High-power hf electromagnetic waves have been used to generate large electron-density cavities in the night-time $F$-region ionospheric plasma. Previous modification experiments have described induced density perturbations of only a few percent. Here we report the first observations of density depletions exceeding 50%, extending hundreds of kilometers along the geomagnetic field. These cavities are thermally driven, with electron temperatures in the depletions increased by factors of 3 to 4. The depletion dynamics exhibits many of the characteristics associated with thermal cavitons.

• Received 22 February 1988

DOI:https://doi.org/10.1103/PhysRevLett.61.239