The Climatology Properties Of The Equivalent Winds In The Ionospheric F Region

Understanding the variation of thermospheric circulation is an important subject involving the coupling system of the upper atmosphere and ionosphere. The neutral meridional winds and the electric drifts are generally observed by Fabry-Perot interferometer (FPI), the incoherent scatter radar (ISR) and the satellite. But these techniques are difficult to carry out routine observation for some reason. Fortunately, we can use the ionospheric characteristic parameters (e.g., F layer critical frequency/peak electron density and its height) to derive the equivalent winds, which includes both the information of the neutral meridional winds and the north perpendicular electric drifts. It can be expected that the ionosonde equivalent winds can have a good distribution with location and time, because the ionosonde stations have been built up all over the world and being routinely operating for a few decades. Further more, the modern ionosonde can provide routinely observed and real-time ionospheric character parameters all over the world, hence study on the technique of deriving the equivalent winds has potential value for the study on the space weather. In this paper, we aim to introduce our achievements in the study of the ionosonde equivalent winds. We firstly introduce a new method for deriving the ionosonde equivalent winds and validated it from comparison with the FPI and ISR winds. We then investigate the diurnal, seasonal and solar cycle variation of the ionosonde equivalent winds over Wuhan (30.6oN, 114.4oE) and analyze the climatology of the equivalent winds along the 120o-150oE logitude with an emphases on their latitudinal dependence. Finally we carry out a statistics analysis for the correlation between the equivalent winds and the solar activity over worldwide ionosonde stations. According to our work, we can conclude as follows: 1. The equivalent winds show evident temporal and seasonal variations. At higher latitudes, the equivalent winds are downward by day while upward at night. At equatorial latitudes, the equivalent winds shift to be equivalent electric drifts, and their directions are opposite to those at higher latitudes. Their daytime speed is relatively larger before noon and so does the nighttime one after midnight. Over Wuhan, we found a marked phenomenon of post-midnight decent companying a large collapse of the F layer peak height, which is similar to that over Arecibo (18.3oN, 293.3oE). The seasonal variation of the equivalent winds is also apparent. For example, the speed of the equivalent winds in spring is much smaller than other seasons over Wuhan. During daytime, the equivalent winds have different latitudinal dependence between summer and winter along the 120o-150oE longitude. 2. The equivalent winds are greatly dependent on the magnetic latitudes and also the magnetic inclination. Both their speed and direction have latitudinal dependence. The equivalent winds are mostly controlled by the electric field drifts in the magnetic equatorial region, and shift to be mostly contributed by neutral winds near mid-latitudes, in consistent with our early knowledge. However, the relative contribution of the two dynamic factors is regulated by the magnetic dip besides their own magnitudes. The latitudinal dependence of equivalent winds has two prevailing trends. One is a maximum at the highest latitudes (as far as the latitudes concerned at present work); the other is a mid-latitude maximum, with peak appearing at magnetic latitudes about 30o-40o. The equivalent winds are also found to be approximate symmetry about the magnetic equator. The degree of the symmetry is better by night than day, and it is also better in equinox than in summer and winter. 3. The equivalent winds and their diurnal tidal components have strong correlation with the solar activity. During both daytime and nighttime, the equivalent winds tend to decrease with the increase of the solar activity over Wuhan and stations along the 120o-150oE longitude. These solar cycle trends are consistent with the observation by the nearby MU radar (34.8oN, 136.1oE), while the daytime trend is different from the results of HWM93 model and those observed by Millstone Hill ISR. At the same time, we further confirm the decrease trend of diurnal amplitude for the equivalent winds with the increase of the solar activity. And we also reveal a general phenominon about the diurnal tidal phase, which nearly delays with the increase of the solar activity at all sations and has marked seasonal variations.These results support the view that the net increase in ion drag is faster than the pressure gradient force, thus restrains the wind speed at higher solar activity. Our results also find the effect of the declination on the solar cycle trend of the equivalent winds, showing that the solar cycle dependence of the equivalent winds becomes weaker with the increase of the declination. To summary, the equivalent winds are derived at various ionosonde stations with long time routine observation around the world using our method. Their dependence on local time, season, latitude, longitude and solar cycle etc., are systemically analyzed to learn more about the coupling system of the upper atmosphere and ionosphere. The results can also help to improve the related empirical models.