Based on the neutral mass density measurements from the on-board accelerometer of Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite, the space-borne atmospheric density detectors on the Atmospheric density detection and Precise Orbit Determination (APOD) satellite and another Chinese satellite (CHN-sat) over almost a solar cycle, this paper presents the annual and semiannual variations of the dawn/dusk thermosphere mass density at 260 km, 460 km, and 630 km altitudes. Density maxima at dawn appear in the local summer at the southern high latitudes, and near equinoxes in the northern poles with the density maximum in March higher than that in October at solar minimum. Neutral density maxima near 630 km at dusk occur at high latitudes under low solar flux condition, and at low latitudes near the equator at medium solar activity. The latitude-seasonal density structure obtained from both the CHN-sat and APOD observations shows a distinct hemisphere asymmetry with a larger magnitude at high latitudes in the summer hemisphere at solar minimum. The semiannual variation dominates from low-middle latitudes at 260 km altitude and decreases with the increase of solar activity, while the annual variations dominate at 460 and 630 km altitudes in solar minimum. The growth of mass densities with increasing solar radiation gradually saturates in June. The amplitude of annual variation in mass densities at middle-high latitudes in the Northern Hemisphere (NH) decreases with increasing solar radiation.
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