The Study Of Relativistic Electron Long-term Dropouts In The Outer Radiation Belt

With the developing of space exploration,more and more spacecrafts are launched,a stable space environment is vital to the safety of satellites.The Earth's radiation belt is a region full of high-energy charged particles,especially MeV energy electrons in the outer radiation belt.These electrons known as “Killer electrons” will pose a huge threat to the safety of satellites,thus improving the understanding and forecasting of behavior of these electrons is important.The Earth's radiation belt is susceptible to the influence of solar wind,interplanetary conditions,and the level of geomagnetic activities.Variations in solar wind conditions and geomagnetic activities in the magnetosphere affect the high-energy electron content of the radiation belt.Although the research on the Earth's radiation belts has a history of decades,we still cannot accurately reproduce the behavior of high-energy particles in the radiation belt under the circumstance of given a specific solar drive and boundary conditions.A particularly extraordinary phenomenon in the outer radiation belt is known as electron flux dropout,which is characterized by the drastic depletion of trapped electron populations over a broad range of energies,pitch angles,and L-shells.Previous research work focused on the stage of significant changes in relativistic electron flux,to study and analyze the internal reasons leading to such significant changes,such as the modulation of solar wind conditions,the influence of magnetic storms on fluxes,and the internal wave-particle interaction mechanisms.Different from previous work:(1)This article first systematically studies the phenomenon of long-term dropout of relativistic electrons.From the distribution characteristics of such events with the solar activity spanning about 1.5 solar cycles,it is concluded that in the maximum and minimum solar sunspots number years of the solar cycle,the level of solar activity cannot completely determine the incidence of relativistic electron long-term dropouts.(2)In addition,based on previous studies,we have introduced more parameters to analyze and discuss the objective law and generation mechanism of the long-term dropouts of relativistic electrons.The results show that the location of plasmapause and magnetopause,and the intensity of the ULF waves have general variation laws before and after the event.This conclusion can provide a good theoretical basis for the subsequent radiation belt prediction modeling.(3)In the statistical work of phenomenon of relativistic electron long-term dropouts,the most important finding is: no intense magnetic storm occurs when the relativistic electron flux on the synchronous orbit maintains a long-term dropout.This result indicates that although the occurrence of a magnetic storm cannot determine the variation of electron flux after a storm,the intensity of a magnetic storm can still affect the level of relativistic electron flux to a certain extent.(4)In addition,this paper also formulates thresholds based on the distribution characteristics of various parameters when the relativistic electron flux long-term dropouts occur,and predicts the relativistic electron flux long-term dropout events from2017 to 2019.When all the conditions are met,most of the relativistic electron flux declines after 1 or 2 days.During the intense geomagnetic storms,no long-term dropouts of relativistic electron occur,and it's also difficult to meet multiple conditions at the same time.(5)When doing the statistical work on the long-term dropouts of the relativistic electrons,there is a special physical phenomenon,that is,the relativistic electron flux can maintain a long-term dropout during two sequential magnetic storms.This phenomenon can only be observed several times in 17 years.And no scientists have ever done this research area.This paper analyzes the detailed physical mechanism of the long-term dropout of relativistic electron fluxes during two sequential magnetic storms from January 31 to February 5,2016.The results show that during weak solar wind disturbances and small/medium geomagnetic storms,the slightly compressed magnetopause and outward radial diffusion was responsible for relativistic electron partial loss at 4<L<5 and complete loss at L>5.During the recovery phase of the magnetic storm,the north distribution of the IMF Bz component suppressed some acceleration activities in the magnetosphere,so that the relativistic electron flux could maintain at the background level.The relativistic electron flux is not significantly enhanced until the enhanced ULF wave activity and chorus waves occur.This case highlights the different variations of the relativistic electron flux during the two magnetic storms,and reflects the different variations of the relativistic electron flux in different L regions without the effect of EMIC wave scattering loss mechanism.(6)In-depth study during the two magnetic storms,the differences between lower energy keV electron flux and the higher energy MeV electron flux suggest that: only MeV electron flux can maintain a long-term dropout.The results also show that in the variation of the location of the maximum electron flux in the outer radiation belt,keV electrons and MeV electrons can show diametrically opposite trends during the two magnetic storms.The lower energy electrons are more sensitive to substorm activities,and can quickly recover to or far exceed the pre-storm level.When there is a combination of chorus waves and no strong loss mechanism occurs,the higher the intensity of the ULF wave,the shorter the time required for keV electrons to accelerate to MeV electrons.The research results in this paper reveal the distribution,formation and maintenance of the long-term relativistic electron flux dropout phenomenon.The longterm dropout of the relativistic electron flux during two sequential magnetic storms also provides many interesting and worthy physical phenomena for future study.