditions, Z‐mode waves can mediate upward energy transport. Using a two‐dimensional finite‐difference time‐domain model, we investigate linear Z‐to‐O mode conversion at the topside plasma resonance in a field‐aligned negative density duct. Simulations show that transmission is strongest when the wave vector is aligned with the geomagnetic field and decreases rapidly for oblique injection. Magnetic field strength exerts stronger control on conversion efficiency than density‐gradient tilt, while the negative duct guides converted wave energy into the outer ionosphere.