The reflection of a linearly polarized plane electromagnetic wave by an idealized cold-plasma ionosphere model, with a constant vertical magnetic field, is examined. A linear electron density profile and a constant collision frequency are assumed. For vertical incidence, the exact time-harmonic solution for the parallel and cross-polarized reflected fields are expressible in terms of Airy functions. Then, using a numerical inverse Fourier transform, the impulse response of the model is obtained. It is shown that causality is satisfied for both the in-polarized and the cross-polarized component of the reflected transient wave. The cross-polarized component owes its existence to the presence of the dc magnetic field.