Study On The Excitation Conditions And Source Evolution Characteristics Of Fast Magnetosonic Waves In The Earth's Inner Magnetosphere

Fast magnetosonic(MS)wave is a common electromagnetic wave in the inner magnetosphere,because it is often bound near the magnetic equator,it is also called equatorial noise.The frequency of MS wave is between the proton cyclotron frequency and the lower hybrid resonance frequency,occurs both inside and outside the plasmapause.MS waves can be locally excited by proton ring distribution.In addition,MS waves play an important role in the evolution of particles in the radiation belt.Through the wave particle interaction,MS wave can effectively heat the seed electrons in the radiation belt and scatter the ring current protons.In this paper,MS waves in the inner magnetosphere are mainly studied.The spatial distribution,excitation conditions and evolution process of MS waves are systematically studied by means of satellite observation,quasi-linear theoretical calculation and computer numerical simulation.Its main conclusions are as follows:1.The global distribution characteristics of MS waves'instability driven by proton rings are revealedBased on particle data observed by the Van Allen Probe-A(RBSP-A)satellite from October 2012 to August 2015,we studied the global distribution of proton rings and MS waves'instability.In order to obtain the distribution characteristics of locally excited MS waves,we first study the global distribution characteristics of MS wave excitation source(proton ring distribution).The statistical results show that both inside and outside the plasmapause,the proton rings mainly appear in the dayside sector and 3<L<6.Combined with the statistics of particle distribution and the quantitative calculation of linear growth rate,the results show that the dayside sector outside the plasmapause can locally excite strong MS waves,while the dusk sector inside the plasmapause can also locally excite low-harmonic MS waves.In low background plasma density area,when the proton ring velocity V_rwith local Alfvén speed V_Aratio is between 1 and 2,both inside and outside the plasmapause,low-harmonic MS waves can be locally excited,when V_r/V_A<1,high-harmonic magnetic waves can be locally excited outside the plasmapause.2.The favorable excitation conditions of narrowband MS waves near the lower hybrid resonance frequency are exploredThe statistical results of proton rings show that the frequency of MS waves decrease with the increase of V_r/V_A.If V_r/V_Ais small enough,narrowband MS waves will be excited near the lower hybrid resonant frequency(f _LHR).In order to explore the favorable excitation conditions for such narrowband MS waves,we selected all events that simultaneously observed proton ring distribution and high-frequency narrowband MS waves using the observation data of RBSP-A satellite between January 1,2013and December 31,2015.The statistical results show that the central frequencies of these narrowband magnetoacoustic waves are mainly concentrated around 0.7f _LHR,with a bandwidth range of 0.2f _LHR.According to the simultaneous observation of energetic particles,the ratio of ring velocity to local Alfvén speed V_r/V_Amainly falls within the range of(0.8,1).As for the spatial distribution of these high-frequency narrowband MS waves,different from the previous statistical results,the narrowband MS waves are mainly distributed in the dayside and the dusk side sector outside the plasmapause,corresponding to the spatial distribution of 0.8<V_r/V_A<1.Combined with the observed narrowband MS wave characteristics and the parameter analysis in calculation of linear growth rate,we theoretically confirm that the most favorable condition of narrowband MS waves'excitation is 0.8<V_r/V_A<1.3.It is verified that the excitation frequency of MS wave is modulated by the ratio of V_r/V_ABased on one-dimensional particle-in-cell simulation,we verify that the excitation frequency of MS wave is modulated by the ratio of V_r/V_A,and the frequencies of MS waves decrease as the V_r/V_Aratio increase.In other words,when V_r/V_Ais large enough,only the first harmonic MS wave will be excited;when V_r/V_Ais small enough,the narrowband MS waves near the lower hybrid resonance frequency will be excited.4.The formation mechanism of proton"double ring"distribution is foundIn order to further investigate the relationship between proton ring evolution and MS excitation.Based on one-dimensional particle simulation and linear theoretical analysis,we find that the high frequency MS waves excited by the initial proton ring distribution can scatter most of the ring protons to a lower energy,resulting in an increase in the ring velocity corresponding to the maximum proton phase space density.These protons have high ring velocity and can excite lower frequency MS waves.Moreover,the newly excited low-frequency MS wave will scatter the high-energy protons to form a new ring distribution,thus forming a"double-ring"distribution structure with the original proton ring.The new ring distribution has higher ring velocity and can provide extra free energy for lower harmonic excitation.However,the suppression of these harmonics by other proton distributions is much greater than the growth generated by the new proton ring,so the lower harmonics cannot be excited in the simulation results.Our simulation results reveal the detailed evolution of proton ring distribution and its effect on MS wave excitation,thus revealing the interaction between MS wave and proton ring distribution.