By observing simultaneously in multiple directions with the middle and upper atmosphere phased-array radar, we have been able to measure the horizontal structure produced by waves in the ionospheric density. We find that our data are most clearly ordered on a contour plot of the horizontal gradient of the electron density versus time and altitude. These plots show a periodic oscillation of the gradient with a downward phase progression below 400 km altitude but very nearly vertical wave fronts above that altitude. A simulation of this effect with a simple wave formulation provides a strikingly good comparison with the measurements. ¿ American Geophysical Union 1993