# Study on the Thermospheric Density Distribution Pattern during Geomagnetic Activity
## 2 Date and Methodology  
### Data Selection  
This study focuses on the Swarm-C satellite accelerometers as the research object. It utilizes the inversion results to discuss the spatial and temporal distribution of thermospheric density during magnetic storms. The Swarm series of satellites were launched on 22 November 2013. Swarm C flies at an altitude of about 460 km (with an inclination of 87.4\({}^{\circ}\), an east-west separation of 1\({}^{\circ}\)-1.5\({}^{\circ}\) in longitude). The GPS receiver provides location- and time-related information for geomagnetic field measurements. It can determine non-conservative forces such as thermospheric drag, solar radiation pressure, and the recovery of thermospheric density [41]. The accelerometers can directly measure these forces with much higher resolution than the GPS receiver. Additionally, by modeling calculations, thermospheric density can be extracted from the Swarm satellite accelerometers. The latitude range of thermospheric density measured by the satellite is from 87.4\({}^{\circ}\) S to 87.4\({}^{\circ}\) N. Since the Swarm-C satellite was fully launched in 2014, this study selected magnetic storm events occurring after 2015. All data, including Swarm accelerometer data and Dst and Ap indices, were provided by the European Space Agency at [[https://earth.esa.int/eogateway/missions/swarm](https://earth.esa.int/eogateway/missions/swarm)]([https://earth.esa.int/eogateway/missions/swarm](https://earth.esa.int/eogateway/missions/swarm)) (accessed on 23 March 2023). Geomagnetic storms are divided into the initial, main, and recovery phases. The primary phase, also known as storm sudden commencements (SSC), lasts tens of minutes., The main phase of a geomagnetic storm usually lasts 2-8 h. The recovery phase may last more than 8 h, sometimes as long as 7 days. Scientists have successively recorded many geomagnetic storms. Three geomagnetic storm events were selected from [[https://www.swpc.noaa.gov/products/real-time-solar-wind](https://www.swpc.noaa.gov/products/real-time-solar-wind)]([https://www.swpc.noaa.gov/products/real-time-solar-wind](https://www.swpc.noaa.gov/products/real-time-solar-wind)) (accessed on 23 March 2023). The main phases of these events were on 23 June 2015, 7 October 2015, and 8 September 2017. We obtained the solar wind parameters during the above three magnetic storms from [[https://omniweb.psfc.nasa.gov/ow_min.html](https://omniweb.psfc.nasa.gov/ow_min.html)]([https://omniweb.psfc.nasa.gov/ow_min.html](https://omniweb.psfc.nasa.gov/ow_min.html)) (accessed on 23 March 2023). Based on the moment when the SYM-H index became negative, we determined the start times of the storms according to the starting time of the magnetic obstacle, which were 23 June 2015 02:23, 6 October 2015 21:35, and 8 September 2017 00:19, respectively. Figure 1 shows the Dst index and Ap index of the three geomagnetic storms on the day before, on, and after the occurrence of the storm's main phase. Figure 1 shows that the Dst index's minimum values for selected storms were less than \(-100\). They reached the level of intense geomagnetic storms (Dst index between \(-100\) nT and \(-200\) nT). Among them, the minimum Dst index of the storm on 23 June 2015 was less than \(-200\), which reached a great geomagnetic storm level (Dst index between \(-200\) nT and \(-300\) nT). The overall trend of the Ap index change was similar to that of the Dst index. However, Dst and Ap characterize different current systems and different types of  
Figure 1: Dst and Ap indices of three geomagnetic storm events. (**a**) the geomagnetic storm event on 23 June 2015; (**b**) the geomagnetic storm event on 6 October 2015; (**c**) the geomagnetic storm event on 8 September 2017.  
geomagnetic disturbances. The vertical dashed lines indicate SSC and the start times of the main phase (MP) and the recovery phase (RP). Figure 1 records the variation trends of the Ap index and Dst index during the three geomagnetic storms.