Equivalent Lonospheric Currents From Ground Magnetometer Observations And Their Responses To Solar Flare And Sudden Stratospheric Warming

The temporal and spatial variations of the geomagnetic field are essential for understanding the dynamic variations of the geospace environment.The geomagnetic field encompasses a vast range of variations on all timescales and of the major factors responsible for ionospheric current variability,which depends on solar activity.The variations associated with geomagnetic field and the ionosphere are what this doctoral thesis investigates using mathematics to give an assessment of the variability content of geomagnetic fields by applying the spherical harmonic analysis(SHA)technique.In this doctoral thesis,the variability of ionospheric current systems relative to sudden stratospheric warming(SSW)as well as solar flares of the same active region(AR)and similar intensity is extensively studied based on the regional and global geomagnetic field observations.The SHA technique has permitted the derivation of both the regional and global equivalent ionospheric current systems.For the SSW impacts on ionospheric and equatorial electrojet(EEJ)currents,the following main results emerged:(1)The strength of the enhancement in EEJ current is associated with the duration of the negative mean zonal wind across all the longitudinal sectors.The significant counter electrojet(CEJ)occurrence associated with significant enhancement in EEJ current occurred in all the sectors before a decelerating zonal wind reverse during the 2005/2006 SSW event.In comparison to the well-investigated 2008/2009 SSW event,a similar feature appeared in all the sectors when the reversing zonal wind is around its maximum.In addition,the results show that most of the complexity in EEJ current is associated with the interaction between the tidal energy and planetary wave variability.(2)The magnitude of varying lunar semidiurnal tide become an important facilitator of CEJ current in the East Asian,African and American sectors during 2005/2006 and 2008/2009 SSW events.The results further confirmed that changes in the value of lunar semidiurnal tide in EEJ strength facilitates and modulates CEJ intensity.The results clearly demonstrate that both SSW events influence the CEJ with a strong longitudinal difference.The strong longitudinal difference in CEJ intensity seems to be related to the basic effective ionospheric conductivity at the longitude concerned.(3)The activity of planetary waves with zonal wavenumber 1(PW1)and zonal wavenumber 2(PW2)showed sudden enhancement during the 2005/2006 SSW event.The potential signatures of these two PWs effects interacting nonlinearly with enhanced lunar-solar semidiurnal tides account for much variability in the CEJ profile during the SSW.(4)Indication for the PWs expansion and tidal variation includes not only the signature in the CEJ occurrence but also the strong variability of the ionospheric current system over the equator.In principle,the ionospheric current shows surprisingly small fluctuations over all the longitudes during the SSWs,which can also be explained by the lunar-solar interactions.On the other hand,we investigated the characteristics of the ionospheric and EEJ currents using the global distribution of the magnetometers during the solar flare event that occurred from September 6 and 10,2017.The following interesting features emerged:(1)The responses of the ionospheric current to the solar flare showed the hemispheric asymmetry,even though the flares happened in the Equinox.During the peak period of X9.33 disk flare,when X-ray and EUV radiation fluxes were significantly enhanced,two enhanced current vortices appeared in 40°N and?30°S,in the northern and southern hemispheres,respectively.For the peak period of X8.28 limb flare,two enhanced current vortices were seen at the latitudes of?16°N and?35°S in the northern and southern hemispheres,and they occurred two hours earlier in local time with respect to those seen in the X9.33 disk flare.(2)The total current the intensity increased from 45 kA at the start flare to 83 kA during the peak of the X9.33 flare and it increased from 38 kA to 73 kA during the peak of the X8.28 flare.The percentage change of the total current intensity of the X9.33 flare is about 84%,which is 8%lower than that of the X8.28 flare.This indicates that,apart from the differences in solar flux,the ionospheric current changes in response to these two solar flares also depend on universal times of the solar flare occurrence.(3)The influence of the X9.33 disk and X8.28 limb solar flares on southern vortex is more than that of the northern vortex.The afternoon CEJ that occurred at?14:55LT over African sector at the peak time of the X8.28 solar flare is likely modulated by a solar flare.In a nutshell,the results of this doctoral thesis will be helpful in any aspects of the atmospheric variability during the SSW events,and will further give deeper insights into the ionospheric electrodynamics.In addition,global studies of the equivalent ionospheric current system relative to the effects of space weather may have significance in terms of monitoring,modelling and prediction of the upper atmosphere.This doctoral thesis concludes with some reflections on the consequences of space weather.