The Formation,Evolution And Disappearance Of Reversed Energy Spectra Of Radiation Belt Electrons:Observations And Simulations

Earth’s radiation belts are a torus-shaped region in geospace,filled with electrons and ions at energies of tens of keV to hundreds of MeV that are stably trapped by the terrestrial magnetic field.Delicate competition among the processes of acceleration,loss and transport of the particles lead to a subtle dynamic equilibrium,which controls the overall structure of the radiation belts.The dynamics of radiation belts is complex,showing strong dependence on the geomagnetic activity,solar wind condition,particle energy,spatial locations,and so on.Almost upon the discovery of Earth’s radiation belts in the late 1950s came the awareness that the nearly ubiquitous energetic particles in the radiation belts pose a potential threat to the electronics and structure materials of a variety of spacecraft facilities as well as human beings in space.Therefore,deep understanding on the source and loss mechanisms as well as the distribution and evolution of the hazardous energetic electron population is required.As two important essential characteristics of energetic particles,the pitch angle distribution and energy spectrum provide essentially important information to extract the underlying physics.Enabled by NASA’s Van Allen Probes launched in August 2012,we can obtain the high-resolution precision measurements of energetic particles,leading to a newly unveiled observation of the reversed energy spectra with abundant high-energy electrons but fewer low-energy electrons between hundreds of keV and~2 MeV in the inner magnetosphere,which revokes the traditional recognition of radiation belt electron energy spectra as steeply falling fluxes with increasing energy.Through a statistical analysis,we find that the reversed electron energy spectrum is prevalent in the plasmasphere,existing mainly within L=2.8-5.5 and during geomagnetically quiet times.Its spatial and temporal distribution exhibits a strong dependence on L and~*but shows little dependence on magnetic local time or geomagnetic latitude.In terms of two-dimensional Fokker-Planck diffusion simulations with a time-dependent,data-driven model of plasmaspheric hiss waves,the main features of observed reversed electron energy spectrum and its evolution in the plasmasphere are well reproduced at L=3-4 during the period of 20-29 March 2015,thereby justifying the dominance of scattering by plasmaspheric hiss accounting for the formation of reversed electron energy spectra.To deepen our understanding of the contributions of plasmaspheric hiss to the formation and evolution of reversed electron energy spectra,we conduct a detailed theoretical parametric analysis through numerical simulations to explore the sensitivity of hiss-induced reversed electron energy spectrum to a number of key parameters including background magnetic field,ambient plasma density,wave frequency spectrum and wave normal angle of plasmaspheric hiss.It is demonstrated that,for given L-shell in the plasmasphere,the formation and evolution of reversed electron energy spectrum is highly sensitive to the variations of ambient plasma density and hiss wave frequency spectrum,while both the variations of background magnetic field(which usually shows small changes in the plasmasphere)and hiss wave normal angle distribution play a less effective role.Additionally,reversed electron energy spectra are more likely to form and evolve more pronouncedly at lower pitch angles.We further find that the reversed electron energy spectrum evolves and can disappear under certain conditions.It is noticed that in the(time,L-shell)space,the distribution of reversed electron energy spectra displays an incontinuous band shape with intervals usually accompanied by intense geomagnetic activities,i.e.,small and slow variations at the lower boundary and dramatic and rapid changes at the upper boundary.Through both a case study and an overall analysis of one-year Van Allen Probes data,we demonstrate that whether the reversed electron energy spectra disappear across all L-shells or to which lower L-shell they can disappear from higher L-shells depends critically on the injection depth of energetic particles around the energy corresponding to the flux minimum of reversed energy spectrum.To summarize,through a combination of observational data analyses and numerical simulations,we have comprehensively studied the reversed energy spectrum of radiation belt electrons,including its spectral characteristics,statistical distribution properties,the formation and evolution mechanism and the disappearance driver.Our results indicate that the reversed energy spectrum of radiation belt electrons has important implications for unveiling the sophisticated energy-dependent nature of wave-particle interactions and energetic particle dynamics in geospace,and also has potential practical applied values to be used for further verification of the empirical models of radiation belt electrons that are currently available and served in aerospace engineering calculations related to on-orbit radiation safety of the satellite as well as its payload and human beings onboard.