Research On The Observation And Excitation Of Plasma Waves During The Disturbed Magnetic Field Structures In The Inner Magnetosphere

Disturbed magnetic field structures are widely observed local magnetic field structures in the inner magnetosphere,including magnetic dips and magnetic peaks,characterized by sudden decrease and enhancement of the magnetic field strength.There are significant differences in the duration and amplitude of different perturbed magnetic field structures.The solar wind dynamic pressure compresses the magnetosphere,as well as the perturbed structures that propagate from the magnetotail to the synchronous orbit and then penetrate into the lower L-shell,may be the sources of these perturbed magnetic field structures in the inner magnetosphere.In addition,the complex plasma environment within a perturbed structure can provide free energy to locally excite plasma waves,such as EMIC waves,MS waves,and whistler-mode waves,which may play an important role in the dynamics of the inner magnetosphere through wave-particle interaction.In this paper,the spatial distribution,particle behavior,and related instabilities of perturbed structures in the inner magnetosphere are studied by combining satellite observations with linear and nonlinear theories.The main conclusions are as follows:1.Statistical analysis of the global distribution characteristics of the perturbed magnetic field structure in the inner magnetosphere.Based on the magnetic field data from the Van Allen A satellite（VAP-A）from December 2012 to December 2019,the spatial distribution of magnetic dips and magnetic peaks in the inner magnetosphere,including the maximum disturbance value,relative disturbance ratio,and duration,was obtained by automatically identifying the structure of the perturbed magnetic field.The statistical results show that the occurrence rate of perturbed magnetic field structures increases with the increase of the AE index,and the covered MLT sector also increases,and can penetrate into the lower L-shell.From the midnight sector to the noon sector（00:00<MLT<12:00）,the occurrence rate of the perturbed magnetic field structure along with the distribution of the L-shell coincides with the drift flux of the injected energy particles.The number of perturbed structural events has a significant correlation with the intensity of substorm activity,gradually decreasing along the drift path of particle injection.In contrast,the region where the magnetic peak structure occurs basically overlaps with the magnetic dip,and the magnetic peak can penetrate the lower L shell on the night side compared to the magnetic dips.2.Provide direct observational evidence for simultaneous excitation of EMIC and MS waves during the magnetic dip structure in the inner magnetosphere.We report for the first time a typical observation event of both EMIC and MS waves observed by the VAP-B satellite during the magnetic dip.Particle observations exhibit important features of the magnetic dip:flux enhancement of ring current protons and flux reduction of relativistic electrons.Based on the observed distribution of ring current protons during the dip and the calculations of linear growth rates,we found that the observed EMIC wave is provided by a warm proton（<10 ke V）with temperature anisotropic distribution,while the MS wave is excited by a thermal proton（>～20 ke V）with partial shell distribution.Our results show that the complex ion distribution of ring current protons during the magnetic dip has the potential to trigger the simultaneous generation of EMIC and MS waves,which may be involved in the evolution of ring current and radiation belt particles.3.Obtained observational evidence of modulation of whistler-mode wave structure by deep magnetic dip in the inner magnetosphere.We report the observation of whistler waves detected by VAP-A satellite in a deep magnetic dip structure on August 2,2016.Detailed wave spectrum analysis shows that composite acoustic waves with rising and discrete structures(～0.2-0.4 f_ce)are observed at the leading and trailing edge stages of the magnetic dip,while whistler mode waves with lower frequencies(～0.05-0.2 f_ce)are observed at the center.The linear growth rate results obtained from the observation and calculation of the spatial density distribution characteristics of the electron phase indicate that hot electrons with anisotropic temperature distribution during the magnetic dip provide free energy for the generation of whistler waves,and the normalized frequency range of the observed whistler waves significantly changes.Based on the nonlinear growth theory of whistler waves,the optimal amplitude and threshold amplitude of the rising-tone structure triggered by whistler waves are calculated.The results show that the observed whistler wave amplitude near the center of the magnetic dip is lower than the threshold amplitude and cannot effectively trigger the rising-tone structure,while the observed whistler wave amplitude near the trailing edge of the magnetic dip is close to the optimal amplitude,thereby triggering the rising-tone structure.This indicates that the inhomogeneous magnetic field within the magnetic dip structure may modulate the nonlinear structural triggering of whistler waves.4.The simultaneous excitation of whistler-mode and MS waves during the magnetic peak structure of the inner magnetosphere was discovered.We report for the first time a typical event of simultaneous observation whistler-mode and MS waves observed by the VAP-A satellite during the magnetic peak structure.The magnetic peak structure is accompanied by flux enhancements of ring current electrons and relativistic electrons,while the medium-energy ring current proton fluxes are reduced and the high-energy proton fluxes are enhanced.Our results indicate that it is precisely due to the different responses of ring current protons and electrons to the magnetic peak structure that instabilities occurs that can simultaneously excite whistler-mode and MS waves.This paper mainly studies the observation and excitation of plasma waves during the perturbed magnetic field structures in the inner magnetosphere.Our work contributes to understanding the spatial distribution of perturbed magnetic structures and their response to substorm activity.At the same time,the observed plasma environment within the disturbance structure can trigger rich instabilities,and these excited wave characteristics are also affected by the disturbance magnetic field structure,which can participate in the dynamics of the inner magnetosphere.