AbstractOn 3 February 2022, 38 satellites launched by SpaceX re‐entered the atmosphere and were subsequently destroyed. An investigation found that a minor geomagnetic storm occurred on 3–4 February 2022 led to a neutral density enhancement and large atmospheric drag. To better understand the responses of the thermosphere to geomagnetic storms, the method proposed by Li et al. (2023, https://doi.org/10.1029/2022ja030988) was employed to extract exospheric temperature (Tex) from ionosonde electron density profiles (∼150–200 km) in Beijing (geolocation: 39.56°N; 116.2°E; geomagnetic location: 30.16°N; 172.08°W) station. The retrieved Tex was plugged into the NRLMSISE‐00 model to calculate the corresponding neutral density. Derived results showed a ∼2%–7% enhancement in Tex and a ∼15%–38% enhancement in neutral density at 430 km. The relative deviation in neutral density on the satellites’ orbital trajectory ranges from ∼10% (210 km) to ∼35% (500 km) on 3 February, and from ∼13% (210 km) to ∼60% (500 km) on 4 February. Furthermore, the neutral density reproduced the variations observed by the SWARM‐C satellite fairly well both on quiet and disturbed days. These results suggest that even a minor geomagnetic storm can cause significant changes in neutral temperature and neutral density at middle latitudes. Additionally, the application of our inversion method, combined with the global, long‐term and real‐time ionospheric observations from ionosondes, provides an opportunity to improve the capability of thermosphere forecasting and nowcasting.

In this article, high spatiotemporal resolution data obtained by the atmospheric density detector carried by China’s APOD satellite are used to study the hemispheric asymmetry of thermospheric density. A detailed analysis is first performed on the dual magnetic storm event that occurred near the autumnal equinox on 8 September 2017. The results show that the enhancement ratio of atmospheric density in the southern polar region (SPR) on the duskside was approximately 1.33–1.65 times that of the northern polar region (NPR), demonstrating a strong hemispheric asymmetry of thermospheric atmospheric density response during the magnetic storm. However, the asymmetry response was smaller on the dawnside, suggesting that the hemispheric density response asymmetry is related to local time (LT). The energy injection in high-latitude regions increases local atmospheric density and forms traveling atmospheric disturbances (TADs). TADs can propagate to low-latitude regions over several hours and affect the global distribution of thermospheric atmospheric density. Similarly, the geomagnetic index fitting slope of SPR relative density difference is greater than that of NPR. The SuperDARN convection pattern indicates that the plasma convection velocity of SPR is significantly greater than that of NPR, indicating that joule heating caused by neutral friction of ions in the Southern Hemisphere may be stronger. Subsequently, an analysis of annual solar activity and seasons was carried out on the thermospheric NPR, SPR atmospheric density, and their differences from December 2015 to December 2020. The results show that thermospheric atmospheric density decreases overall as the number of sunspots decreases. The differences between the NPR and SPR atmospheric densities in the thermosphere exhibits a noticeable annual periodicity. The NPR and SPR atmospheric densities appear to have different distribution characteristics in different seasons. The NPR density peak is mainly in March or April. In particular, the “double-peak” phenomenon occurred in 2017, with peaks in March and September, while the most obvious feature of SPR atmospheric density is that its minimum value occurs in the summer months of June and July. This paper reveals the annual, seasonal, and magnetic storm response characteristics of the hemispheric asymmetry of thermospheric atmospheric density, which has significant implications for the study of multilayer energy coupling of the magnetosphere–ionosphere–thermosphere.

The longitudinal distribution of upper atmospheric density has been broadly studied. However, the studies mostly focused on 24 h averaged distribution. This study presents the longitudinal distribution of thermospheric density at dawn and dusk, using observations collected by the atmospheric density detector onboard the Chinese satellite APOD (Atmospheric Density Detection and Precise Orbit Determination) during low solar activity. The APOD observations show a significant relative longitudinal variation of thermospheric density with global maxima (Δρrmax) near the geomagnetic pole, especially in the winter hemisphere. The annual maximum of Δρrmax appears in the Southern Hemisphere around the June solstices and reaches 26.3% and 39.6% at dawn and dusk, respectively. The auroral heating and meridional wind might play a significant role in the longitudinal variation of thermospheric density. We further compare the APOD observations with the semi-empirical atmospheric model MSIS (Mass Spectrometer Incoherent Scatter Radar) 2.0 predictions under low solar activity conditions. The MSIS 2.0 model reproduces similar longitudinal variations to the observations, with hemispheric asymmetry. The longitudinal variation of thermospheric density from APOD should be related to the distribution of the atmospheric average molecular weight from the model. More observational data are needed to verify the results of this study further.

