Measurement of the Turbopause Level and Turbulent Velocity in the Lower Ionosphere Based on the Creation of Artificial Periodic Irregularities in the Ionospheric Plasma

We have developed a new method for remote sensing of the ionosphere and neutral atmosphere based on the creation of artificial periodic irregularities (APIs) in ionospheric plasma. This method is used, in particular, to study atmospheric turbulence, which can affect GNSS signal delays by causing disturbances in the radio wave propagation medium. We applied the API method to determine the turbopause height and turbulent velocity at altitudes of 60–130 km in the lower ionosphere. Turbulence is one of the most important phenomena in the Earth’s atmosphere. This article presents the results of turbulence parameter measurements based on the resonant scattering of APIs. These irregularities are created in the ionosphere by powerful high-frequency radio waves. The altitude profile of turbulent velocity is obtained from measurements of the relaxation time of the API scattered signal after the end of the powerful radio emission on the ionosphere. The turbopause level is defined as the altitude at which turbulent mixing of atmospheric gases gives way to diffusive separation. During the transition from diffusive separation to turbulent mixing (below the turbopause level), irregularities decay more rapidly than during ambipolar diffusion, and their relaxation time is determined predominantly by turbulent diffusion. This fact allows us to determine the velocity of turbulent motion as a function of the diffusion time and the measured relaxation time of the scattered signal. The turbopause level is determined from the altitude profile of the relaxation time. Measurements were carried out using the SURA heating facility (56.15° N, 46.11° E). The altitude resolution of the API method is 0.15 km–1 km with a time resolution of 15 s, which allows us to study both fast and slow processes. According to our data, the turbopause level varied in the altitude range from 85 km to 110 km. Significant variability in the turbopause level was observed during the day and from day to day. The minimum turbopause level was located at mesospheric altitudes. The average turbulent velocity over a 5 min interval varied from near zero at the turbopause altitude to 5–6 m/s below it. Such high velocities were typically observed during natural ionospheric disturbances. The average velocity of regular vertical plasma motion, measured using the API method, reached 10 m/s and varied with altitude and direction. Temporal variations in turbulent parameters were observed, with periods ranging from 15 min to several hours. The effect of atmospheric wave propagation on the characteristics of scattered signals and environmental parameters was confirmed.