1. Introduction
Twenty three years ago, Perkins and Kaw [1971]predicted that absorptive parametric instabilities [DuBois and Goldman, 19651 could be excited in the F layer of the ionosphere by powerful radio (HF) transmitters. Soon thereafter, Carlson et.al. [1972] and Wong and Taylor [1971]observed HF enhanced plasma lines using the Arecibo incoherent scatter radar. Two decades of intense observational and theoretical work followed. The 1980’s saw the development of new theoretical models and improved observational diagnostics.
In this review I will concentrate on observations, mainly carried out at Arecibo over the last five years, which have attained unprecedented temporal and spatial resolution of the HF induced Langmuir turbulence as observed by the incoherent scatter radar. This work was motivated by new predictions of the so called strong Langmuir turbulence (SLT) theory and the observations, in turn, contributed to the further development of the theory. This theory predicted properties of the turbulence induced near reflection density which appeared to be in substantial agreement with the observations and which were totally different than the predictions of the traditional weak turbulence approximation (WTA).
Despite this apparent success of SLT theory a number of workers expressed doubts concerning the value of this theory. Stubbe et.al. [1992] expressed doubts concerning the fundamental correctness of the model and alleged that its predictions did not agree with observations at TromsG. However, Kohl et.al. [1994] have recently reported VHF radar observations at Tromsg which may have identified the SLT-predicted continuum plus free- mode plasma line features as well as the w = 0 ion line feature. [See also Kohl et.al. 1993, especially Fig. 101. Also Isham et.al. [1994], using their chirped radar technique, observed up-shifted plasma line features in the UHF radar spectrum near reflection altitude at Tromso; the upshift is consistent with a free-mode line. Muldrew [1994, Comment] alleged that SLT theory could not explain certain longer time heating phenomena at Arecibo and promoted a field aligned duct hypothesis which he claimed could explain essentially all the observations. It should be emphasized that no other quantitative, nonlinear calculations on the same level as the SLT predictions have been made using alternative theories. Several allegations stated that SLT theory could not explain certain observational regimes where, in fact, no SLT predictions had been attempted because of technical difficulties.
In this review I will summarize some of the recent apparent successes of SLT theory but I will concentrate on the problems remaining in formulating accurate models and in calculating the predictions of these models for ionospheric conditions which are relevant to the observations.
In Section 2, I will summarize recent work [DuBois et.al. 1994bl in which we have attempted to derive improved, but numerically tractable, reduced SLT models starting from kinetic theory. Validity conditions specifying an extended “Zakharov” ordering are given. This has resulted in a more accurate description of the low frequency ion response for the case of nearly equal electron and ion temperatures appropriate to the ionosphere. The predictions of spectral simulations using the improved model are qualitatively similar to those of the older generalized Zakharov model. We have also introduced a new local quasilinear theory which gives a kinetic description, on the slow time scale (compared to the HF period), of the collisionless damping of cavitons and the concomitant evolution of the electron velocity distribution.

Anonymous,

The short answer? YES.