Latitudinally dependent Trimpi effects: Modeling and observations

Modeling studies show that the exclusion of the propagating VLF wave from t he ionospheric region results in the decline of Trimpi magnitude with patch altitude. In large models such as Long Wave Propagation Capability (LWPC) this exclusion does not occur inherently in the code, and high-altitude pre cipitation modeling can produce results that art: not consistent with obser vations from ground-based experiments. The introduction to LWPC of realisti c wave attenuation of the height gain functions in the ionosphere solves th ese computational problems. This work presents the first modeling of (Born) Trimpi scattering at long ranges, taking into account global inhomogeneiti es and continuous mode conversion along all paths, by employing the full co nductivity perturbation matrix. The application of the more realistic heigh t gain functions allows the prediction of decreasing Trimpi activity with i ncreasing latitude, primarily through the mechanism of excluding the VLF wa ve from regions of high conductivity and scattering efficiency. Ground-base d observations from Faraday and Rothera, Antarctica, in September and Octob er 1995 of Trimpi occurring on the NPM (Hawaii) path provide data that art: consistent with these predictions. Latitudinal variations in Trimpi occurr ence near L = 2.5, with a significant decrease of about 70% occurrence betw een L = 2.4 and L = 2.8, have been observed at higher L shell resolution th an in previous studies (i.e,, 2 < L < 3).