| The Earth’s radiation belts are the important circle layers in the near-Earth space,which have many coupling effects with other regions such as the magnetosphere and the ionosphere.The communication and operation of the spacecraft are also significantly affected by the environment of radiation belts.Thus,it becomes a very hot research topic in spacec physics research and space weather forecasting that attracted a lot of attention of many space scientists.For decades,many conuntries have launched some satellites to measure the physical characteristics of the radiation belts and provide more accurate and detailed observations and forecasts for space exploration,manned spaceflight,and satellite communication applications.In August 2012,NASA successfully launched the Van Allen Probes mission and conducted long-term and multi-point observations on the environment of the Earth’s radiation belts.It can archieve different tempoal scale and spatial scale observations,already accumulated a large number of effective data,and will further improve the study of the physcical processes and dynamics of the radiation belts.This mission also provided a good opportunity for the data analysis and theoretical modeling of the radiation belts.In previous studies of the energy electron flux in the radiation belts,Summers,Fox,and Mauk et al.have made a series of original advances in theoretical analysis.Mauk et al.(2010)proposed that the strongly magnetized planets(such as Earth,Saturn,Jupiter,Uranus,and Neptune)have a quasi-radiation belt structure.Through the observational data,Mauk et al.found that the energy electron differential fluxes in some regions have the“breaking point”phenomenon and gave the corresponding theoretical model.For example,when L=4,5,and 6,the electron fluxes tend to show the"breaking point",but they still have many differences.In order to better understand the dynamic changes of the energetic electron flux in radiation belts,in this paper,we used Van Allen Probes data to conduct detailed case study and statistical analysis of the characteristics of the enegetic electron differential fluxes during a typical magnetic storm.We selected three typical magnetic storm events from 2012 to 2015 and carefully performed our data processing and analysis from the Magnetic Electron Ion Spectrometer(MagEIS)and Relativistic Electron Proton Telescope(REPT).(1)Firstly,we performed statistical analysis on the energetic electron differential fluxes during typical magnetic storm events,especially before and after the occurrence of magnetic storms.The results show that the electron fluxes for dieffrent L-shell will have different changes during a magnetic storm.The fluxes at L=2 are relatively stable and fluctuate within a very small range during the whole magnetic storm event.The electron flux at L=3,4,5,and 6 will vary greatly with the intensity of the magnetic storm.When L=3,the electron flux at higher energy will not change much and stay at a low level,but the electron flux at the lower and medium energy can be boosted up to 10~4 times.When L=4,in a quiet period of a storm,the electron fluxes always show a decrease at the medium energy and present a“groove"there;but after the magnetic storms,the slope of low-energy electron fluxes is in the form of1/E as Mauk et al.predcted.When L=5 and 6,lower-energy electron fluxes somehow appear in the form of 1/E,and the the slopes will increase as the L-Shell??increases.The slope of high-energy electron fluxes is decreasing very fast.We noticed that the positions of radiation belts are not always constant.With the occurrence of magnetic storm,the boundaries of the radiation belts will erode inward and thus change the position of the radiation belts relative to the L-Shell.In addition,there is indeed a"breaking point"phenomenon in the energetic electron fluxes in the outer radiation belt.(2)The magnetic storm will be accompanied by the occurrence of some substorms.We found that,when the AE index is large,the electron flux curves have some significant difference.Thus,we choose the period from March 17 to March 30,and further analyzed the relationship between this difference of electron flues with MLT,AE and DST.We found that the relationship between the variation of electron fluxes and the MLT is not so close.But large Dst will casue the clear difference of electron fluxes for lower-medium energy at a given L-Shell.In the future,we will try to convert the electron differential flux into omnidirectional electron flux to present detailed study in different energy band.(3)In order to further study the"breaking point"of the energetic electron flux,we use the least square method to fit the observation data and present out verification and error analysis of the fitting results.After doing careful data analysis for several hundreds of the fitting results during three magnetic storm events,we found that the range of constant C is in the range of 10~6~10~122 and the minimum is 10~4 when L=5 in quiet period.The parameter E0(breaking point)is around 1500~2500 keV and the upper and lower limits are slightly floating.The parameterγ1(slope of lower energy fluxes)is around 1~3 but those at L=6 will be larger that at L=5(γ1 is around 0.5~1at L=5).The parameterγ2(slope of higher energy fluxes)generally varies smoothly between 6~9.Finally,we found that there is a positive correlation between the parameters c andγ1 with DST.Our statistical analysis and theoretical fitting results show that,at L=4,5,6,the breaking point of the energetic electron differential fluxes does exist,but it is not fixed.The parameter E0 is the the energy that represents the breaking point and will vary in the range of 1500~2500 keV.The statistical results in this paper have very important research significance on the theoretical modeling of the electron flux in the Earth’s radiation belts,which will improve the forecast of the radiation belt environments in the future. |
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