Time and spatial variations in the ratio of nighttime and daytime critical frequencies of the F2 layer

The ratio of daytime and nighttime values of the foF2 critical frequency is analyzed on the basis of the data of 28 ionospheric stations of the Eastern Hemisphere. It is found that after about 1980, three types of behaviors of this ratio with time are observed: an increase with time (a positive trend), a decrease with time (a negative trend) and the absence of pronounced changes (a zero trend). The sign of this trend is shown to be governed by the signs of the magnetic declination D and magnetic inclination I at the given ionospheric station. This fact makes it possible to assume that the above-indicated trend is caused by long-term variations in the zonal component Vny of the horizontal wind in the thermosphere, the latter component contributing into the vertical drift velocity W. The causes of the systematic changes in the thermospheric circulation regime after 1980 are not yet known.