ABBY NORMAL Model

Model Description Details:
The AbbyNormal model is a multi-level model, which includes the following components:

1. An empirical neutral atmosphere model (MSISE00)
2. Physics-based model of the electron-ion densities through the D and E region altitudes
3. High Frequency (HF) signal absorption calculation for HF ray paths through the ionosphere
4. Ionospheric conductivity calculation for vertically integrated Pedersen and Hall conductivities.

AbbyNormal combines these 4 components to predict the attenuation of an HF signal in transit through the earth’s ionosphere. The attenuation depends on the electron and neutral density in the D and E regions. The electron gas of the ionosphere ‘sees’ the electromagnetic wave and damps the signal magnitude if the electron-neutral collision rate is similar to the signal frequency. The electron-neutral collision rate is near HF frequencies (1 to 30 MHz) at D and E region altitudes (60-105 km). Most HF signal propagation codes include this ‘non-deviative’ signal absorption by treating the D region as a thin shell, which attenuates a trans-ionospheric signal. The D-region absorption is often determined through empirical relations with solar angle dependence (1/cos c), HF frequency (1/f 2), and solar indices (F10.7). AbbyNormal uses physics-based models and formulae to calculate the attenuation.

We assume straight-line propagation within AbbyNormal for the HF frequencies of interest (3-50 MHz) because the low densities of the D region do not affect the HF propagation direction. The absorption calculation is performed for a single vertical transit of the HF signal, which is then adapted to non-vertical propagation by using the angle of incidence (1/cos I) for oblique transit absorption (Figure 2).

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