# Study on the Thermospheric Density Distribution Pattern during Geomagnetic Activity
## 2 Date and Methodology
### Correlation between Dst, Ap, and Atmospheric Density  
The periodicity of solar activity determines the basic structure of the thermosphere and the regular long-term changes. In contrast, the influence of geomagnetic activity on thermospheric density is short-term and severe [48]. Therefore, the thermospheric density, Dst index, and Ap index obtained from the Swarm-C satellite accelerometer are compared to study the relationship between the intensity of geomagnetic activity and thermospheric density. A point 72 h before the beginning of the geomagnetic storm is taken as the abscissa's initial value. The density of the thermosphere and Dst is used as the ordinates to draw a biaxial map. The Ap index is recorded every 3 h. The atmospheric density of the thermosphere and Ap index are used as the ordinates to draw a biaxial map. The final results are shown in Figure 2.  
Figure 2a,c,e show the relationship between thermospheric density and the Dst index. These three pictures show that the amplitude of thermospheric density and the magnitude of geomagnetic index Dst negatively correlate during geomagnetic storms. The thermospheric density value is in the trough when the Dst index peaks in the three geomagnetic storms. The peaks and valleys of the two curves are opposite. The correlation coefficient between the Dst index and thermospheric density value is \(-0.805\) in the geomagnetic storm on 23 June 2015, \(-0.87\) in the geomagnetic storm on 7 October 2015, and \(-0.795\) in the geomagnetic storm on 8 September 2017. The absolute value of the correlation coefficient between the Dst index and thermospheric density is more than 0.8 in two storms, which has reached a negative significant correlation level. Although the peak valley of thermospheric density is opposite to that of the Dst index, the maximum absolute value is not necessarily in the same position. For example, on 8 September 2017, peaks appeared at the 74 h and 90 h. However, the maximum absolute value of the Dst index appeared at the 74 h, and the maximum absolute value of thermospheric density appeared at the 90 h. Moreover, the peak value of the Dst index is not synchronous with the peak of thermospheric density; instead, there will be a specific time delay. Among the three geomagnetic storm events, the time delay between the Dst index peak and the peak of thermospheric density on 22 June 2015 is the largest. Figure 2b,d,f show the relationship between thermospheric density and the Ap index. It can be seen from the three pictures that the Ap index shows a positive correlation with thermospheric density. The value of thermospheric density rises and falls with the rise and fall of the Ap index. The difference is that the maximum value of the Ap index appeared earlier than the maximum value of thermospheric density in the three geomagnetic storms. In the geomagnetic storm on 23 June 2015, the maximum value of the Ap index appeared about 10 h earlier than the maximum value of thermospheric density. In the geomagnetic storm on 7 October 2015, the maximum value of the Ap index appeared earlier than the maximum value of thermospheric density. In the geomagnetic storm on 8 September 2017, the maximum value of the Ap index appeared about 4 h earlier than the maximum value of thermospheric density. Moreover, unlike the thermospheric density curve showing a stable upward or downward trend, the curve fluctuation of the Ap index is more evident than that of the thermospheric density curve. The Dst index lags behind thermospheric density, while the Ap index is ahead of thermospheric density. The different cadences may cause this difference. The change in thermospheric density and the Dst index always lags behind the Ap index for a while, so the Ap index is a more sensitive data choice when studying the change in thermospheric density. However, the disadvantage of the Ap index is that the correlation between the Ap index and thermospheric density is not as high as that between the Dst index and thermospheric density.