Sustained heating of the ionosphere above thunderstorms as evidenced in “early/fast” VLF events

Quasi‐electrostatic (QE) thundercloud fields are proposed to maintain the ionospheric electrons at a persistently heated level well above their ambient thermal energy. Changes in the thundercloud charge (e.g., in lightning discharges) lead to heating/cooling above/below this quiescent level, and are registered as sudden (i.e., ‘fast’ <20 ms) subionospheric VLF signal changes, occurring simultaneously (i.e., ‘early’, <20 ms) with lightning discharges, and referred to as early/fast VLF events [Inan et al., 1993].