Parameters of traveling ionospheric disturbances: results from spaced oblique HF sounding
Abstract
Urgency. A characteristic feature of the ionospheric radio channel is that its parameters depend on season, local time, solar activity, geographic coordinates, the state of the sun, the state of atmospheric-space weather, etc. The state of the atmosphere and ionosphere and thus the state of the corresponding radio channels is rarely quiet. Usually, they are disturbed by high-energy sources of natural or anthropogenic nature. It is the perturbation of the parameters of the radio channel that limits the potential tactical and performance characteristics of radar, radio navigation, radio astronomy, radio communication systems, and remote radio sensing of the Earth from space. To improve the radio link performance, the non-stationary and random disturbances need to be diagnosed, and the link characteristics should be adapted to the disturbed conditions. The dynamic processes in the atmosphere are studied with many radio techniques, viz. ionosonde, incoherent scatter radar technique, signals from Global Positioning System satellite constellation, observations of HF radio waves at oblique incidence, etc. The oblique incidence technique can use dedicated transmitters, intercept signals from non-dedicated transmitters, utilize the transmitters of opportunity, those used for broadcasting or for dedicated services. As a result, the continuous monitoring of the ionosphere becomes cheaper when shortwave radio communication links at HF frequencies are used for overseas broadcasting. The technique of oblique sounding is a passive technique, and it is capable of monitoring large, up to global-scale, areas of the planet Earth. The technique shows high sensitivity since it employs measurements of the Doppler shift and the signal amplitude. Its time resolution is 10 s, and the Doppler resolution is 0.01–0.1 Hz, yielding the Doppler shift error of better than 0.01 Hz. These errors translate into the capability of monitoring electron density variations of order of 10–4–10–3, and vertical movements of 0.1–1 m/s.
The aim of this work is to present the results of determining the parameters of traveling ionospheric disturbances over the People’s Republic of China by making use of the database measurements made with the multiple path oblique HF radio waves probing the ionosphere.
Methods and Methodology. The multifrequency multiple path coherent system located at the Harbin Engineering University campus has been used to acquire spaced measurements along 14 oblique HF radio wave propagation paths of different lengths and orientations. Based on the harmonic model of traveling ionospheric disturbances and making use of three propagation paths, the phase shifts along two propagation paths were determined in relation to the reference path. This shift equals to the dot product of the wave vector on the known reference base, which is defined to be the vector from one transmitter to another. Generally, a superposition of traveling ionospheric disturbances with different values of wave vector and frequency occurs instead of a single traveling ionospheric disturbance. To determine the parameters of these traveling ionospheric disturbances, the cross-spectra are first calculated, which are then used for calculating the periods and phases of the cross-spectra.
Results. Regarding spaced measurements taken along oblique HF radio wave propagation paths with the multifrequency multiple path coherent system, the technique has been developed for determining periods, horizontal wavelengths, and the azimuths of arrival of the traveling ionospheric disturbances. The cross-spectral analysis was invoked to determine the periods, horizontal wavelengths, and the azimuths of arrival of the traveling ionospheric disturbances with predominant amplitudes. The ~16–40-min period traveling ionospheric disturbances are confirmed to be generated by atmospheric gravity waves, whereas the ~10–13-min period waves are electromagnetic in nature. Usually, the relative amplitude of disturbances in the electron density was observed to be ~1–10%. As a rule, the ionospheric disturbances traveled along magnetic meridians, most often from the north to the south.
Conclusions. The system performance and working of the proposed techniques have been demonstrated through the determination of salient parameters of traveling ionospheric disturbances.
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References
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