GNSS Computerized Ionospheric Tomography And Its Applications In The Study Of Space Weather Events

Space weather changes will not only affect the morphological structure of the ionosphere,but also further affect the application of the Global Navigation Satellite System(GNSS)and the accuracy and reliability of radio communications.Therefore,monitoring the abnormal changes of ionosphere during space weather events,mastering its change laws and revealing its physical mechanisms are of great significance to study the coupling between different atmospheric layers and provide ionospheric prediction and early warning during space weather events.GNSS three-dimensional Computerized Ionospheric Tomography(CIT)can realize large-scale and high-precision three-dimensional inversion of the ionosphere,and it has been applied to the analysis and researches of ionospheric anomalies during space weather events.Although the accuracy of the three-dimensional CIT algorithm has been greatly improved after continuous improvement,the accuracy and reliability of the solution still need to be improved due to the ill-posed problem of the CIT equation.Moreover,GNSS stations have poor coverage in some regions,which also leads to limited CIT accuracy.Therefore,the existing CIT algorithm needs to be improved to improve its accuracy,reliability and universality,so as to realize higher precision ionospheric three-dimensional monitoring.At present,a variety of ionospheric detection methods can realize large-scale and multi-dimensional monitoring of abnormal changes in the ionosphere,but there is still a lack of deep mechanism explanations.Therefore,combining the ionospheric multisource measured data with the Thermosphere-Ionosphere-Electrodynamics General Circulation Model(TIEGCM),phenomenon observation and mechanism simulation complement each other,which is a necessary means for ionospheric refinement,largescale monitoring and studying the coupling mechanism of ionosphere to space weather events.In view of the above problems,the research work of this paper mainly includes the following aspects:(1)A three-dimensional CIT algorithm that integrates multi-source data is proposed.The algorithm improves the accuracy and stability of CIT by adding the vertical constraints provided by ionosonde and occultation data.The number of CIT layers can be adjusted adaptively according to actual needs.At the same time,the reliability of the algorithm is verified by the measured GNSS data and ionosonde data under three different geomagnetic conditions(geomagnetic storm,magneto-static,and slight geomagnetic disturbance)in Europe region.(2)Through the combination of phenomenon observation and mechanism simulation,three typical space weather events occurred in high latitude,middle latitude and low latitude were analyzed and their mechanisms were discussed.Some new phenomena were discovered,and preliminary mechanism explanations were given.(1)The three-dimensional inversion and anomaly mechanism analysis of ionospheric evolution during the geomagnetic storm on January 7,2015 are carried out.The CIT images not only monitor the horizontal expansion process of the ionospheric anomaly from east to west with the development of the storm,but also track the key process of triggering ionospheric high-level disturbance and spreading to the bottom in the initial stage of the storm.At the same time,the ionospheric changes were simulated by using the TIEGCM physical model,and it was found that the response of ionospheric electron density disturbances at different heights to the storm is inconsistent,which is caused by the action of Prompt Penetration Electric Fields(PPEF)during the storm.(2)The three-dimensional inversion and simulation analysis of the Tongue of Ionization(TOI)structure during the moderate geomagnetic storm on October 11,2010 were carried out.The CIT results show the formation and development of the TOI structure during the storm,and it is found that TOI is more significant in the upper ionosphere,but not in the lower ionosphere,which can’t be effectively observed in other ionospheric detection technologies;The simulation results of TIEGCM show the same TOI structure as that of CIT.Combined with the measured data such as the ionosonde and Super DARN and the TIEGCM,it is believed that the main formation mechanism of TOI is electric field transport,which will be weakened by ambipolar diffusion and neutral wind transport.(3)The three-dimensional inversion and mechanism of ionospheric changes during the total solar eclipse in South America on December 14,2020 are studied.The CIT results show that during the solar eclipse,the ionospheric TEC decreases by up to 10 TECU,and different ionospheric heights have different responses to solar eclipses,with the greatest impact at the altitude of 200-400 km.The TIEGCM simulation results show that the solar eclipse will cause changes in temperature and pressure gradients,and the resulting neutral wind disturbance is the dominant factor in the change of ionospheric TEC.It is also found that the solar eclipse inhibits the generation and development of equatorial ionospheric anomaly(EIA),and the trans-equatorial transmission between hemispheres will enhance the EIA in the conjugate region of the northern hemisphere,indicating that the response of the ionosphere to the solar eclipse is not only the response to the change in solar radiation,but also involves the chemical,dynamic and electric effects in the ionosphere at mid-and low-latitudes.(3)Developed a multi-source and multi-dimensional ionospheric monitoring system based on GNSS observations.The system can realize multi-dimensional modeling of the ionosphere,detect ionospheric anomalies and track the evolution process of anomalies.On this basis,combined with TIEGCM,the evolution of ionospheric anomalies is simulated and analyzed.The system can perform multi-source and multi-dimensional monitoring of the ionosphere’s response to space weather events from two aspects: measured data processing and physical model simulation.