# Near-Real-Time Global Thermospheric Density Variations Unveiled by Starlink Ephemeris
## 1 Introduction  
The thermosphere is a layer of Earth's atmosphere situated between the mesosphere and the exosphere, extending from approximately 80 km to 600 km (or higher) in altitude. This region is a critical zone for human space activities. Despite the extremely low density(on the order of 10\({}^{-12}\) kg/m\({}^{3}\)), the residual gas molecules within the thermosphere exert drag on high-speed satellites [1; 2]. This drag, known as atmospheric drag, acts in opposition to the satellite's direction of motion. As a result, atmospheric drag causes mechanical energy loss in satellites, leading to gradual orbital decay, particularly for those in LEO. Variations in thermospheric density, frequently disturbed by space weather activities, significantly impact spacecraft operations, including orbit determination and prediction, space debris monitoring and mitigation. Thermospheric density exhibits complex spatiotemporal variations, including diurnal, multi-day, seasonal, and long-term changes, driven primarily by solar radiation, geomagnetic activity, and Earth's rotation and revolution [3; 4; 5; 6; 7; 8]. Modeling and predicting the thermospheric variations are challenging, requiring extensive observational data. Currently, thermospheric density retrieval primarily relies on two approaches: (1) calculating orbital decay based on orbit tracking data and (2) measuring non-conservative forces using onboard satellite instruments. The first approach, based on orbit tracking data, utilizes two-line element (TLE) sets provided by the North American Aerospace Defense Command. TLE data cover a wide range of space objects and have long-term tracking records, making them valuable for studying long-term thermospheric variations and atmospheric modeling [9; 10; 11; 12]. However, the irregular and low update frequency of TLE data, coupled with frequent data gaps during geomagnetic storms, limit their ability to capture detailed thermospheric density variations. The second approach, based on satellite payload data, can be further divided into two categories: (1) direct measurement of non-conservative forces using high-precision accelerometers and (2) indirect calculation of non-conservative forces through precise orbit determination (POD) data obtained from GNSS receivers [13]. These efforts have focused on a limited number of satellites equipped with GNSS receivers or accelerometers, such as CHAMP [14], GRACE [15], GOCE [16], Swarm [17], and GRACE-FO [18]. High-precision accelerometers directly measure non-conservative forces acting on a satellite's center of mass during orbital motion, enabling the detection of subtle variations of atmospheric drag. Numerous studies have utilized accelerometer-based thermospheric density observations to analyze the responses of thermospheric neutral density to extreme geomagnetic storms and investigate the fine structure of the thermosphere [19; 20; 21; 22; 23; 24]. However, accelerometers are expensive and prone to technical failures, limiting their widespread use. As orbit determination accuracy has improved, POD data from GNSS receivers have increasingly become a valuable complement to accelerometer-based observations in thermospheric density retrieval. By combining POD data with high-fidelity dynamical models, researchers can estimate orbital decay and non-conservative forces. This approach enables the derivation of thermospheric density with resolution and accuracy comparable to accelerometer measurements [25; 26; 27; 28; 29; 30]. Despite the above advancements, the limited orbital coverage of these satellites restricts continuous global observations, while delayed updates of POD data (typically several days) and accelerometer data (often a few months) hinder the real-time monitoring of thermospheric density variations. With advancements in space technology and the expansion of the space industry, satellite launches are no longer limited to scientific research.