## 5 Climatology (2002-2022) of Longitudinal Structures in the Ionosphere
### TEC WN2 Structure  
The climatology of the TEC WN2 structure is calculated by the superposition of the same climatologically mean nonmigrating tides (and SPW2) as those in the lower thermosphere, i.e., DE1, DW3, S0, SW4, and TW1. Figure 14a shows the LT variability of the climatologically mean (2002-2022) TEC WN2 structure at different latitudes, shown in the upper side of each plot, during October for the NH and April for the SH. Similarly to the TEC WN3 structure, here, also, the seasonal dependence of the TEC WN2 structure is different from that of the SABER temperature WN2 one. The reason is again the prevailing ionospheric semiannual variability that has not been removed in advance. The main features of the climatologically mean TEC WN2 structure can be summarized as: (i) the immediately striking feature of this TEC structure in the latitude range \(\pm 40^{\circ}\) is that it is significant mostly in the afternoon and evening hours, somewhere between 14-15 LT and 22-23 LT; (ii) in general, there is hemispheric asymmetry, as this time the NH structure is stronger than the SH one with an exception at \(\pm 20^{\circ}\); if we consider the strongest TEC WN2 at all latitudes, then this asymmetry is valid as well again without \(\pm 20^{\circ}\); (iii) this TEC structure reveals a second type of asymmetry in that the significant structures in the two hemispheres are out of phase in April and October; while the TEC WN2 maxima are centered at \(-150^{\circ}\) and \(30^{\circ}\) in the SH, those in the NH are at \(-60^{\circ}\) and \(120^{\circ}\); (iv) besides the above-considered structures with the largest magnitude of 5.4 TECU, there are also weaker structures observed predominantly during early morning and daytime, between 04-05 LT and 14-15 LT; these TEC WN2 structures demonstrate the third type of asymmetry expressed in the reverse tilt of the structures; while they are tilted to the west in the NH, those of the SH are oriented to the east; (v) here, also, there are WN2 structures observed between 00 LT and 03-04 LT, but in general are not strong; however, at some latitudes, particularly in the NH, they are stronger than the daytime ones; and (vi) there is a feature, not shown in Figure 14a but which has to be mentioned, that at latitudes of \(\pm 50^{\circ}\), the strongest structures are the daytime ones seen between 06 LT and 18-19 LT, where again the NH WN2 magnitude (3.1 TECU) is larger than that of the SH (2.6 TECU). In addition to October and April, large TEC WN2 magnitudes are also seen in March and November; the latter, however, are very similar to those in April and October. That is why Figure 14b presents the latitudinal dependence of the tidal and SPW2 contribution to the TEC WN2 structure only in October (left plot) and April (right plot). These results show that both the DE1 and SPW2 are the main contributors for the production of this structure, as the SPW2 is a larger than the DE1 contributor in April. The next contributor is DW3, followed by SW4, S0, and finally TW1. Figure 14c demonstrates the latitude-time climatological structures of the first four contributors to the TEC WN2 structures; these are the SPW2 (upper left plot), the DE1 (upper right plot), the DW3 (bottom left plot), and the SW4 (bottom right plot). The both factors connected with the hemispheric asymmetry of these components and their contributions to producing the TEC WN2 structure (Figure 14b) define the above-mentioned asymmetric features of this structure. Figure 14b,\(c\) also demonstrate why the TEC WN2 structure does not have small magnitudes at latitudes of \(\pm 50^{\circ}\) (as 3.1 TECU) in comparison with those of WN4 and WN3 ones (as 0.9 TECU and 1.35 TECU, respectively).