There are two distinctly different causes of phase and amplitude perturbations of subionospheric VLF transmissions (termed Trimpis): (1) ionization enhancement in the ionospheric D region due to whistler-induced electron precipitation (WEP) and (2) modifications to the ionosphere induced directly by lightning. The latter appear to be subdivided into ionization anomalies produced by cloud-ionosphere electrical discharge (CID), or red sprites, and heating anomalies which may not involve ionization at all. Here we consider only Trimpis (WEP and CID) produced by ionization and find that the magnitude of the Trimpi perturbation (which includes both the amplitude and phase perturbations) of both Trimpi types decays logarithmically rather than exponentially with time. While this has been previously shown for CID Trimpis, the decay of WEP Trimpis was previously thought to be exponential. ¿ 2001 American Geophysical Union