Using total electron content (TEC) maps of the ionosphere (http://www.aiub.unibe.ch/ download/CODE/) for the northern hemisphere, except the polar region, in the winter seasons of 2012—2018, ionospheric effects of powerful seismic events were considered. It is shown that seismic-ionospheric effect is global, which is superimposed by local effects on epicenters of individual earthquakes (EQ). Temporal TEC variations near time of strong EQs at a great distance from their epicenters (global effect) consist of two maxima: the precursor and «aftershock» maximum. In TEC variations over the EQ epicenter (local effect), only a precursor is usually registered, the amplitude of which at night (on average 8 %) is about twice as high than during day. Always (locally and globally) after a positive surge in TEC, its reduced values are observed for several days. The maximum amplitude zone of the seismic-ionospheric effect belongs to the middle latitudes, especially 35—40° N, and within this zone atlongitudes near 30° W (Mid-Atlantic ridge) and 140—150° E (Japanese islands and adjacent waters of the Pacific Ocean). Latitudinal amplitude maxima of the seismic-ionospheric effect are in good agreement with the latitudinal maxima of the EQ number in both geographic and geomagnetic coordinate systems. Changes in the EQ number and, consequently, the effect in the ionosphere on geomagnetic coordinates are more ordered, which indicates a significant impact on seismicity of the same processes at the boundary of the liquid core and lower mantle, which form the Earth’s magnetic field. In addition to seismic belts and zones of mid-ocean ridges, increase in TEC has been recorded along the so-called lineaments, marking the weakened zones of the Earth’s crust with increased flows of deep gases. The correspondence between the spatial features of seismicity and seismic-ionospheric effect testifies in favor of the «radon» mechanism of lithosphere-ionosphere coupling and indirectly confirms the role of deep gases in formation of seismicity planetary features.