First optical observations of energetic electron precipitation at 4278 Å caused by a powerful VLF transmitter

A summary is presented of experimental optical observations at 4278 Å from close to a powerful (~150 kW) VLF transmitter (call-sign JXN) with a transmission frequency of 16.4 kHz. Approximately 2.5 seconds after transmitter turn-on, a sudden increase in optical emissions at 4278 Å was detected using a dedicated camera/CCD monitoring system recording at a frequency of 10 Hz. The optical signal is interpreted as a burst of electron precipitation lasting ~0.5 seconds, due to gyro-resonant wave-particle interactions between the transmitted wave and the magnetospheric electron population. The precipitation was centered on the zenith and had no detectable spatial structure. The timing of this sequence of events is in line with theoretical predictions and previous indirect observations of precipitation. This first direct measurement of VLF-induced precipitation at 4278 Å reveals the spatial and temporal extent of the resulting optical signal close to the transmitter.