Characterizing Ionospheric Gradients From Oblique Angle-of-Arrival Ionosondes

Ionospheric variability exists on a range of spatial and temporal scales, and can routinely alter or degrade high-frequency (HF) radio wave propagation characteristics, often in an unpredictable way. Real-time data-assimilative models of the ionosphere, such as the one used in Australia's Jindalee Operational Radar Network (JORN), capture a great proportion of the day-to-day and hour-to-hour variations in electron density by spatially mapping between a fixed set of conventional ground-based ionosondes. However, with each sounder estimating electron density profiles under the assumption of a symmetric great-circle propagation path, it is of interest to evaluate the ability of the model to characterize medium- to large-scale gradients.

This presentation shows observations from an oblique-incidence angle-of-arrival (AoA) ionosonde network, which have been transformed using an off-angle reflection point geometry to give estimates of the zonal and meridional ionospheric tilt angles about the path midpoint. Direct comparisons of tilt angles estimated from individual AoA paths against tilt angles extracted from the spatially mapped model (i.e. without AoA inputs) display an encouraging level of agreement, particularly about the dawn/dusk terminators and in the signatures attributed to traveling ionospheric disturbances. As well as being used for model assessment, such AoA-derived tilt estimates also offer valuable ionospheric information in their own right; this may be particularly beneficial for regions of the Earth where conventional ionosondes are more sparsely distributed (e.g. over oceans) and spatial mapping techniques are thus less effective.