# Study on the Thermospheric Density Distribution Pattern during Geomagnetic Activity
## 1 Introduction
In addition to the solar radiation index, the geomagnetic index is usually used to represent the intensity of geomagnetic activity [27; 28; 29]. Geomagnetic activity indices are used to measure the level of disturbance in the Earth's magnetic field caused by solar activity [30; 31; 32; 33]. There are several different types of geomagnetic activity indices, including:  
K-index: This index measures the maximum deviation of the Earth's magnetic field in a 3 h period. A-index: This index is based on the average of the K-indices measured at a single station over a 24 h period. Ap-index: This index is similar to the A-index, but it is based on a global average of K-indices measured at several stations around the world. Dst-index: This index measures the strength of the ring current in the Earth's magnetosphere. AE-index: This index measures the level of geomagnetic activity caused by high-energy particles from the solar wind. PC-index: This index measures the level of geomagnetic activity caused by the interaction of the solar wind with the Earth's magnetic field. In this study, the Dst index and Ap index were selected as the research objects. The choice of these two factors is based on their ability to provide complementary information on the state of the Earth's magnetic field. The Dst index provides a measure of the strength of the ring current, while the Ap index provides a measure of the overall level of geomagnetic activity. Geomagnetic storms can be divided into initial, main, and recovery phases. The initial phase is characterized by the Dst component decreasing by \(-20\) to \(-50\) nT in a few minutes. The initial phase with a rapid decrease in Dst is also referred to as a storm sudden commencement (SSC). However, not all storms have an initial phase, and not all SSCs are followed by a storm. The Dst index's decrease to less than \(-50\) characterizes geomagnetic storms' main phase stage, scientists have determined after years of observation. The Dst index's minimum value will be between \(-50\) and \(-600\) during the main phase of the geomagnetic storm, which usually lasts 2-8 h. This change in the Dst index characterizes the recovery phase of a geomagnetic storm. This change is from its minimum value to a quiet period of \(-20\) to \(20\). The recovery phase may last more than 8 h, sometimes even as long as seven days [34; 35; 36]. Since the 20 th century, scientists have recorded many geomagnetic storms that have caused substantial economic loss and infrastructure malfunction [37]. For example, on 29 October 2003, a geomagnetic storm led to the wide-area augmentation system (WAAS), operated by the Federal Aviation Administration (FAA), being unfunctional for about 30 h [38]. In addition, Japan's ADEOS-2 satellite was seriously damaged, and the communication industry of many other satellites was interrupted by the geomagnetic storm [39]. This study selected two geomagnetic indices, the Dst index and the Ap index, to analyze the relationship between geomagnetic indices and thermospheric density in order to explore the relationship between geomagnetic storms and thermospheric atmospheric density. Finally, three recent typical geomagnetic storms were selected as research objects. Empirical orthogonal function analysis was used to perform a spatiotemporal decomposition of the latitude-time distribution of thermospheric density during the storms. The first-order time coefficients and EOF [40] (empirical orthogonal function) base functions obtained from the decomposition were used to study the temporal and spatial distribution of thermospheric density during the storms, and the spatiotemporal variation patterns of thermospheric density during geomagnetic storms were summarized and generalized. The study reveals a complex interplay between geomagnetic storms and the density of the thermospheric atmosphere. By using two different geomagnetic indices, this study can provide a more comprehensive understanding of the relationship between geomagnetic storms and thermospheric density.