Incoherent backscatter radar studies were conducted of the HF ionospheric modification interaction region using the facilities of the Arecibo Observatory. Very high relative spatial and temporal resolutions were achieved. These measurements resolve the miniovershoot (2--10 ms after HF on) and main overshoot (20--40 ms after HF on) plasma wave excitation features. The main overshoot phenomenon is observed to develop very reproducibly with enhancements of a factor of 10 greater than the steady state HF-enhanced plasma line intensity. However, this main overshoot effect is strongly suppressed for HF off times of less than 100 ms. In general, the miniovershoot excitation is found to occur 800-1000 m below the main overshoot excitation height. In addition, in one case, a small plasma line enhancement is detected at very early times (<1 ms) distinct from the miniovershoot and originating at a greater height. The main overshoot and the steady state enhanced plasma line both develop at essentially the same altitude. However, the interaction layer is observed to broaden by several hundred meters during the main overshoot relaxation. Possible explanations for these effects are proposed using a combination of the theories of soliton formation and collapse, wave ducting, profile modification, parametric instabilities, and other interactive processes.