Global Distribution Of Exohiss Waves During Different Level Of Geomagnetic Activity

Low-frequency plasma waves play an important role in the dynamic changes of highenergy electron fluxes in the radiation belts and the ring current of the Earth’s inner magnetosphere.Most of the previous studies focused on the two low-frequency waves,hiss and chorus,and more and more attention has been paid to the study of exohiss now.Exohiss is a broadband,unstructured whistler-mode wave existing outside the plasmapause,which plays an important role in the evolution of chorus and hiss waves,as well as the related electron dynamics.In order to deeply understand the global distribution of exohiss waves and to further understand its formation process,this paper mainly uses the observation data of Van Allen Probe A satellite from January 1,2013 to June 30,2017,to aim at the following two related researches:1.Firstly,the geomagnetic activity is classified according to the AE index,and the global distribution characteristics of the exohiss under different conditions of geomagnetic activity are discussed.The research analysis found:(1)The distribution of exohiss has obvious diurnal asymmetry and the amplitude of exohiss is stronger on the dayside.The amplitude of exohiss is weaker on the nightside.In addition,the exohiss on the morning side and the afternoon side showed different evolution characteristics with the enhancement of geomagnetic activity.With the enhancement of geomagnetic activity,the exohiss amplitude on the afternoon side shows a significant increase.The overall amplitude of the exohiss on the morning side did not change significantly.(2)Since the Poynting flux can reflect the source of the plasma wave,we divided the exohiss into two categories according to the direction of the Poynting flux: the unidirectional propagation exohiss and the mixed propagation exohiss.The analysis found that most of the exohiss propagated mixed,suggesting that most of the exohiss evolved from the hiss within the plasmapause.At the same time,with the increase of geomagnetic activity,the amplitude change of the mixed-propagating exohiss is not obvious.In addition,the one-way propagating exohiss mainly appears on the afternoon side of the Earth,and its energy mainly comes from the cyclotron resonance outside the plasmapause.Statistical results show that,along with the strengthening of geomagnetic activity,the amplitude of unidirectionally propagating exohiss is significantly enhanced.2.Since the mixed-propagated exohiss mainly evolves from the hiss wave in the plasmapause,in order to analyze the evolution characteristics of the mixed-propagated exohiss with the geomagnetic activity,we further analyzed the distribution characteristics of hot electron flux and exohiss wave under different geomagnetic activity conditions.Statistical results show that,on the morning side,the amplitude enhancement of the mixed propagating exohiss is smaller compared to that of the hiss inside the plasmasphere.Analysis of the epithermal electron flux associated with geomagnetic activity indicates that,during strong geomagnetic activity,the exohiss may have very strong Landau damping so that the enhancement of wave amplitude is not significant during strong geomagnetic activity.The results of this paper further clarify the global distribution characteristics of the exohiss outside the plasmasphere and its relationship with the level of geomagnetic activity.By comparing the global distribution,the evolution process of the exohiss outside the plasmasphere under different geomagnetic conditions is shown.The research results will further promote the understanding of the plasma waves and waveparticle interaction in the Earth’s inner magnetosphere,and have great significance for future space exploration programs and space weather forecasting.