Dayside Statistical Distributions And Electron Scattering Effects Of Electrostatic Electron Cyclotron Harmonic Waves

Diffuse aurora is the major source of energy input into the polar ionosphere.Electrostatic electron cyclotron harmonic（ECH）waves and whistler-mode chorus waves are two major drivers of the diffuse aurora by precipitating plasma sheet electrons into the upper atmosphere via wave-particle interactions.The precipitating electrons can collide with the atmospheric molecules to emit the light.It has been reported that nightside ECH waves mainly pitch angle scatter electrons and then cause the higher latitude diffuse aurora.Therefore,ECH waves can significantly affect the dynamic evolution of plasma sheet electrons.In addition,the diffuse aurora precipitation induced by ECH waves can influence the ionospheric conductivity and the magnetospheric convection,suggesting that ECH waves play an important role in the magnetosphere-ionosphere couplings.Due to the observational limitations,it was difficult to distinguish diffuse aurora from the sunlight on the dayside.However,as the increase of auroral observation stations and the development of measurement technique,dayside diffuse aurora has received increasing attentions.Recent studies revealed that the dayside diffuse aurora exists in a broad MLT sector and its effects on the dayside magnetosphere-ionosphere couplings can be significant.Whistler mode chorus waves are believed to play an important role in the excitation of dayside diffuse aurora,while the contribution of ECH waves to dayside diffuse aurora has not been determined yet,despite their extensive existence in the dayside magnetosphere.Therefore,in this dissertation,we aim to comprehensively investigate the spatial distribution features of dayside ECH waves,and the scattering effects of magnetospheric electrons（including diffuse auroral electrons）induced by ECH waves.One the one hand,we aim to clarify the vital contribution of ECH waves to the formation of dayside diffuse aurora.On the other hand,we are devoted to deepen the current understanding of the role of ECH wave-induced magnetospheric electron precipitaion in the magnetosphere-ionosphere couplings.Firstly,we use the high resolution satellite data to perform a statistical analysis of dayside ECH waves.Secondly,based on quasi-linear theory,we quantitatively evaluate the electron scattering rates induced by ECH waves,and estimate the effect of ECH waves on the formation of dayside diffuse aurora.Thirdly,we perform a detailed parametric analysis to explore the sensitivity of the linear instability and electron scattering effects of ECH waves to the wave and ambient plasma parameters.The principal results are summarized as follows:1.In this study,we analyze high-quality wave data from the four MMS satellites between 1 September 2015 and 30 August 2018 to investigate the statistical properties of dayside ECH emissions.The results show that dayside ECH waves are preferentially observed on the prenoon side in the outer magnetosphere（L-shell=8-12）,with average wave amplitude E_w>0.1 m V/m.In addition,besides the typical near-equatorial（|MLAT|≤15°）region,dayside ECH waves exhibit moderate occurrence rate and wave amplitude in higher latitudinal regions（i.e.,15<|MLAT|≤40°）,possibly due to the off-equatorial geomagnetic field minimum.Our reported double peaks of dayside ECH wave occurrence zone and considerable occurrence rates of prenoon side ECH waves suggest that dayside ECH waves can be a potentially important contributor to the formation of dayside diffuse aurora.In addition,we provide the statistically averaged values of wave amplitude and peak wave frequency for different L-shell,MLT,and geomagnetic activity levels.This provides an improved statistical model of ECH wave spatial distribution in the Earth’s magnetosphere on the dayside,which can be readily adopted as an input for diffusion codes to reasonably evaluate the efficiency of ECH wave-driven pitch angle scattering and to determine the contributions of ECH waves to the magnetospheric electron dynamics and diffuse aurora electron precipitation.2.Based on the statistical analysis,we further perform a detailed survey of a typical dayside ECH wave event observed by MMS1 spacecraft on 11 Dec 2015.Using the observations from the spacecraft,combining with a realistic magnetic field model and a latitudinal varying wave normal angle model based on well-fitted electron velocity distribution,we calculate the quasi-linear bounce-averaged scattering rates of electrons,and quantitatively evaluate the efficiency of dayside ECH wave scattering in producing the electron diffuse aurora precipitation.We find that dayside ECH waves can efficiently pitch angle scatter diffuse auroral electrons on timescales of a few hours to～1 day over a broad range of electron energy（from～300e V to 10 ke V）and equatorial pitch angleα_eq（from the loss cone to 45°）.For～300 e V-2 ke V electrons,the scattering rates can even approach the strong diffusion limit,resulting in an almost fully filled loss cone.In addition,the precipitating fluxes are comparable to the observed fluxes outside the loss cone.Our results confirm the significant role of ECH waves in driving the dayside diffuse aurora.3.To deepen our understanding of excitation and scattering effect of ECH waves,we conduct a detailed parametric study to analyze the dependence of the linear instability and scattering effects of ECH waves on a number of key parameters including the ambient plasma density,magnetic field strength,ratio of hot electron density to the cold electron density,and loss cone depth and width.Our results demonstrate that both the temporal growth rate and dispersion relation are highly sensitive to the variations of electron number density,ambient magnetic field strength and ratio of hot electrons,while the loss cone depth and width mainly affect the value of temporal growth rate.As the electron number density increases,the frequency corresponding to the peak growth rate in each harmonic band increases,while the peak frequency decreases for increasing magnetic field strength and ratio of hot electrons.In addition,the peak frequency is not sensitive to the variations of loss cone size.ECH waves mainly scatter electrons with energy ranging from hundreds of e V to several ke V.The waves primarily pitch angle scatter electrons at low pitch angle and energy diffuse electrons at high pitch angle.As the electron number density increases,the scattering of electrons induced by ECH waves tends to be more efficient.As the magnetic field strength and loss cone depth increase,the scattering rates of electrons decrease.For increasing ratio of hot electrons,the scattering rates first decrease and then increase.For increasing loss cone width,the scattering rates first increase and then decrease.In addition,compared with pitch angle scattering rates,energy scattering rates are more sensitive to the variations of the wave and ambient plasma parameters.