The influence of the terminator (day/night boundary) and long-term trends

The terminator is the boundary between the illuminated (day) and unilluminated (night) hemispheres. At this boundary the ionization, and thus the conductivity of the lower ionosphere, changes abruptly, which affects the geometry and electrical properties of the Earth-ionosphere cavity. The passage of the terminator therefore modifies the conditions of ELF wave propagation and contributes to the diurnal variability of the resonance parameters; it is one of the factors why the behavior of the cavity differs between day and night.

Alongside these fast (diurnal) effects, long-term trends are also sought in the measurements. These tend to be associated with large-scale climatic oscillations, especially with the ENSO phenomenon (El Niño / Southern Oscillation — see cluster G), and with solar activity within the roughly eleven-year solar cycle (see cluster H). Distinguishing a genuine trend from natural variability is methodologically demanding, and so part of the interpretation of long-term changes remains a subject of active research (🧪), whereas the physical influence of the terminator on the cavity is well established (🔬).

*Cluster F file — part of the Schumann resonance knowledge base.*