Influence Of Hot Plasma On The Dispersion Properties And Electron Scattering Effects Of Plasmaspheric Hiss

Plasmaspheric hiss is a broadband electromagnetic emission with frequencies ranging from?10 Hz to several kilohertz.This emission is typically observed in the high-density regions,i.e.,the plasmasphere and plasmaspheric plumes.Plasmaspheric hiss can exist for a long time during geomagnetically quiet times and its power intensifies during geomagnetic storms and substorms.Hiss-driven pitch angle scattering of electrons has been proposed to account for the formation of the slot region between the inner and outer radiation belts and the precipitation loss of energetic and relativistic outer zone electrons.Recent studies reported that scattering by plasmaspheric hiss is responsible for the formation of a reversed energy spectrum.Therefore,plasmaspheric hiss plays an important role in the dynamic evolution of radiation belt electrons.The cold plasma approximation is a common treatment to study the resonant interactions between plasmaspheric hiss and magnetospheric electrons.However,during periods of disturbed geomagnetic activity this approximation could break down due to the enhanced injections of hot plasmas into the inner magnetosphere from the magnetotail.Combining observations and numerical modeling,this dissertation is aimed at understanding the influence of hot plasma on dispersion relation of plasmasphric hiss and hot plasma effects on the scattering loss of radiation belt electrons by hiss waves.The investigation can help us understand the dynamic evolution of radiation belt electrons in actual plasma environment and further establish a more reliable model of radiation belt dynamics.The principal results of this dissertation are summarized as follows:1.To investigate the influence of hot plasmas on the dispersion properties of plasmaspheric hiss,we adopt the cold plasma theory to calculate the wave magnetic field intensities from the electric field intensities observed by Van Allen Probe A from01 October 2012 to 28 February 2018.Comparisons between the observed and converted hiss magnetic field intensities show that as the substorm activity intensifies,the influence of hot plasma on dispersion properties of hiss waves becomes more significant.In addition,the variance of the ratio between the observed and converted hiss wave intensities exhibits an evident day-night asymmetry,mainly peaking on the nightside and extending to the dawnside and duskside with enhanced substorm activity levels.While the variances at higher frequencies of hiss waves increase significantly with geomagnetic activity,the variances at lower frequencies remain slightly affected.2.During geomagnetically active conditions,the free energies provided by injected hot plasma during geomagnetic storm can excite the instability of hiss waves.Therefore,by solving the hot plasma dispersion relation based on the kinetic linear theory,we obtain the linear growth rate of hiss waves.Subsequently,we investigate the influences of different hot plasma parameters on the maximum growth rate and corresponding frequency of hiss waves.We find that the maximum linear growth rate increases with the hot electron temperature anisotropy,plasma beta(?_||he)and hot electron abundance.The corresponding real frequency does not change significantly with temperature anisotropy,but increases with the reduced?_||he.Finally,we develop analytic models of maximum linear growth rate and corresponding real frequency which are functions of three independent dimensionless hot plasma parameters,i.e.,hot electron temperature anisotropy,?_||heand hot electron abundance.The results of normalized differences show that our model matches well with the numerical solutions.This work is of great significance for understanding the linear excitation process of hiss waves and its spectral distribution properties.3.Based on observations and numerical calculations,we prove that hot plasma effects can modify the dispersion relations of hiss waves,and subsequently influence the electron scattering effects.Three typical hiss events during different substorm conditions are used to study the differences between cold plasma and observed data based dispersion relations.We find that hot plasma effects will modify the hiss dispersion relation and result in changes in the electron pitch angle diffusion coefficients.And the differences between cold plasma and hot plasma dispersion relations increase with substorm activity intensification.The cold plasma approximation leads to an overestimate of the electron scattering rates,especially during medium and active geomagnetic activities.Compared with cold plasma dispersion relations,the scattering effects of hiss waves on electrons below 100 ke V are weaker under the hot plasma dispersion conditions,and the resonant pitch angle range is also smaller.4.To improve the understanding of hiss wave-induced scattering of radiation belt electrons in a realistic plasma environment and its sensitivity to hot plasma parameters,we solve the hot plasma dispersion relation using both the bi-Maxwellian and bi-Kappa velocity distributions and perform a detailed parametric analysis of the hot plasma effects on the bounce-averaged pitch angle scattering rates due to plasmaspheric hiss.The results show that the dependence of the bounce-averaged pitch angle scattering rates on the L-shell is not only reflected in the energy range of the resonant electrons and the pitch angle,but also in the peak value of the scattering rates.The difference,however,in the pitch angle scattering rates between the cold plasma and the hot plasma dispersion relations does not change significantly with L-shell,but it increases significantly with the decrease of?*=?_e²/?_pe²(where?_eis the electron gyrofrequency and?_peis the plasma frequency).For the electrons below100 ke V,in almost all the range of electron pitch angles,hot electron injection causes the decreases of scattering rates,and the differences of the scattering rates between the cold plasma and the hot plasma increases with the enhancement of the hot plasma parameters.For the electrons above 100 ke V,the scattering rates increase at smaller pitch angles and decrease at larger pitch angles which may result from the enhancement of hot plasma effect.As the spectral index?of kappa distributions increases,the dispersion relations of hiss waves approach the curves of standard Maxwellian distribution,and the differences of scattering rate between different?-values decrease with increasing electron energy.For the electrons below 100 ke V,the differences of scattering rates between different?-values become more pronounced with the enhancement of hot plasma effects.For the electrons above 100ke V,as the?-values of kappa distributions increase,the scattering rates increase at small pitch angles but decrease at large pitch angles.As the L-shell increases or the parameter?*decreases,the scattering rates of low energy electrons increase for all?-values.It is also shown that the increase of the temperature anisotropy and hot electron abundance can weaken the scattering efficiency of?30 ke V electrons at almost all pitch angles for?=2,while the scattering rates of?100 ke V electrons decrease for??2.