Simulation of Focusing Effect of Traveling Ionospheric Disturbances on Meter‐Decameter Solar Dynamic Spectra

Abstract

AbstractFor the first time we present simulation results of the focusing effect of the ionospheric plasma density irregularities, namely, medium‐scale traveling ionospheric disturbances (MSTIDs), on solar radio emission by applying a ray tracing method to the Earth's ionosphere with MSTIDs. With this technique we investigate the focusing effect that manifests itself in the form of peculiar spectral disturbances in intensity with specific morphology, so‐called Spectral Caustics (SCs), occasionally appearing in dynamic spectra of different solar radio instruments operating in the meter‐decameter wavelength range. We show that the simulated spectral shapes of SCs are in good agreement with the ones detected in real solar radio spectrograms. In particular, SCs that are classified as inverted V‐like, V‐like, X‐like, and fiber‐like types have been reproduced. It is also found that the seasonal dependence in the occurrence of SCs, which has been discovered recently, can be understood through a strong relation between the focusing frequency—the most important characteristic point in most SC patterns—and the elevation angle of the Sun. We find that under typical parameters of MSTIDs with spatial and temporal periods set to be 300 km and 40 min, respectively, the focusing frequency decreases with the growth of the elevation angle. Physical interpretations of the results and implications on the analysis of solar radio data with SCs are discussed.