Observations and Modeling of Traveling Ionospheric Disturbance Signatures From an Australian Network of Oblique Angle‐of‐Arrival Sounders

A network of oblique angle‐of‐arrival (AoA) ionosondes was installed as part of the Elevation‐scanned Oblique Incidence Sounder Experiment (ELOISE) in September 2015. The ELOISE experimental campaign was designed to study the spatial and temporal structure of ionospheric variability at midlatitudes, of which traveling ionospheric disturbances are a key component. The new AoA sounder makes use of Defence Science and Technology Group's direct‐digital high‐frequency transmitter and receiver technology, to enable multichannel collection of both ionograms and channel scattering functions (Doppler spectra) on a common 2‐D array. In this paper, the array design and onboard signal processing for the AoA sounder is described, along with a sample of results showing typical disturbance signatures across the delay, Doppler frequency, bearing, and elevation measurements. Realistic parameterized models of electron density perturbations, along with geometric ray tracing, were used to synthesize the effects of medium‐ to large‐scale traveling ionospheric disturbances on the sounder observables and aid in interpreting the measured signatures. Plain Language Summary The ionosphere is an outer charged layer of the Earth's atmosphere that is used by technologies such as over‐the‐horizon radar to “bounce” (refract) high‐frequency radio waves out to very long distances beyond the horizon. However, as a propagation medium, the ionosphere is far from uniform, and to improve the performance of such radars, it is necessary to better understand the nature and sources of variability that affect high‐frequency propagation. One of the aims of the Elevation‐scanned Oblique Incidence Sounder Experiment (ELOISE), conducted by Defence Science and Technology Group in September 2015, was to study the wave‐like perturbations known as traveling ionospheric disturbances by receiving a network of low‐power ionospheric sounder (or ionosonde) transmissions on two new antenna arrays. These were designed to measure path delay, angle‐of‐arrival and Doppler shifts simultaneously on multiple paths. This paper describes the ELOISE array and its signal processing and provides examples of typical observations. It is shown that the characteristic signatures of traveling ionospheric disturbances in the measurements, combined with realistic synthetic models, can provide information about the structure and behavior of such ionospheric disturbances in space and time. Key Points Two interferometric receiving ar