# Evaluation of the Mid-Latitude Ionospheric trough Using GRACE Data
## 2 Database and Method
### Data Processing
Figure 2 shows a graphic representation of the MIT detection approach. The left plots (see Figure 2a,c) show examples of satellites passing between \(40^{\circ}\) and \(70^{\circ}\) MLAT in the northern and southern hemispheres (red line). The right plots (see bottom panels of Figure 2b,d) show the original Ne variation (see yellow curve) and the background Ne variation along the MLAT (black dashed line). The detected MIT characteristic points are marked in the Ne variation plot. The detrended Ne variation is shown in the upper panels of b and d, where a constant line indicates the threshold level at \(-0.2\) (red dashed line). The MIT detection approach was applied to all available GRACE KBR Ne profiles, and we detected approximately 44,930 Ne profiles with MIT signatures. Among them, approximately 16,210 profiles were found in the northern hemisphere and approximately 28,720 in the southern hemisphere. The number of detected profiles varies yearly (see Figure 3). The blue-colored bars in Figure 3 show the number of Ne profiles observed during each year from 2002 until 2015 over the northern (see Figure 3a) and southern hemispheres (see Figure 3b). The yellow bar plots show the number of profiles detected with MIT signatures in each hemisphere. Figure 3: Number of total and detected MIT profiles for each year in the northern hemisphere (**a**) and the southern hemisphere (**b**). Figure 2: Examples of the GRACE trajectory over the northern (**a**) and southern (**c**) hemispheres on 14 April 2002 between 40\({}^{\circ}\) and 70\({}^{\circ}\) magnetic latitudes. Concentric circles are plotted in 10\({}^{\circ}\) intervals. The blue lines correspond to magnetic latitudes 40\({}^{\circ}\) and 70\({}^{\circ}\). The red line is the footprint of the satellite path. The magnetic pole is marked with a star. Corresponding Ne profiles with detected trough minimum position and trough characteristic points for the northern (**b**) and southern (**d**) hemispheres (see below plots). Detrended logs of Ne are represented in upper plots for the northern (**b**) and southern (**d**) hemispheres. The gray dashed line represents the Ne running average. The yellow line is the Ne profile, and the red dashed line shows the threshold for trough detection. A comparison of the blue and yellow bar plots during the complete period from 2002 to 2015 revealed relatively fewer MIT profiles detected during the solar minimum period of 2007-2010, indicating that the probability of MIT occurrence is related to the solar cycle variation. As previously mentioned, if an MIT occurrence was detected, the locations of the trough minimum (Tm), the equatorward and poleward breakpoints (Be and Br), and the equatorward and poleward half points (He and Br) were determined. The breakpoints were determined as the intersection points of the Ne profile and the background density. Then, the trough width and trough depth were determined as shown in Figure 4. The trough width is defined as the spatial distance between the breakpoints (Be and Br), whereas the trough depth is defined as the electron density difference between the value of the breakpoint and the trough minimum point.