Detailed analysis of Trimpi phenomena (Trimpis) revealed two different regimes in their temporal behavior. One time regime is the initial nose part of Trimpis of about 1--5 s, and the second is the tail of Trimpis of about 50--100 s. Using a rather simple theoretical model of VLF wave scattering from abating ion and electron density perturbations, we discovered two fitting formulas to describe the temporal evolution of Trimpis. An exponential-hyperbolic evolution in amplitude a/a0=(exp{-t/&tgr;0})/[1+b/2(1-exp{-t/&tgr;0})> for weak scattering and a logarithmic dependence a/a0=[ln &tgr;2/(t+&tgr;1)>/(ln &tgr;2/&tgr;1) for strong scattering. Parameters b, &tgr;0, &tgr;1, &tgr;2 were calculated for all the observed 50 Trimpi events. Maximal ion and electron density perturbation (Δnmi and Δnme) were estimated together with the background ion and electron density (ni0 and ne0) and vertical length of the perturbation column (ΔH). For the conventional model of background electron density and horizontal scale of perturbations L≃5--10 km, we find the pertubation parameters to be as follows: Δnme≃103--104 cm-3, Δnme/n&egr;0~Δnmi/ni0≃10--100, ni0≃3¿103--3¿104 cm-3, and ΔH~10--15 km. ¿ 1998 American Geophysical Union