Research On Temporal And Spatial Distribution And Evolution Of Storm Enhanced Density

The polar ionosphere is the "natural window" for the earth to open into space and the natural "Display Screen" for many space weather phenomena.Various dynamic processes caused by solar wind-magnetosphere coupling,such as large-scale convective electric field and particle sedimentation,can be directly mapped to the polar ionosphere along the magnetic field lines,forming many heterogeneous structures with different scales,including large-scale Storm Enhanced Density(SED)and Tongue of Ionization(TOI).Medium-scale Polar Cap Patch,etc.The formation and evolution of these iregularities are accompanied by physical processes such as velocity shear instability and gradient drift instability,and then many smaller-scale iregularities are formed,which often cause ionospheric scintillation,thus leading to serious consequences such as signal lock-out and navigation and positioning failure of spacecraft.For example,during a magnetic storm,a long and narrow plume moving from mid-latitude to high latitudes,with high density and zonal distribution——Storm density enhancement(SED),which causes the total electron content(TEC)in the ionosphere to be dozens or even hundreds of times higher than the background density,can often cause severe disturbance or even interruption of signals such as satellite communication and navigation and over-the-horizon radar.Therefore,the polar ionospheric iregularities have always been a research hotspot at home and abroad.However,the formation mechanism and evolution characteristics of ionospheric iregularities in the polar region are still far from clear due to the lack of large-scale continuous observation data due to the harsh natural environment in the polar region.Focusing on the morphological characteristics and temporal and spatial distribution of SED,this paper explores the evolution characteristics of SED in polar ionosphere and the main influencing factors of SED cutting to form plasma clouds through the joint observation of various space-based and ground-based equipment and the simulation analysis of typical events combined with TIEGCM model.The main results of this paper are as follows:1.Through the combination of observation and simulation,a typical conjugate event of SED in the northern and southern hemispheres was deeply studied,and the formation mechanism of different evolution processes of conjugated SED in the northern and southern hemispheres was revealed.On January 4th,2015,the global TEC data recorded that there were storm-time density enhancement structures(SED)in both hemispheres,but the evolution angle of SED plume in both hemispheres exceeded 90.Through observation and simulation,it is found that the dynamic balance between the plasma density supplement and the density decrease caused by the earth’s rotation at the location of the SED plume is the main reason for the evolution difference between the northern and southern hemispheres.In the northern hemisphere,the enhancement of plasma density by the net production rate and electric field transport in the photochemical reaction on the solar side can offset the density decrease caused by the earth’s rotation;In the southern hemisphere,the net production rate of photochemical reaction is offset by the enhancement of plasma density caused by electric field transportation and the decrease of plasma density caused by bipolar diffusion,but it is not enough to offset the local density decrease caused by the earth’s rotation.As a result,the Sedplum in the northern hemisphere has been tilted to the morning side,while the Sedplum in the southern hemisphere has been stretched to the east-west arrangement with the rotation of the earth,which eventually leads to the evolution angle of Sedplum in the two hemispheres exceeding 90.The physical mechanism of the difference in the evolution process of SED plume between the northern and southern hemispheres is as follows:first,the difference between geographical poles and geomagnetic poles in the northern and southern hemispheres leads to different electric field transport effects;second,the neutral wind behaves differently in the northern and southern hemispheres.2.By analyzing the data of multi-means cooperative observation,two typical cases of SED structure being cut into plasma clouds before entering the polar cap region under different magnetic storm conditions are deeply studied,and the physical mechanism of ionospheric convection enhancement caused by solar magnetic reconnection and then cutting SEC to form plasma clouds is revealed.(1)During the great magnetic storm on February 27th,2014,a typical storm-time density enhancement structure appeared at high latitudes in the northern hemisphere,which was successively cut to form several large plasma clouds.The observation shows that the rapid northwest ionospheric convection with high ion\electron temperature and low plasma density(speed up to 1.5 km/s)plays a key role in this cutting event.We characterize the change of reconnection rate by Kan-Lee electric field,and find that the generation of this fast northwest current is closely related to the modulation of ionospheric convection by reconnection of magnetospheric top magnetic field.In addition,the size of the plasma cloud formed By cutting the SED in this event is large in the latitudinal space,which may be caused by the fact that the distribution area of magnetic reconnection at the top of the magnetosphere on the Japanese side is wider and wider than usual under the condition of the Interplanetary Magetic Field,IMF)with a strong continuous positive by component(By>10nT).(2)During the small magnetic storm on December 21,2014,there was also a storm-time density enhancement structure in the high latitudes of the northern hemisphere,but the density(TEC)of this structure was only half of that of the event on February 27,2014,and it was also cut into a small plasma cloud before entering the polar cap region.The observation results show that there is also northwest convection enhancement at the cutting position of the SED structure,which may also be caused by the transient magnetic field reconnection at the top of the magnetosphere.3.Through numerical simulation,the three-dimensional plasma characteristics and internal physical mechanism of the above two typical SED plasma cloud events formed by reconnection electric field cutting are revealed.(1)First of all,we simulated the SED event during the great magnetic storm on February 27th,2014 by using the TIEGCM model,and the results showed the formation process of the SED plume structure which was in good agreement with the observation.However,due to the high overall density of the SED in this event,the process of cutting the SED into plasma clouds was not clearly shown in the simulation.Therefore,we added an additional horizontal electric field of 0.08V near the latitude of the cusp region in the simulation to reflect the influence of solar magnetic reconnection.It was found that the TEC of SED plume at the latitude near the cusp region decreased significantly,which was similar to the characteristics of the cutting region observed.Therefore,it is proved that the polar ionosphere mapped by the reconnection electric field formed by the magnetic reconnection at the top of the magnetosphere on the solar side causes the enhancement of the polar ionosphere convection in the polar gap region,and cuts the SED plume to form a polar cap patch.(2)Secondly,we also simulated the SED event during the small magnetic storm on December 21st,2014 by using the TIEGCM model,which also showed the formation and evolution process of SED consistent with the observation.We comprehensively compare the contributions of several related parameters to the reduction of oxygen ion number density at different ionospheric heights,and find that at the ionospheric height of 290km,the net loss rate of photochemical reaction and the density loss caused by electric field transport play a major role in cutting SED;At the height of 350km,the density loss caused by electric field transport and bipolar diffusion is dominant;At the height of 400km,neutral wind and bipolar diffusion play the main role.At the same time,we also added an extra horizontal electric field of 0.08V to the latitude attachment of the polar gap region in the simulation,which has reflected the influence of solar magnetic reconnection.It is found that the cutting effect of plasma clouds formed by SED at different heights is obviously improved under the action of this electric field.The research results in this paper will enhance our understanding of the formation and evolution process of ionospheric inhomogeneities in polar regions such as storm density enhancement and plasma clouds,and help us to deeply understand the dynamic process of solar wind-magnetosphere-thermosphere-ionosphere coupling,and provide data and theoretical basis for polar ionospheric theory/empirical modeling model.