# Evaluation of the Mid-Latitude Ionospheric trough Using GRACE Data
## 3 Results and Discussion
### Seasonal Variation  
In this section, we address the dependence of the MIT parameters on seasonal variation. Figure 9 shows the percentage occurrence of the MIT during each month separately in the northern (left plot) and southern hemispheres. The northern plot shows that the MIT occurs more often in winter months (Nov-Feb) and equinoxes than in summer (May-Aug). The southern plot shows a similar (i.e., opposite, as seasons are opposite) trend, although the seasonal dependence is less pronounced. The high MIT occurrence during equinoxes may be linked to the increased auroral activity around the equinoctial months. Increased auroral activity facilitates the buildup of the main trough's poleward wall, leading to easier identification of trough signatures around equinoxes. Furthermore, during geomagnetic activities, the ionospheric convection flows increase, and as a result, due to the frictional heating mechanism, more troughs eventually form [18]. Seasonal fluctuations are more pronounced in the northern hemisphere than in the southern hemisphere, which indicates hemispheric asymmetry. This could be related to the offset of the magnetic and geographic poles [18]. Figure 10 shows the MIT distribution during December solstice (Nov-Feb), June solstice (May-Aug), and the equinoxes (Mar, Apr, Sep, Oct) over the northern (see top panel) and southern hemispheres. The scatter plots show the MIT minimum positions on global maps. An ellipse approximates the mean position of the MIT minimum for each season. The mean values were first determined by binning trough location with one-degree geographical latitudinal and longitudinal resolution; the mean values were then fitted to the ellipses. We used a similar algorithm to fit the ellipsoids to data described by [PERSON] et al. [27]. We further investigated the spatial distribution of the MIT minimum depending on the season (see Table 2). We computed the mean position of the MIT minimum and corresponding STD values during the December solstice, June solstice, and the equinoxes. The statistical estimates are presented in Table 2 (STD values are given after \(\pm\) sign). We found that the mean trough minimum position varies slightly depending on the season. Compared to the values during equinoxes, the mean trough minimum moves equatorward during local summer (June solstice for NH and December solstice for SH). In contrast, during local winter (December solstice for NH and June solstice for SH), the mean trough minimum moves poleward. The standard deviation values are lower for the NH than for the SH. Figure 9: Seasonal variation in the MIT occurrence rate for the northern and southern hemispheres. (**a**) MIT monthly variation in the northern hemisphere; (**b**) MIT monthly variation in the southern hemisphere. In order to determine the seasonal variation in Tm, width, and depth, their monthly mean and STDs were computed. The monthly mean variations in Tm, width, and depth are plotted in Figure 11. The corresponding STDs are plotted as error bars. The plot for the southern hemisphere (blue line) shows that the MIT is deeper during the winter months (May-Aug) than during the summer months. The plot for the northern hemisphere (yellow line) shows a similar trend, although the seasonal dependence is less pronounced. The Tm variation shows that during winter months, the Tm position is located at slightly higher latitudes compared to summer months (see first panel). In the southern hemisphere, the MIT is located at a higher latitude in the summer period. The trough width varies from 8 to 10 degrees in the northern hemisphere, and in the southern hemisphere, the trough width is smaller, and in the range of 5 to 7 degrees [18]. The trough width broadens in wintertime, with an opposite correlation in the southern hemisphere. This could be caused