# Evaluation of the Mid-Latitude Ionospheric trough Using GRACE Data
## 2 Database and Method  
### Data Sources  
To investigate the MIT occurrence probability and characteristics, we used electron density data from the GRACE satellite mission covering a period of 14 years from 2002 to 2015. The period includes both high- and low-solar-activity conditions. The GRACE constellation consists of two satellites (GRACE-A and GRACE-B) launched on 17 March 2002 into a near-circular polar orbit with an initial altitude of about 490 km. The twin GRACE satellites flew in sequence with a separation distance of roughly 200 km and at an initial altitude of 490 km and an orbit inclination angle of 89\({}^{\circ}\). The electron densities were retrieved from GRACE K-band ranging system (KBR) [20] measurements. The KBR instrument is capable of precisely measuring the range change between the two GRACE satellites. The average electron densities were estimated based on the differential total electron content (TEC) derived from the range change between the two satellites [20]. The quality of the KBR densities was investigated by [PERSON] et al. (2015) [21], who found the KBR densities to be in agreement with incoherent scatter radar (ISR) measurements taken at Arecibo, Millstone Hill, Jicamarca, and EISCAT.  
The KBR density data used in this investigation were processed by the German Research Centre for Geosciences (GFZ), Potsdam, Germany, and downloaded from the GFZ archive ([[ftp://isdcftp.gfz-potsdam.de/grace/IONOSPHERE/KBR_Electron_Density/01/](ftp://isdcftp.gfz-potsdam.de/grace/IONOSPHERE/KBR_Electron_Density/01/)]([ftp://isdcftp.gfz-potsdam.de/grace/IONOSPHERE/KBR_Electron_Density/01/](ftp://isdcftp.gfz-potsdam.de/grace/IONOSPHERE/KBR_Electron_Density/01/)), accessed on 5 July 2022). The data are stored in ASCII format, and each file contains data for a whole day with a resolution of 5 s. Each GRACE satellite has an orbital period of about 94.5 min and rotates around the Earth approximately 15 times during the span of a day. Both GRACE satellites encircled the Earth many times during their lifetime, which lasted from March 2002 until October 2017. Figure 1 shows the evolution of the orbit height of the satellites during the GRACE mission. GRACE data, as well as the evolution of the GRACE orbital altitudes, were validated by [PERSON], [PERSON], et al. [21]. As shown in Figure 1, from 2011 to 2015, the height of the GRACE satellites decreased by 92 km. The long-term Ne data derived from GRACE KBR measurements are generally suitable for analysis of MIT characteristics.  
A literature study [18] showed that space weather conditions impact MIT occurrence probability and characteristics. [PERSON] et al. (2006) [16] found that trough occurrence is correlated with the interplanetary magnetic field (IMF) structure. Space weather conditions can be represented by several indices, such as the solar radio flux index (F10.7); solar wind (SW); ana; interplanetary magnetic field (IMF) components Bx, By, and Bz; geomagnetic index SYM-H; and the Hp30 index of global geomagnetic activity. In order to characterize the dependence of the MIT on space weather conditions, we collected data on the SYM-H, Bx, By, Bz, F10.7, SW index, and flow pressure [22] from the OmniWeb high-resolution service [23]. Hp30 index data were processed by the Geomagnetic Observatory Niemegk, GFZ German Research Centre for Geosciences [24]. The time resolution of each index is presented in Table 1.