Study On Generation And Propagation Characteristics Of Dayside Chorus

The Earth’s radiation belt is composed of a large number of energetic particles trapped in the Earth’s magnetic field,mainly located between 1.2-8 Earth radii.These energetic particles can cause a variety of faults in the in-orbit space system and pose a serious threat to the astronauts’health.Chorus and energetic electrons interaction is an important mechanism controlling the radiation belt electron dynamics.Previous works have indicated that nightside chorus is generated through interactions with an anisotropic distribution of energetic electrons from～keV-100 keV,but the precise generation mechanism of dayside chorus still requires thorough investigation.In this thesis,we shall combine analysis of multi-satellite data,theoretical analysis and numer-ical simulation,and deeply study the generation,propagation and instability of dayside chorus.Firstly,we study generation mechanism of dayside chorus at lower L shells in two invnents.When Van Allen Probes observed intensification of dayside chorus,～keV-100 keV energetic electrons displayed enhancements in fluxes and anisotropy as they were injected from the plasma sheet and drifted from midnight through dawn toward the dayside.Calculations of oblique chorus growth are performed under the fitting parameters of kappa distributions and realistic plasma parameters.The calculations results show that the growth rate γ drops with increasing wave normal angle θ,with a scaled peak growth rate γ/Ωce=2.6 × 10-3 for θ=0°(Ωce is the electron equatorial angular gyrofrequency).The upper cutoff and peak wave frequencies display similar patterns to the observations.The current results confirm that the observed lower L shell dayside chorus can be excited by anisotropic electrons originating from the plasma sheet in drifting from the nightside to the dayside.secondly,using correlated data from the Van Allen Probes and EGR satellite in two invnents,We adopt the observed wave parameters and three-dimensional ray tracing method to study the propagation of dayside chorus.It is found that in cases of various wave ferquencies,initial wave normal angles and azimuthal wave angles,chours can spread from low MLT(magnetic local time)area to high MLT area.The propagation ranges in two events occur from 6.6 to 11.1 MLT and from 7.2 to 10.8 MLT.The initial wave normal angle controls the range of the wave in the MLT direction.In the cases of higher wave frequency and higher normal angle,dayside chours is present over a wider region of MLT.The simulation results show that chorus can propagate from the dawnside to the dayside along the latitude and MLT direction.Finally,we simulate the evolution of electrons and dayside chorus.Using the ob-served wave power spectrum and storm-time evolution of electron radiation belt model,we calculate the bounce-averaged diffusion coefficients and the corresponding phase space density(PSD)evolution of energetic electrons driven by dayside chorus.The simulation results show that pitch angle diffusion coefficients<Dαα>/p2 are about 10-3 s-1,momentum diffusion coefficients(Dpp)/p2 are about 10-5 s-1,The maximum values of diffusion rates are in the region of large pitch angles.The PSD of the lower energetic electrons decreases by tens to hundreds of times,and the higher energetic electrons increases by tens to hundreds of times in 4 hours.Dayside chorus can produce pitch angle scattering of lower energetic electrons and acceleration of higher energetic electrons.These results confirm that dayside chorus plays an important role in both stochastic acceleration and pitch angle scattering of electrons.