Effects of Solar Extreme Ultraviolet Radiation on Thermospheric Neutral Density

The Solar Extreme Ultraviolet (EUV) radiation is well known as the major source of energy of driving the Earth atmospheric motion and transformation. The 10.7 cm solar radio flux (F_10.7) instead of EUV measurement is often used in the thermospheric neutral density models, to calculate the thermosphere density and temperature. In this paper, the continuous measurements of EUV from SEM instruments onboard the SOHO satellite during 2001－2021 are directly compared with the densities derived from accelerometers onboard the CHAMP, GRACE-A and SWARM-C satellites, to reveal the essential relation between the solar EUV radiation and the thermosphere density. It is found that the correlation coefficients between the EUV data and the densities are obviously greater than those between the F_10.7 indexes and the densities, which proves that the solar EUV radiation is the dominant energy source of heating the thermosphere rather than the radio radiation. From the liner regression slopes at different local time, it is shown that the maximum value occurs at 15:00 LT (corresponding to 14:00－16:00 LT), while the minimum value at 01:00 LT (00:00－02:00 LT), which confirms that the EUV radiation derives the diurnal variation of the thermosphere. According to the statistical results from three satellites with different orbital altitudes, it is indicated that in the 350~500 km altitude region, the heating effect of the EUV radiation at lower altitude is more intense than that at higher altitude (considering absolute variation). Because of the high spatial resolution of the observation of three satellites, the difference in the effect of the EUV radiation at various local time and geographic latitudes can be concluded: the slope in summer hemisphere is greater than that in winter hemisphere; in the dayside, the slope at the mid-latitudes is greater than that at the equator and high latitudes; in the nightside, the maximum of the slope appears at the high latitude of summer hemisphere, which is not described by the thermosphere models such as DTM2000 and NRLMSISE00 yet. In order to improve the modeling of the effect of the EUV radiation, the fitting method of six-order spherical harmonics is proposed. Compared with the formulas of the present models, the spherical harmonics are more valuable in improving the model construction or correction of the EUV effect and the thermosphere diurnal variation. At last, the physical mechanism of statistical results is discussed by considering the energy transmission and meridional circulation in the thermosphere.