Rocket observations of the high-power HF modified ionosphere

Radiowave based diagnostics of the ionosphere over a high power radiowave transmitter show that field-aligned irregularities are easily generated. However, until the work reported here, the complex organization and detailed shape and size of these irregularities in and around the heater was not known. Nor was the role that the filament shape plays in radar backscatter completely understood. We find after analysis of data from a 1992 NASA rocket flight through a high power transmission from the Arecibo, Puerto Rico facility that after a few seconds of continuous wave transmission, the HF-illuminated ionosphere is filled with meter-scale field-aligned density depletions which are grouped in kilometer-scale patches throughout the beam. Additional grouping (bunches) inside the patches also exists. While these irregularities are small in cross-section, having scale sizes around 15 m, they are very deep with depletion depths up to 12%, and are very elongated along the magnetic field. The organization of the irregularities into patches results in a kilometer scale average depletion which is much shallower than the filaments themselves. The individual depletions are each associated with a temperature enhancement of a few hundred degrees Kelvin. This temperature elevation could possibly be larger near the center of the beam but our experiment only skirted the edges of the disturbed region.; Structure with smaller scales than the filaments themselves exists within the heated volume and is revealed through the spectral analysis of the rocket data. The spectra of both the Langmuir probe and double-probe detector data change shape to a shallower spectral slope at high frequencies. This change in spectral index cannot be explained merely by the filament shape. Spectrograms reveal that the power at these frequencies is contained inside the filaments. We hypothesize that this high frequency structure is due to drift-like waves which are driven by the steep temperature and density gradients of the filaments.; A radar scattering cross-section model based on the power spectrum of the Langmuir probe data is consistent with the results of radar studies and shows the role of the filaments in radar backscatter. Comparing our model with a scattering cross-section plot generated from Platteville data, we are able to understand the difference between measured backscatter at the two sites. At Arecibo, VHF radars receive backscatter from what we believe are drift-like waves on the edges of the filaments. The filaments at Platteville were smaller, so there, VHF radars would detect backscatter from the filaments themselves.