A large (100 km N/S), stable, smoothly-structured, and significant depletion (15% in number density) of the daytime Tromsϕ F-region was produced inadvertently during a Heater polarisation and beam-forming procedure. We show that the depletion coincided with the passage of an atmospheric gravity wave of natural origin which served as a large-scale 'seed' depletion for the self-focussing instability. ¿ American Geophysical Union 1988