| The ionosphere is the part of the upper atmosphere where the number of free electrons is sufficient to affect the propagation of radio waves.The topside ionosphere is the region above the peak of F2 layer,connected to the fully ionized magnetosphere and the peak region of the ionospheric F2 layer.The topside ionosphere is located between two regions with different mechanisms of dominant physical processes and is very important for the ionospheric magnetospheric coupling process.Due to the relatively scarce observational data of the topside ionosphere,the characteristics and physical mechanism of the topside ionosphere have not been fully studied.Plasma source is one of the most fundamental problems in the earth’s magnetosphere.The phenomenon of ions in the ionosphere accelerated and upflowing along the magnetic field line is called ion upflow,which is affected by the particle precipitation,convective electric field,etc.Ion upflow is an important way of ionospheric-magnetospheric coupling interactions.Some ionospheric upflowing ions can escape to the magnetosphere along the open magnetic field lines,and become an important supplement to the magnetospheric plasma.These incident ions also influence the magnetosphere electrodynamics process,causing disastrous space weather,magnetic storms and sub-storms.Therefore,it is of great significance to study the factors controlling the ionospheric ion upflow incidence and ion upflow flux for ionospheric magnetospheric coupling.The observed ion upflow generally confined to narrow latitude regions close to the aurora zone.Ions in this region are modulated by solar and geomagnetic activities and can enter the magnetotail along the open magnetic field lines.No large upflowing ions flux were observed in the closed magnetic field line region.The acceleration mechanism of ionospheric ion upflow mainly includes ion heating at the lower height and electron heating at the top ionosphere.In addition,where O~+ions in the magnetosphere originate from and what mechanism plays a major role in ionospheric ion upflow during storms are still worth further to study.The characteristics of ionospheric electron density variation are hot topics in the research of space physics.The fluctuation of electron density will delay the radar signal and affect the accuracy of high frequency communication and gps positioning.The detection of electron density in the ionosphere is not only helpful to reveal the internal mechanism of various physical phenomena in the ionosphere,but also helpful to improve the performance of wireless communication.The detection of electron density near and below the peak height of the ionosphere F2 layer have attracted the most attention,and there are a lot of measurement methods,while the observational methods of the topside ionosphere are relatively limited.Ground radar observations are mainly restricted by the station coverage area and the high observation cost of radar.Although the satellite in-situ measurements can obtain continuous data at all latitudes,satellites can only provide information on the electron density at orbital altitudes.TEC data from low orbit satellites make up for the lack of topside ionospheric data,and ionospheric tomography can obtain a wide range of electron density structures.The ground-based ionospheric tomography is mainly affected by the electron density at the peak of F2 layer,and space-based ionospheric tomography can better respond to the electron density variation at topside ionosphere.However,due to the ill condition of the observed data,the primary tomography is sensitive to errors.The evolution process at topside ionosphere was limited to the field-aligned structure and lacked the information of adjacent longitude.Limited by sparse data,the space-based ionospheric tomography is mainly used to observe large-scale ionospheric phenomena.The observations of meso-scale ionospheric phenomena during storms hve not been realized.1.Statistical analysis of dayside ionospheric ion upflow.A large number of observations show that ionospheric ion upflow occurs mainly in a narrow region close to the auroral zone.The features are strongly dependent on solar and geomagnetic activities.In order to distinguish the influence of solar activity and geomagnetic activity on dayside ionospheric ion upflow,DMSP satellites observations during solar maximum(2000-2002,F13 and F15)and minimum(2007-2009,F13;2007-2010,F15)are binned into three groups according to the Kp index:geomagnetic quiet(Kp<3),moderate disturbances(3≤Kp<5)and strong geomagnetic activity(Kp≥5),then spatial and temporal distributions of ion upflow at dayside ionosphere are analyzed,respectively.The statistical results show that there seems to be two regions at the dayside ionosphere with comparable efficiency of bulk upflowing ions.One of them lies at the widely accepted cusp/cleft regions.The other one is situated in dawn-side sub-auroral regions.The results are as follows:a)Ion upflow in cusp/cleft regions:the main regions of ion upflow in the ionosphere are cusp/cleft regions,where the upflowing ions move upwards along open field lines towards magnetic tail;with the increase of geomagnetic activity level,the high incidence regions extend to the low latitude,and the latitude span of the high incidence regions increase.b)Ion upflow in dawn-side sub-auroral regions:in addition to cusp/cleft regions,the other regions of ion upflow in the ionosphere are dawn-side sub-auroral regions,where ionospheric ions upflow along closed field lines and possibly reaching boundary layers of dayside plasmasphere;there was no significant correlation between the ion upflow flux and the level of solar activity and geomagnetic activity in dawn-side sub-auroral regions.2.The acceleration mechanism of ion upflows during geomagnetic storms.The ionospheric upflow O~+ions are the source of the ion outflow.The ion heating caused by frictional heating occurs mainly in the low ionosphere while the electron heating caused by soft electron deposition occurs mainly in the top ionosphere.Therefore,only observations by DMSP satellites at the top ionospheric height cannot directly distinguish their contributions to ionospheric ion upflow during storms.Considering that O~+density depletion usually occurs in the upflowing source region for a short time;accordingly,there will be an increase in O~+density on the upflowing path.Therefore,the increase/depletion of O~+density during ion upflow can be used to roughly distinguish the height of the ionospheric ion upflowing source region.Combined with the ion/electron heating observed by DMSP satellites during the magnetic storm,the acceleration mechanism of ionospheric ion upflow during storms can be discussed.The results are as follows:a)At the topside ionospheric height,the upflowing ions are accompanied by more O+density depletion,indicating that the observed uplinking O~+ions were mainly originated from the top ionosphere.b)During storm the the upflowing ions associated with electron/ion heating are frequently observed at topside ionosphere,and the frequency of electron heating was much higher than that of ion heating.Combined with O~+density depletion and the electron heating,we believe that the bipolar diffusion enhancement plays a major role in the ion upflowing process during storms.3.Computerized ionospheric tomography study of three-dimensional electron density distribution in the top ionosphere.Local measurements of three-dimension electron density in topside ionosphere were achieved by means of computer tomography based on GPS/TEC data from the Swarm A/C satellites.The computerized ionospheric tomography were carried out simultaneously in the two spacecraft orbit panels.Tikhonov regularization was employed in the inversion,during which constraints of spatial density relative variations and their weights were applied with help of empirical model.Numerical validations indicate strong tolerance to observational errors of our method,with an average density deviation better than 10%.Comparisons with in-situ observations under various geomagnetic conditions were carried out.It was verified that our method is capable of well reproducing the characteristics of field-aligned irregularities with scale of several hundred kilometers in the top ionosphere,as well as the density discrepancy between the two spacecraft orbit panels.The results are as follows:a)We have realized the observations of three-dimensional electron density distribution in the top ionosphere based on Swarm satellites GPS/TEC computerized ionospheric tomography.b)Numerical verification results showed that the method had a strong tolerance to common observational errors.c)During a strong magnetic storm,the regularized inversion method reproduced the ionospheric irregularities,and was able to distinguish the difference in electron density between the two satellite orbital planes with a longitude of～150km apart,indicating that our method could be used to observe meso-scale ionospheric irregularities. |
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