Two characteristic heights (electric and magnetic)

Because the conductivity grows smoothly with height, there is no single sharp upper boundary of the cavity. The propagation of ELF waves is therefore described using two characteristic heights:

• the electric height h_e (≈ 50 km), where predominantly the vertical electric field component is bound;
• the magnetic height h_m (≈ 90-100 km), where the horizontal magnetic component is "anchored".

These heights correspond to two different conductivity levels, to which different field components are sensitive. Their difference h_m - h_e (on the order of tens of km) is a key parameter: it governs the propagation of ELF waves and enters into the complex phase velocity (see #23). The larger the difference, the more pronounced the losses and the lower the effective phase velocity relative to c.

The concept of two heights is the standard way of replacing the continuous conductivity profile with two physically intelligible numbers without losing the essence of propagation. It is followed by the "knee" model (see #22), which approximates the profile more accurately.