Middle and upper atmosphere radar observations of ionospheric horizontal gradients produced by gravity waves

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.