Chinese space station has entered its construction stage, starting a new journey of manned spaceflight. According to the operation requirements of Chinese space station, an atmospheric density multidirectional detector for the core module was independently developed to obtain density changes in spatial and temporal distribution at operating altitude. The detector could provide services of flight control management, precise positioning, and analysis for the space station and accumulate long-term autonomous detection data to provide basis for studying physical models and effects in neutral geospace environments. The atmospheric density detection for the core module of the space station was achieved by two atmospheric density multidirectional detectors that were symmetrically installed, each equipped with sensors in three directions. The basic working principle of the detector is as follows. The in-track gas inflow entered the sensor at high speed. According to the theory of molecular gas dynamics, the atmospheric density could be calculated from the pressure and temperature in the sensor. Furthermore, special techniques, e.g., nanoscale thermal cathode coating, were used to extend the working life of the detector beyond 15 years. This thesis mainly introduced the scientific objectives, payload configuration, and long-life design. In addition, preliminary results of in-situ detection were used to analyze the variations of atmospheric density on the operating orbit, showing the changes on the dayside and nightside orbits. The dayside and nightside peak-to-valley ratio during the quiet time of the space environment from September 11 to 14 and November 19 to 22, 2021, was between 2 and 3. During the strong geomagnetic storm on November 4, 2021, the measured peak atmospheric density increased to 1.966 times, whereas the peak model value increased to 1.483 times. The model of the geomagnetic storm recovery period was 6 h, while the measured data was 12 h later than model. During the geomagnetic storm event on March 13, 2022, the measured atmospheric density increased to 1.424 times, and the model value increased to 1.250 times. The global rise of atmospheric density began in the southern hemisphere and extended to the northern hemisphere, which was opposite the location of the disturbance source on November 4, 2021. According to statistics of six geomagnetic storm events in 2021 and 2022, the increase in the ratio of atmospheric density was positively related to the intensity and duration of the geomagnetic storm, and the maximum ratio could reach 96.6%. The disturbance source of atmospheric density varied its location with the injection position of energy particles, which might appear at the poles or near the equator. During the quiet time from December 11 to 31, 2021, the relative deviation between the measured value and the daily mean of the model was less than 10%. Compared to model, measured data exhibited a larger variation amplitude, wider propagation range, longer duration, more sensitive response, and smaller wave structures, which could reflect more abundant details in the change. The measured data would constitute a long-term independent database for the study of space environment physics and effects.

我国空间站已经进入建造阶段,开启了载人航天新征程.根据我国空间站试验任务需求,自主设计研制了空间站核心舱大气密度多向探测器,探测获取空间站运行高度上大气密度的时空分布变化,为空间站的飞控管理、精密定位和分析服务,并积累长期自主探测数据,为地球空间中性环境物理模式和效应研究提供基础数据.本文主要介绍了探测器的探测目标、载荷配置、长寿命设计等情况,同时使用在轨探测初步结果进行了运行轨道上大气密度的变化特性分析,呈现出日侧和夜侧轨道圈上大气密度变化, 2021年9月11～14日和11月19～22日空间环境平静期日侧和夜侧的峰谷比在2～3之间; 2021年11月4日的较强地磁暴期间实测大气密度峰值增加至1.966倍,而模式值峰值增加至1.483倍,磁暴恢复期模式为6 h,而实测数据晚12 h; 2022年3月13日的磁暴事件期间,实测大气密度增加至1.424倍,模式值增加至1.250倍,大气密度全球抬升从南半球开始,扩展到北半球,与2021年11月4日扰动源位置相反;统计了2021～2022年期间6次磁暴事件,大气密度抬升比例与磁暴强度、持续时间呈正相关; 2021年12月11～31日空间环境平静期,实测值和模式的日均值相对偏差小于10%.实测数据与模式数据相比,增变幅度大、传播范围广、持续时间长、响应更加灵敏.实测数据将为地球空间环境物理与效应研究提供长期自主数据库.

