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Two solar flares X2.2 and X9.3 erupted over the active region 2673 on 6 September 2017, and the second flare is the strongest since 2005. In order to investigate the ionospheric and thermospheric responses to the two solar flares, the global total electron content and the critical frequency of F2 layer obtained from GPS stations and ionosondes are used. The results indicate that the ionosphere in the sunlit hemisphere increased significantly with magnitudes of 0.1 and 0.5 total electron content units for the X2.2 and X9.3 solar flares, respectively. The electron density, thermospheric neutral density, and neutral temperature simulated by the Thermosphere-Ionosphere Electrodynamics Global Circulation Model show that the behavior of ionospheric and thermospheric responses is different. The ionospheric disturbances occurred at the altitude ranges of 150À300 km, and the thermospheric responses occurred at the altitudes of 250À400 km are caused by solar extreme ultraviolet and ultraviolet photons, respectively. Both ionospheric and thermospheric responses are proportional to the height within their corresponding altitude ranges. Observations and simulations reveal that the ionospheric and thermospheric responses are nonlinearly dependent on the solar zenith angle. The disturbances caused by the X2.2 solar flare are symmetric, but the X9.3 solar flare are not. The O/N 2 density ratio simulated by Thermosphere-Ionosphere Electrodynamics Global Circulation Model increases from lev0 to lev5.0 pressure surface with a magnitude of 0.1-1.8, while the ratio decreases in the American sector with a magnitude of À0.6 to À0.3. The longitudinal asymmetry of O/N 2 density ratio is a major contributor to the longitudinal asymmetry of ionospheric and thermospheric responses. (CMD = 83°) was smaller than that caused by the X17.2 flare on 28 October 2003 (CMD = 8°), which was

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