TEC perturbation values collected during the storm of 05 August 2011 have shown the formation of LSTIDs that originated on the northern auroral oval and propagated southward into the opposite hemisphere. No LSTID was observed propagating northward. The TEC perturbation is derived by subtracting TEC – TEC(fitted). As mentioned above, TEC perturbations, also called delta TEC, consist of the ionospheric signature of large-scale GWs that propagate from both auroral ovals toward the equator and intersect near the geographic equator. However, we observed that LSTIDs propagated only from the northern hemisphere on this day. LSTIDs from the southern hemisphere were nonexistent, too weak to be seen, or overrun by the northern hemisphere LSTIDs near Antarctica. It is suggested that this particular circumstance was produced by the considerable positive value of the IMF Bx component. According to antiparallel merging, a significant positive Bx makes the merging point move above the ecliptic plane. This configuration makes the distance between the merging location and the northern auroral oval much shorter than the southern one.

TEC perturbation values collected during the storm of 23 August 2016 showed the formation of typical large-scale TIDs (LSTID) that moved equatorward and propagated into the opposite hemisphere. The TEC perturbation is derived by subtracting TEC – TEC(fitted). This formula is applied to TEC values collected by each GPS/GNSS receiver and each PRN satellite. TEC perturbations, also called delta TEC, consist of the ionospheric signature of large-scale GWs that propagate from both auroral ovals toward the equator and intersect near the geographic equator. The train of LSTIDs originated in the Southern hemisphere (SH) overrun and destroyed the NH LSTID first at 21:35 and then later at 22:15 UT. The SH MSTIDs continue propagating further north and likely fading at later times. This movie demonstrates that TIDs can interfere with each other and produce smaller-scale structures that are a source of “noise” in the thermosphere and ionosphere. However, their north-south motion seems to prevail and remain during their transit and later after getting fragmented.