Based on the in-situ observed mass densities derived from the space-borne atmospheric density detectors on the Atmospheric density detection and Precise Orbit Determination (APOD) and another Chinese satellite (CHN-sat), from 2008–2018, the neutral density variations of the dawn/dusk thermosphere mass density at 460 km, 630 km altitudes are presented in this paper. Our results reveal: (1) The densities observed by APOD show consistent variations with that in empirical model results, and the latitudinal- seasonal structure shows that the largest density maxima appear near equinoxes in the northern high latitudes with the higher maximum near March than that in October at solar minimum. (2) Equatorial observations of CHN-sat are in good agreement with model results, while significant differences between observations and MSISE00 densities occur at high latitudes under the very low solar activity conditions. In solstices, the measured densities by CHN-sat at high latitudes in the summer hemisphere are distinctly higher than those in the winter hemisphere, which indicates that the relative importance of the conjunct contribution from oxygen atom and temperature, and the winter helium bulge maybe should be adjusted in the empirical model. (3) The weak geomagnetic disturbance can strengthen the high-latitude density at dawn and the low-latitude density maxima at dusk at CHN-sat altitudes.

This study presents the longitudinal distribution of thermospheric density around the terminator (in the dawn and dusk sectors), using observations collected by the atmospheric density detector onboard the Chinese satellite APOD (Atmospheric density detection and Precise Orbit Determination) from 2017 to 2018. The APOD observations show a significant relative longitudinal variation of thermospheric density with global maxima (Δρrmax) near the geomagnetic pole, especially in the winter hemisphere. The annual maximum of Δρrmax appears in the Southern Hemisphere around the June solstices and reaches 26.3% and 39.6% at dawn and dusk, respectively. Compared with at dawn, Δρrmax occurs at a higher latitude with a larger value at dusk. The auroral heating and meridional wind might play an important role in the longitudinal variation of thermospheric density. We further compare the APOD observations with the NRLMSIS 2.0 model predictions under low solar activity condition. The NRLMSIS 2.0 model reproduces similar longitudinal variations to the observations, with hemispheric asymmetry and local time difference.

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.

On September 20th, 2015, twenty satellites were successfully deployed into a near-polar circular orbit at 520 km altitude by the Chinese CZ-6 test rocket, which was launched from the TaiYuan Satellite Launch Center. Among these satellites, a set of 4 CubeSats conform the atmospheric density detection and precise orbit determination (APOD) mission, which is projected for atmospheric density estimation from in-situ detection and precise orbit products. The APOD satellites are manufactured by China Spacesat Co. Ltd. and the payload instruments include an atmospheric density detector (ADD), a dual-frequency dual-mode global navigation satellite system (GNSS) receiver (GPS and Beidou), a satellite laser ranging (SLR) reflector, and an S/X-band very long baseline interferometry (VLBI) beacon. In this paper, we compare the GNSS precise orbit products with colocated SLR observations, and the 3D orbit accuracy shows better than 10 cm RMS. These results reveal the great potential of the onboard micro-electro-mechanical system (MEMS) GNSS receiver. After calibrating ADD density estimates with precise orbit products, the accuracy of our density products can reach about 10% with respect to the background density. Density estimates from APOD are of a great importance for scientific studies on upper atmosphere variations and useful for model data assimilation.

The radiative transfer softwares such as MODTRAN or 6S can be directly used to compute remote sensing radiances of the sensors which illuminated by the sun. But lunar irradiance must be integrated in the computation for the glimmer sensors which detect the lunar radiation reflected by the objects. The lunar surface albedo and extraterrestrial solar irradiance already been known and published were used to calculate the lunar irradiance. Till now only 6S can easily include the lunar irradiance in radiative transfer computation. By description of how to compute lunar irradiance in detail then obtained extraterrestrial moon surface irradiance in the range of 0.250μm-1.500μm.