Different Responses of O-Mode and X-Mode in Ionograms to Medium Scale Traveling Ionospheric Disturbances: Observations and Simulations

U-shaped structures (or transient cusp-shaped features) are special phenomena in ionograms, related to the electron density perturbations in the ionospheric F region. However, the mechanisms that trigger distinct shape structures, such as U-shaped and hook-shaped ones, are not fully understood yet. In this study, the ray tracing method is utilized to simulate the synthesized ionograms over Sanya under the medium-scale traveling ionospheric disturbances (MSTIDs) whose propagation parameters are extracted from total electron content observations of BeiDou geostationary (GEO) satellites. It was illustrated that the simulated ionograms are generally consistent with the observations from Sanya ionosonde. Interestingly, X-mode radio waves exhibited stronger responses to MSTIDs, especially at lower frequencies. It was found that the multiple reflection of ionosonde radio wave among different MSTID wavefronts could be the primary cause of the U-shaped structures. The different responses between O-mode and X-mode are influenced by the angles between the geomagnetic field and the MSTID propagation direction. The O-mode (X-mode) traces become more prominent when the geomagnetic field is parallel (perpendicular) to the MSTID propagation direction. Additionally, the MSTID parameters can significantly influence the morphology of MSTID-induced structures in ionograms. For instance, an increase in relative amplitude of MSTID can cause the echo trace structures in ionograms to transform from U-shaped to hook-shaped. Moreover, the downward movement durations of these structures are modulated by the periods and vertical wavelengths of MSTID.