# Study on the Thermospheric Density Distribution Pattern during Geomagnetic Activity
## 3 Results
### Analysis of the Spatial Distribution of Atmospheric Density during Geomagnetic Storms  
Similarly, using empirical orthogonal decomposition, Figure 5 shows the first-order EOF spatial distribution curves of the day before, on, and after the three geomagnetic storms.  
From the longitudinal comparison in Figure 5, for the spatial distribution of thermospheric density in the daytime area, the spatial distribution characteristics of the thermospheric density in the daytime region of the three geomagnetic storms are similar. Their curves all present a bimodal structure. In other words, there are two atmospheric density peaks at 40\({}^{\circ}\) to 60\({}^{\circ}\) north-south latitude, and the minimum thermospheric density appears in two polar regions. The minimum values of the geomagnetic storms on 23 June 2015, and 7 October 2015, appeared near the south pole. The minimum values of the geomagnetic storm on 8 September 2017 appeared near the North Pole. Although there are peaks in both the northern and southern hemispheres, the two peaks are not symmetrical. The peak values of the southern hemisphere were higher during the geomagnetic storms of 23 June 2015 and 7 October 2015, and the peak values of the northern hemisphere were higher on 8 September 2017.  
Another similarity is a trough near the equator for the spatial distribution of thermospheric density in the daytime area. Although the equatorial region's atmospheric density is not the minimum value, it is slightly lower than nearby regions. Compared with the daytime region's time coefficient curve, the atmospheric density's spatial distribution curves before and after the geomagnetic storm are similar. There will not be a sudden change in one day, like the time coefficient, but instead there will be a gradual change.  
There are some similarities in the spatial distribution of atmospheric density in the three geomagnetic storms' thermospheres for the spatial distribution of atmospheric density in the nighttime. The curves show a bowl-like structure, and a depression appears in the middle of all the curves. This pattern means that the lowest value appears near the equator. Comparing the spatial distribution curve of atmospheric density on the day before the geomagnetic storm with the day after the storm, the curve before the storm is the gentlest,  
Figure 5: EOF basis functions the day before, on, and after the geomagnetic storm. (**a**) On 23 June 2015, the daytime basis function; (**b**) the nighttime basis function on 23 June 2015; (**c**) the daytime basis function on 7 October 2015; (**d**) the nighttime basis function on 7 October 2015; (**e**) the daytime area basis function on 8 September 2017; (**f**) the night time basis function on 8 September 2017.  
and the fluctuation of the atmosphere is the lowest. With the coming of the geomagnetic storm date, two asymmetric peaks of atmospheric density gradually appeared in the mid-latitude region of the northern and southern hemispheres.  
From the horizontal comparison in Figure 5, the spatial distribution of the atmospheric density in the daytime and nighttime has a common point in the same geomagnetic storm. This is because a trough appears near the equator, a common feature of the three geomagnetic storms. The difference is that the spatial distribution curve of atmospheric density in the nighttime changes more obviously than in the daytime before and after the onset of the geomagnetic storm. That is to say, the influence of geomagnetic storms on the spatial distribution of atmospheric density in the night region is more significant than in the daytime region. As a result, daytime regions are slower to respond to geomagnetic storms.