Observation and Research of Strong Ionospheric Disturbances Using a Chaotic Coding Digital Ionosonde

The ionosphere is part of the space above 60 to 1000 km from the ground and is an important part of near-earth space. The study of the ionosphere is conducive to providing better understanding the coupling interaction features within the lithosphere, atmosphere and ionosphere, exploring the possible associations between earthquake precursors and ionosphere disturbances, providing services for human activities and more approaches for disaster prevention and mitigation.

A chaotic coding ionosonde was developed in Yinchuan, Ningxia Hui Autonomous Region, China, in 2021. The ionosonde scans in 1~30 MHz frequency range, with the distance resolution of 1.5km and detection height from 67.5 km to 560 km. It utilizes technologies of pulse compression and chaotic coding, suppresses clutter interferences successfully and obtains high quality ionograms. This ionosonde operates automatically and produces an ionogram every 15 minutes. A multiscale transformer neural network is utilized for the extraction of echo traces and accurate inversion of the ionospheric parameters, such as the critical frequence of F2 layer, the minimum reflection height, the separation of traces of the F layer's O/X waves as well as the electron density profile based on an improved bottom inversion model of the International Reference Ionosphere.

Several strong ionospheric disturbances were observed in 2023 and 2024. In April and November 2023, massive solar flare eruptions caused geomagnetic disturbances, and the F2 layer responded to the disturbances obviously in term of the critical frequence and the traces of the F layer's O/X waves. In December 2023 two earthquakes with ML ≥ 4 happened in Gansu province, and also there were solar flare eruptions during that period. Some ionospheric disturbances were observed by the ionosonde approximately two or three weeks before the earthquakes. Besides, the fluxgate sensors and magnetometers installed on the geomagnetic stations in Gansu province and Ningxia Hui Autonomous Region also recorded the disturbances in the daily curves, synchronous with the ionosonde records. In January and February 2024, some typical U-shaped and sickled-shaped traces are observed in the ionograms, which are considered to be the phenomena caused by traveling ionospheric disturbances (TIDs). Some other disturbance phenomena are also recorded by the ionosonde, including the traces diffusion, partial disappearance, abnormal shapes, etc., worthy of research in multiple fields combining the lithosphere, atmosphere, ionosphere and space physics.