<正>[1]The equatorial mass anomaly(EMA) in the thermosphere and equatorial ionization anomaly(EIA) in the ionos...
Jing Liu,Libo Liu,Biqiang Zhao,Jiuhou Lei,and Weixing Wan 1 Beijing National Observatory of Space Environment,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing,China. 2 Also at State Key Laboratory of Space Weather,Center for Space Science and Applied Research,Chinese Academy of Sciences,Beijing,China. 3 Graduate University of Chinese Academy of Sciences,Beijing,China. 4 School of Earth and Space Sciences,University of Science and Technology of China,Hefei,China.
In this paper, globally-averaged, thermospheric total mass density, derived from the orbits of -5000 objects at 250, 400, and 550 km that were tracked from 1967 to 2006, has been used to quantitatively study the annual asymmetry of thermospheric mass density and its mechanism（s）. The results show that thermospheric mass density had a significant annual asymmetry, which changed from year to year. The annual asymmetry at the three altitudes varied synchronously and its absolute value increased with altitudes. The results suggest that there is an annual asymmetry in solar EUV radiation that is caused by the difference in the Sun-Earth distance between the two solstices and the random variation of solar activity within a year. This change in radiation results in an annual change in the thermospheric temperature and thus the scale height of the neutral gas, and is the main cause of the annual asymmetry of thermospheric mass density. The annual asymmetry of mass density increases with altitude because of the accumulating effect of the changes in neutral temperature and scale height in the vertical direction.
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