A numerical and analytical study is made of the approach to steady state of density depressions formed at the reflection height of HF waves propagating in a daytime iososphere. A time-dependent transport code including the effect of dissociative recombination is used to study the time evolution of these thermal cavitons. The time-asymptotic results are compared with the predictions of a steady state model developed by Vas'kov and Gurevich. It is found that their model overestimates the depth of density cavities by a factor of 2. Correcting for this factor leads to the conclusion that for realizable heating values of ΔTe/Te=0.4, the maximum attainable 1Δn1/n0 under daytime conditions range from 1% to 6.5% depending on values of the recombination rate. ¿ American Geophysical Union 1989