Contribution of the Czech-lead international network of ionospheric Doppler shift sounders to vertical coupling studies

This paper summarizes contribution of the Czech-lead international network of ionospheric Doppler shift sounders to vertical coupling studies. The network consists of Doppler sounders in the western half of Czechia (5 measuring paths, 3 frequencies with central receivers in Prague), northern Taiwan (3 transmitters, two separated receivers, 1 frequency), and two similar systems (3 measuring paths with 1 receiver and 1 frequency) in Tucuman (north-western Argentina) and Hermanus (the southernmost South Africa). Main results: (1) The theoretically expected dominance of gravity wave propagation against wind has been confirmed. (2) Impact of a train of seismic waves (P, S, SS, Rayleigh) generated by the Tohoku 2011 M9.0 earthquake was registered in the ionosphere over the Czech Republic as a long-period infrasound at the distance of about 9000 km from epicenter. (3) Nonlinear effects result in transformation of wave train into N-shaped pulse disturbance in the upper atmosphere /ionosphere above strong earthquakes as documented by the M8.3 Illapel 2016 earthquake. (4) There are three mechanisms of Doppler shift - vertical motion of the reflecting layer (gravity wave effect), compression and rarefaction of the atmosphere/ionosphere (infrasonic wave effect), and rapid change of ionization rate (solar flare effect). (5) Short period gravity waves and rarely infrasound are observed in the ionosphere above large convective systems. (6) The Doppler shift during solar flares responds to the rate of change of ionizing flux, not to its magnitude, and the sounding signal is remarkably attenuated. (7) Typhoons near Taiwan produced long-period infrasound for several hours. (8) Effect of solar eclipse is bipolar - first a negative Doppler shift (upward motion, i.e. decrease of electron density), then a positive Doppler shift (downward motion, i.e. increase of electron density).