Studies On Indexation And Simulation Of Ionospheric Disturbances

Ionospheric disturbances are an important problem of ionospheric physics.It’s also one of the main tasks of space weather forecast.The ionosphere is coupled with the co-existing thermosphere,and influenced by the solar and geomagnetic activities from the upper layer and atmospheric activities from the lower layer.Therefore,the ionospheric disturbance is characterized by complex,multi-scale and irregular variations.When the ionosphere is disturbed,the electron density changes,which affect the short-wave communication,satellite communication,navigation and positioning,high-frequency skywave over-the-horizon radar system and the measurement and control of spacecraft.It is necessary to study the physical mechanism and index forecast of ionospheric disturbance to avoid the economic damage of space weather events and to maintain the safety of human space activities.The separation method of ionospheric disturbances from the background has always been the core and key point of researches on ionospheric disturbances.The application of spectral whitening method（SWM）in ionosphere provides a new mathematical method for extracting the characteristics of ionospheric disturbance.The ionospheric index Js,Jr and Jp constructed based on SWM can well reflect the single-station,regional and global ionospheric disturbances.The Js map can visually reflect the two-dimensional spatial variation characteristics of the ionospheric disturbance.Based on the three indices,the ionospheric disturbances response to typical geomagnetic storms and typhoon were studied in this paper.The feasibility and advantages of SWM under different disturbance intensities are verified.Firstly,the ionosphere indices Js,Jr and Jp,as well as the solar interplanetary and geomagnetic parameters,are used to analyze the processes of two G4 level geomagnetic storms in the“St.Patrick’s Day”event in March 2015 and the“Zhongyuan Festival”event in September 2017.The intensity of local ionospheric disturbance in the two events was comparable.However,the ionosphere disturbed in large range and lasted longer in the“St.Patrick’s Day”event due to the shock wave rapid compression of southward component of interplanetary magnetic field.In the“Zhongyuan Festival”,the ionosphere disturbances show regional phase-type characters caused by southward component of interplanetary magnetic field shortly increased for three times with three ICMEs arriving at the Earth.Besides,we attempt to extract the disturbances of solar,geomagnetic,ionospheric and neutral atmospheric density by using the SWM,and to construct new indices of each region.The responses of space weather chain from upstream to downstream can be seen from the new indices,which have been extracted from the background trends.The decline trend of solar cycles in F10.7 index,Sunspot Number（SSN）,SA（Solar Active Area）and GTEC（Global Total Electron Content）was obviously suppressed in the new indices J_F10.7,J_SSN,J_SSA and J_{p TEC} by SWM.For Dst,Ap and AE,which background have been removed before,the disturbance characteristics shows little differences after SWM processing.Dst,AE and corresponding new indices J_Dst,J_AE in“St.Patrick’s Day”event were greater than those in“Zhongyuan Festival”event,while Ap and J_Ap were greater in“Zhongyuan Festival”event.The different performance of three geomagnetic indices may attribute to the different latitude distribution of observation stations,the different types of geomagnetic data,and the different time accuracy of the data.Although the solar activity in the“St.Patrick’s Day”event was much stronger,the overall ionospheric disturbance was smaller than that in the“Zhongyuan Festival”event,and the atmospheric density disturbance around 300km was larger than that in the“Zhongyuan Festival”event.This may be due to a sudden increase of the topside ionospheric plasma density in the“St.Patrick’s Day”event.The solar wind energy was captured by the topside ionosphere,which result in low electron density below the F2 layer.Based on the high-resolution TEC data of CMA and the 2-D Js map reflecting the spatial variation characteristics of the ionosphere,this paper studied the ionospheric disturbance during the super typhoon“Usagi”in September 2013,and showed the ionospheric spatial disturbances characteristics ionosphere responded to typhoon for the first time.The result showed that the ionospheric disturbance is strongest in the area near the typhoon path,and the ionospheric activity may change as the typhoon approaches the mainland（including large islands）.In addition,the Global Ionosphere–Thermosphere Model with local-grid refinement（GITM）was used to examine the influence of typhoon-induced gravity waves on the ionosphere and thermosphere.In this study,gravity waves induced by the typhoon Meranti in September 2016 have been simulated.Information of the TC shape and moving trails is obtained from the TC best-track dataset and the gravity wave patterns are specified at the lower boundary of GITM-R（100 km altitude）.The horizontal wavelength and phase velocity of wave perturbation at the lower boundary are specified to be consistent with the TEC observations.The simulation reveals a clear evolution of CTIDs,which shows reasonable agreement with the GPS-TEC observations.This is the first time the typhoon-driven TEC perturbation has been simulated in a general circulation model.To further examine the dependence of the CTIDs on the wavelength and frequency of the gravity wave perturbation at the lower boundary,different waveforms have been tested as well.The magnitude of CTIDs has a negative correlation with the period and a positive correlation with the wavelength when the horizontal phase velocities are sufficiently fast against the critical-level absorption.