Feasibility study for reconstructing the spatial‐temporal structure of TIDs from high‐resolution backscatter ionograms

Abstract

Over‐the‐horizon radar (OTHR) utilizes the reflective “sky wave” property of the ionosphere for high‐frequency radiowaves to illuminate targets at ranges extending to several thousand kilometers. However, the ionospheric “mirror” is not static but exhibits geographic, diurnal, seasonal, and solar cycle variations. NorthWest Research Associates has developed an ionospheric data assimilation capability called Global Positioning Satellite Ionospheric Inversion (GPSII; pronounced “gypsy”) that allows real‐time modeling of the ionospheric structure for the purpose of accurate coordinate registration (CR; OTHR geolocation). However, the ionosphere is routinely subjected to traveling ionospheric disturbances (TIDs), and the deflection of HF sky wave signals by unmodeled TIDs remains a troubling source of CR errors (tens of kilometers). Traditional OTHR tools for ionospheric sounding (vertical and backscatter ionograms) do not resolve the fine spatial structure associated with TIDs. The collection of backscatter ionograms using the full aperture of the OTHR was recently demonstrated, thus providing enhanced resolution in radar azimuth in comparison with conventional OTHR backscatter soundings that utilize only a fraction of the OTHR receiver array. Leading edges of such backscatter ionograms demonstrate prominent spatial features associated with TIDs. We investigate the feasibility of recovering TID perturbations of ionospheric electron density from high‐resolution backscatter ionograms. We incorporated a model of naturally occurring TIDs into a numerical ray tracing code that allows the generation of synthetic OTHR data. We augmented GPSII to assimilate time series of full‐aperture backscatter ionogram leading edge data. Results of the simulation show that GPSII is able to reproduce the TID structure used to generate the backscatter ionograms reasonably well.

An overview of methodologies for real-time detection, characterisation and tracking of traveling ionospheric disturbances developed in the TechTIDE project

“…At 400 km the perturbation marginally exceeds the background levels. These results are in agreement with earlier studies reporting that TID amplitudes maximize near or below the background electron density maximum hmF2 (e.g., Morgan & Ballard, 1978).…”

“…The accuracy of ground-based single-site-location (SSL) HF radio wave direction finding is severely compromised by the passage of TIDs through the ionospheric reflection area (Nickisch et al, 2016). Small amplitude TIDs, occurring virtually all the time with varying amplitudes, similar to cloud occurrence in the troposphere, can tilt the reflecting isodensity contours by as much as 3°-5°.…”

Tomographic Imaging of Traveling Ionospheric Disturbances Using GNSS and Geostationary Satellite Observations

“…This type of model has been implemented to model TIDs in several recent studies (Cervera & Harris, 2014;Mitchell et al, 2017;Nickisch et al, 2016), and the model itself was tested by Morgan and Calderón (1978) who found relatively good agreement with measurements. The main drawbacks of this model is that there is no viscosity effect to damp out the wave at higher altitudes, and that the scale height is constant with altitude.…”

Simulated high frequency ray paths considering traveling ionospheric disturbances

“…In the present work, the effect of medium-scale TIDs (MSTIDs) on some characteristics of HF ray paths is analyzed. MSTIDs are often associated with the action of atmospheric gravity waves (GWs) [3,4], with periods ranging from several minutes to less than one hour and velocities from ~ 50 to 300 m s −1 . TIDs effect on ray paths can result in Doppler shift of the received signal and changes of the ray trajectories, which in turn can produce apparent fluctuations on return echoes from target in radar applications, and cause coordinate registration errors for over-the-horizon radars OTHR [5].…”

“…In this study, ray path modifications by a simulated TID are obtained using a 3-D ray tracing algorithm developed by Jones and Stephenson [7], which was modified to include the 2016 version of the International Reference Ionosphere (IRI-2016) model [8], the US Naval Research Laboratory Mass Spectrometer and Incoherent Scatter radar model (NRLMSISE-00) [9], a realistic Earth's magnetic field using the International Geomagnetic Reference Field 12th generation (IGRF-12) model [10], and Hooke's TID model [11]. In addition to the links to these models, a cubic interpolation method was implemented to guarantee continuous first derivatives in the electron density profiles [4]. A set of simulations are analyzed to demonstrate the robust performance of this ray tracing technique.…”