In recent years reports have appeared from various ionospheric observatories concerning the appearance of a third magneto‐ionic component or a so‐calledztrace. The phenomenon has also been called magneto‐ionic triple splitting. After having recorded a number of such triple splits at the new Kiruna observatory, we became very interested in the problem as a whole. The entire matter was examined theoretically and as a result it has been possible to show that strong coupling often exists between the ordinary andzcomponents as a critical level. It has further been possible to show that thezwave becomes strongly excited when the collisional frequencyvapproaches the critical collisional frequencyvcof Appleton and Builder. Whenvis larger thanvctheoecho rapidly disappears and only thezandxcomponents remain (ifvcis sufficiently small,i.e.at very high latitudes). A useful expression for the transmission coefficient of theo —zwave has been derived.