![]() ![]() in the upper atmosphere and ionosphere at certain regions, zones, and times have been reported 4, 5, 6, 7, 8, 9, due to the lack of global and uniform three-dimensional (3D) observations, the response of the ionospheric F2-peak height (hmF2) to the double bulge of upper atmospheric lunar tides owing to the Moon’s gravitational force has not yet been globally observed. Although semidiurnal lunar tide signatures of electron density, wind, temperature, etc. Unlike the ocean water with a constant density of about 1 g/cm 3, the atmospheric density varies based on hydrostatic equilibrium and the ideal gas law, decreasing exponentially with altitude, which results in the atmospheric density at about 300 km altitude being about one thousand billion times smaller than that on the ground 3. However, for the semidiurnal tide, the times of tidal high water can vary due to global continental distribution, as the tides spread from the oceans onto the surrounding continental shelves, bearing no simple relationship to the ideal double bulge 1, 2.īy contrast, since the upper atmosphere is essentially a continuous fluid without the continental pattern effect superimposed, the free surface of the Earth’s ionosphere should respond by adapting to the shape of the Equilibrium Tide 1. Based on the Equilibrium Tide developed from Newton's theory of gravitation, two symmetrical tidal bulges of the sea level should appear with amplitudes of about 0.5 m at equatorial latitudes directly under and directly opposite the moon 1. The time interval between the next adjacent maximum tide is one half of the moon’s revolution period of 14.76 days, which has been also observed for several centuries and is known as the semimonthly lunar tide. When the three objects line up during either new moon or full moon, the lunar tide attains maximum amplitude. This oceanic surface height further varies according to the lunar phase of new moon, first quarter, full moon, and third quarter, which reflects the relative positions between the Sun, the Moon, and the Earth. The span of time that it takes for the Moon’s disk to change from all dark to all light to all dark again is called the lunar month or phase, which is about 29.53 days. The lowest oceanic surface height between the two tidal bulges is termed low tide. ![]() ![]() For thousands of years, human beings, especially those of maritime trades such as sailors and fishermen, have known that the oceanic surface height (i.e., tidal bulge, high tide) yields prominent variations with a period of about 12.42 h, termed the semidiurnal lunar tide 1 due to the Moon’s gravitational force and the Earth’s rotation. ![]()
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