Characterising spatial and temporal ionospheric variability with an oblique angle-of-arrival and Doppler ionosonde
Abstract
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 the 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. These instruments are low-power HF atmospheric radars designed to measure the path loss and delay of ionospheric propagation modes as a function of frequency. However, without direct angle-of-arrival (AoA) or Doppler (radial velocity) observations, the ionospheric model must rely on electron density profiles that have been fitted to ionosonde data assuming a symmetric great-circle propagation path which is smoothly varying in time. The Elevation-sensitive Oblique Incidence Sounder Experiment (ELOISE) involved the deployment of two 19-element ionosonde receiving arrays in central Australia, designed to measure both AoA and Doppler on nine one-way oblique paths of between 900 and 2700 km. Over a period of several weeks in late 2015, many different aspects of ionospheric variability were observed, including large-scale horizontal gradients, travelling ionospheric disturbance signatures, night-time mid-latitude spread-F, responses to geomagnetic storm conditions, and sporadic-E. One of primary aims of the experiment was to evaluate the ability of a conventional spatially-mapped model to characterise such features. In this presentation, propagation modes observed in the ELOISE data are compared against the results synthesised by ray-tracing through the ionospheric model. Aggregated plots reveal geophysical features in the diurnal pattern of ionospheric variability which are well-captured in both observations and model. Individual peaks can also be transformed using an off-angle reflection point geometry to give estimates of the East-West and North-South tilt angles about the path midpoint. Again, there is an encouraging level of agreement with the model, particularly about the dawn/dusk terminators and in the signatures attributed to travelling ionospheric disturbances; this is despite the model being constructed without any AoA information. 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.
- Publication:
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43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E.573H