Abstract:
Dynamic information （neutral winds and electric fields） near the peak of the ionospheric F-layer can be obtained from ionospheric critical parameters （f0F2 and hmF2） with different approaches. The physical basis of these approaches is the fact that neutral winds and electric fields strongly control the evolution of the ionosphere near and above the F-layer peak, especially the peak height of the F-layer. These approaches can be classified into three categories: traditional and revised servo approaches, data assimilation and approaches with the aid of ionospheric models, and the new approach introduced by Libo Liu. The first kind of approaches is based on the servo theory which was originally proposed by Rishbeth et al. （1978）. A key point of this kind of approaches is that the equivalent wind is assumed to be proportional to the displacement of the peak height of the ionospheric F-layer from its equilibrium height without wind. Although it has achieved great success （Titheridge, 1995）, the servo theory approach needs the no-wind equilibrium height, which is difficult to be accurately determined due to the large time constants for the ionosphere reaching an equilibrium state, especially in the day time. The second one includes those with the aid of an ionospheric theoretical model. Some need only an ionospheric model to provide the relationship between the neutral winds and the height displacements. In essence, they should belong to the serve theory approaches. The others make use of the idea of data assimilation, i.e., making the parameters from the model consistent with the observations as well as possible. The adjusted atmospheric parameters during the calculations are assumed to be the values at the actual situations. The approach proposed by Libo Liu avoids deducing equivalent winds from the displacement of the observed peak height from the no-wind equilibrium height, thus it is very simple and does not need to determine the no-wind equilibrium height which may limit the accuracy of the deduced winds as the servo theory approaches did.Equivalent winds derived from F-layer critical parameters by using these approaches are compared with the measurements with Millstone Hill Incoherent Scatter Radar during day 177-180 of the year 1990. Another example shows the consistence of an observation of the FPI wind at Beveridge （37°S, 144°E）, Australia, with the equivalent winds derived from the Digisonde 256 observation at the same site. Both case comparisons can qualitatively evaluate the validity of the third approach.
