Geographical distribution of long‐term changes in the height of the maximum electron density of the F region: A nonmigrating‐tide effect?

The geographical distribution of trends in the height of the maximum electron density of the F 2 region ( h m F 2 ) seemed to indicate that ionospheric stations showing decreasing (i.e., negative) h m F 2 trends are mostly located on or in the vicinity of seashores. In contrast, ionospheric stations in continental areas exhibit generally increasing (i.e., positive) h m F 2 trends. On the basis of the present study, it is shown that considering the whole world, the geographical distribution of h m F 2 trends is affected essentially by the nonmigrating tidal wind. Moreover, besides the field‐aligned drift, the elecrodynamical drift must also be considered in the development of h m F 2 trends. Taking into account both the field‐aligned and the electrodynamical drift, northward (eastward) tidal winds and southward (westward) tidal winds would create downward and upward drifts, respectively, in the Northern Hemisphere but southward (eastward) and northward (westward) tidal winds in the Southern Hemisphere. An upward drift is accompanied by an increase and a downward drift is accompanied by a decrease of h m F 2 . It has been suggested that because of the thermal driven origin of the nonmigrating tides, the distribution may also be connected with ocean currents warming or cooling the neighborhood of the coastline. The effect of ocean currents would create downward drift due to cooling at the western coastlines of the continents but upward drift at the eastern coastlines as a result of the warming. This has been shown by comparing the distribution of h m F 2 trends with the map of ocean currents. The corresponding h m F 2 trends may develop because of changes in the land/sea‐warming ratio, where the land‐surface air temperature increases faster than the sea‐surface air temperature.