Electric and magnetic field measurements have been made in the ionosphere over an active thunderstorm and an optical detector onboard the same rocket yielded an excellent time base for the study of waves radiated into space from the discharge. In addition to detection of intense, but generally well understood whistler mode waves, very unusual electric and magnetic field pulses preceded the 1–10 kHz component of the radiated signal. These pulses lasted several ms and had a significant electric field component parallel to the magnetic field. No known propagating wave mode has this polarization nor a signal propagation velocity as high as we have measured. We have investigated and rejected an explanation based on an anomalous skin depth effect. Although only a hypothesis at this time, we are pursuing a more promising explanation involving the generation of the pulse via a nonlinear decay of whistler mode waves in the frequency range 10–80 kHz.