| The thermal self-focusing instability, which manifests itself in the formation of density striations in the ionosphere, is studied. Two separate ionospheric modification experiments were conducted at the National Astronomy and Ionosphere Center (NAIC) in Arecibo, P.R. for the purpose of discerning the spatial dependence of the striations on incident HF power flux and angle between {dollar}rmvec ksb{lcub}HF{rcub}{dollar} and {dollar}rmvec B.{dollar} Both experiments made use of the HF heating facility in Islote, P.R., which operated at 5.1 MHz in the O-mode. The ionosphere was overdense to 5.1 MHz in both experiments. The orientation of the striations with respect to the HF heater beam and the earth's magnetic field is also considered. The first experiment was a two part campaign in which maps of the HF heated volume electric field intensity were made. The HF beam was vertical for the first part and tilted 12{dollar}spcirc{dollar} to the north for the second part, which allowed for the experimental variation of {dollar}theta{dollar} (the angle between {dollar}rmvec ksb{lcub}HF{rcub}{dollar} and {dollar}rmvec B).{dollar} The second experiment was the AA-3A rocket of the CRRES campaign. This rocket injected three clouds of barium into the heated ionosphere, each 20 km apart in altitude, centered on the middle of the HF heated volume. Incoherent Scatter Radar plasma line data and CCD optical data was taken to study the geometry of the evolution of the striations. Ray tracing, ionization modeling, and thermal self-focusing theoretical and computer simulation results were compared to experimental data. Conclusions were made that suggest that the striations are aligned with the HF beam below the HF reflection height and are {dollar}rmvec B{dollar} field aligned above reflection due to thermal conduction. The experimental data supports Perkins & Valeo's and Cragin & Fejer's theories which both state that minimum striation width decreases with increasing incident power flux. It is also concluded that the barium cloud acts as a divergent lens which focuses HF energy away from the area centered above the cloud. |
