# Near-Real-Time Global Thermospheric Density Variations Unveiled by Starlink Ephemeris
## 4 Discussion  
From the above results, the mechanical energy losses derived from individual Starlink satellites demonstrate great consistency with the thermospheric density variations. More significantly, the true strength of Starlink lies in its dense LEO constellation, which provides comprehensive coverage across global and different altitudes, as illustrated in Figure 1a and Table 2. Based on this perspective, in this section, we shift our analysis to the constellation level, and discuss the distinct advantage of utilizing Starlink ephemeris data from the entire satellite network. Using data from Shell 1 of the Starlink constellation as an example, we selected three distinct periods--before, during, and after the geomagnetic storm--to project the mechanical energy loss of all satellites on a global scale at identical UTs. Contour maps were generated separately for ascending and descending orbits, as shown in Figure 6. Note that the data coverage is constrained to 53 degS-53 degN due to Shell 1's orbital design parameters. The corresponding Dst indices for the three periods are marked by vertical lines of different colors in Figure 6g. Figure 6: The mechanical energy loss of Starlink satellites projected onto longitude–latitude coordinates for the (**a,b**) pre-storm, (**c,d**) during the storm, and (**e,f**) post-storm periods at identical UTs. The left panels represent ascending nodes, while the right panels correspond to descending nodes. Solid black line indicates the terminator. (**g**) The Dst index is provided for reference, with the three periods demarcated by vertical lines in different colors. The dashed lines in (**a,b**) represent samples of the orbit trajectories. First, comparing contour maps of ascending and descending orbits at any period reveal similar patterns of high and low values at the dayside and nightside. However, at finer spatial scales, the mechanical energy loss values for the same geographic location do not fully align. For instance, in Figure 6a,b, corresponding to UT 12:00, 1 October, the high-value region near the prime meridian (driven by solar radiation) exhibits distinct edge patterns: the ascending orbit shows a southwest-to-northeast arc, while the descending orbit displays a northwest-to-southeast arc. Note that the edge of the high-value region is aligned with orbit trajectories. This discrepancy results in a twofold difference in mechanical energy loss at 60\({}^{\circ}\)W, 53\({}^{\circ}\)S between the two orbits. In reality, the mechanical energy loss experienced by a satellite passing through the same region should be nearly identical, regardless of whether it is in ascending or descending orbit. We attribute this artifact to the usage of predicted ephemeris data rather than post-processed POD data. Since each data point is derived from the preceding one, the results for ascending and descending segments exhibit continuity. This finding is further supported by the closer agreement between the southwestern data of the ascending segment and the southeastern data of the descending segment, as well as between the northwestern data of the descending segment and the northeastern data of the ascending segment, as these regions are temporally contiguous but artificially divided into separate orbital segments. Second, examining the data for the same orbital segment across the three periods reveals variations in the mechanical energy loss. For any given region, the mechanical energy loss during the storm exceeds that before the storm, which, in turn, exceeds that after the storm. The first inequality is expected, as the energy injection from the storm heats and expands the thermosphere, increasing its density and thereby enhancing the atmospheric drag. The second inequality, however, stems from the aforementioned post-storm overcooling phenomenon. Next, we compare the mechanical energy loss across different shells of the first-generation Starlink constellation, with the results presented in Figure 7. As before, data from each shell are divided into ascending and descending nodes.