Combined measurements of Langmuir or broadband whistler wave intensity and lower‐frequency electric field waveforms, all at 10‐µs time resolution, were made on several recent sounding rockets in the auroral ionosphere. It is found that Langmuir and whistler waves are partially rectified in the plasma sheaths surrounding the payload and the spheres used as antennas. This sheath rectification occurs whenever the HF potential across the sheath becomes of the same order as the electron temperature or higher, for wave frequencies near or above the ion plasma frequency. This rectification can introduce false low‐frequency waves into measurements of electric field spectra when strong high‐frequency waves are present. Second harmonic signals are also generated, although at much lower levels. The effect occurs in many different plasma conditions, primarily producing false waves at frequencies that are low enough for the antenna coupling to the plasma to be resistive.
