A Statistical Model Of Inner Magnetospheric Electron Density:Van Allen Probes Observations

As an important part of the inner magnetosphere,the Earth’s plasmasphere plays a very important role in the link of the occurrence and development of each space weather process.The Earth’s plasmasphere is dynamic,and the abundant particles in the plasmasphere change highly with interplanetary and geomagnetic activity.The large-scale structure evolution of the plasmasphere during the geomagnetic storm controls the generation and propagation of waves in the plasmasphere,thus affecting the wave-particles interaction,resulting in the change of the spatial distribution of electrons and ions in the plasmasphere,and then affecting other magnetospheric and ionospheric processes.The dynamic characteristics of the plasmaspheric density structure in space are useful indicators to reflect the extent of the disturbance of the space environment and to reveal the mass transport and energy transfer in the coupling process of solar wind-magnetosphere-ionosphere.Therefore,the plasmaspheric density has undoubtedly become an important parameter in space weather.It is particularly important to conduct an in-depth statistical analysis and model the distribution of plasmaspheric electron density.Based on electron density observations between September 2012 and July 2019 from the Electric and Magnetic Field Instrument Suite and Integrated Science（EMFISIS）on board the Van Allen Probes（VAP）spacecraft,this paper makes a statistical investigation on the distribution of inner magnetospheric electron density in the magnetic equatorial plane（MEP）under different Kp levels,and further develops a new model of inner magnetospheric electron density in the MEP with multi-parameters(Kp,SYM-H,P_dyn,F10.7).The statistical results show that the plasmaspheric electron density in the inner magnetosphere in the MEP has an obvious Magnetic Local Time（MLT）distribution characteristic.Under the same Kp index（Kp≥2）,the distribution of plasmaspheric electron density in the MEP reflects the asymmetry between the day-side and night-side in each MLT intervals.The distribution of plasmaspheric electron density in the MEP extends to the high L-value region in the afternoon and shrinks to the lower L-value region in the midnight.Such an asymmetric distribution should be related to the convection electric field distribution in the inner magnetosphere and the mass transport caused by the interaction of ionosphere-plasmasphere coupling system.With the increase of Kp index,the asymmetry of plasmaspheric electron density distribution in the MEP becomes more significant.The erosion caused by geomagnetic activity reaches the maximum near the midnight.Furthermore,in most MLT intervals,the distribution of plasmaspheric electron density in the MEP will erode towards the Earth with the increase of Kp index.In the interval near 14 h MLT,the degree of this erosion is relatively small due to the formation of a plume.In the region of low L-value（L≤2.8）,the distribution of plasmaspheric electron density in the MEP is less affected by the geomagnetic activity.Although the electron density has no order of magnitude change,the solar cycle effects should not be ignored in this region.In the region of high L-value（L≥3.6）,especially in the region near the plasmapause,the distribution of plasmaspheric electron density in the MEP changes greatly during the geomagnetic disturbance.Based on the statistics,the basic evolutions of inner magnetospheric electron density in MEP with different Kp indices can be well reproduced with two simple formulas with smooth L-value and MLT variations.Both the modelled distributions for each Kp level and time series prediction agree well with observations.In addition,we comprehensively consider the contribution of SYM-H,P_dyn and F10.7 to plasmaspheric electron density in different L-value intervals,which makes the model more accurate and more dynamic in time series.The event analyses and long-term comparison also indicate that,compared with previous empirical models（e.g.,Sheeley model and GCPM v2.4）,the newly built model can well reproduce the dynamic evolution of the plasmaspheric electron density during geomagnetic storms.The plasmaspheric electron density profiles of the newly built model under different geomagnetic conditions are in good agreement with the observations.In addition,the modeling process also answers some scientific and application questions.The relationships between space weather parameters and plasmaspheric density are not linear,and both types of parameters have their own effect on the plasmasphere dynamics.Development of the model has shown that solar wind and geomagnetic parameters can be used to accurately quantify the plasmasphere dynamics.The newly built model has great application potential in mechanism research and application.Its performance in the structure and evolution of the inner magnetosphere can reflect the dynamic characteristics of the plasmaspheric electron density,and help to reveal the mechanism of mass and energy transfer in the coupling process of solar wind,magnetosphere and ionosphere,improve the delay effect of radio waves passing through the ionosphere and the plasmasphere when the single-frequency GPS receiver is used for navigation and positioning,and advance the precise prediction and model improvement of the topside ionosphere.