| Ion cyclotron waves are widely present upstream of the Martian bow shock,and their frequencies are close to the local proton gyrofrequency in the spacecraft frame.They are left-handed polarized waves propagating quasi-parallel to the background magnetic field,and are likely the product of the interaction with the solar wind to excite right-handed resonance instabilities after the ionized extended exospheric hydrogen has been picked up by the solar wind.Particles(especially hydrogen)escaping from planetary atmospheres carry important information about the composition and evolution of the atmosphere during the life cycle of the solar system.For non-magnetized planets like Mars,which has a neutral exosphere extending into the solar wind plasma,losses due to pickup ions upstream from the bow shock play an important role in the escape of the Martian atmosphere.This inextricable relationship between ion cyclotron waves and pickup ions makes the study of ion cyclotron waves upstream from Mars important for the research on the Martian atmospheric escape processes.The first chapter introduces the Martian dayside space environment,the excitation mechanisms of upstream ion cyclotron waves and pickup ions,as well as the current research status of Martian upstream ion cyclotron waves and the research contents of this thesis.Chapter 2 of this thesis first introduces the MAVEN spacecraft and the instruments used to study ion cyclotron waves:the magnetometer investigation(MAG)and the Solar Wind Ion Analyzer(SWIA),then details the minimum variance analysis method,often used to determine the normal direction of the discontinuities in the solar wind,followed by a description of the common criteria used in most studies to identify upstream ion cyclotron waves on Mars,and finally details the application of transport ratios to identify low-frequency waves in high-β plasmas.Chapter 3 in this thesis derives the relationship between the three wave-mode plasma perturbations and the magnetic field perturbations on the basis of the three wave dispersion relations of MHDtheory,and analyses in detail a typical ion cyclotron wave event upstream from Mars to show that the ion cyclotron wave has an amplitude around 0.5 nT,a power spectral density of the transverse component of the magnetic field 17.28 times higher than that of the longitudinal component.The frequency with the power spectral density peak is 0.0827 Hz,which is close to the local proton cyclotron frequency(0.0740 Hz).The analysis results of the MVA method show that the magnetic field perturbations in this ion cyclotron wave event are left circularly polarized(λ1/λ2=1.13,λ2/λ3=13.16)and propagate quasi-parallel to the background magnetic field(the propagation angle θ is approximately 12.37°).In the field aligned coordinate system,the theoretically expected values of the density perturbations and the velocity perturbations in the direction of the background magnetic field do not match the observed values severely if θ=12.37°is taken;a search of the θparameter is conducted to minimize the deviation between the values expected by the MHDtheory and those of observations,which yields another wave vector k’(i.e.，θ=63°,φ=95°).The angle between k’ and the wave vector k determined by the MVAmethod is 64.73°.Combining the characteristics of the plasma velocity and magnetic field perturbations in the three directions and of the density perturbations,these perturbations of the ion cyclotron wave event can be interpreted as the superposition of fast magnetosonic waves with Alfvén waves.The final chapter of this thesis summarizes the analysis results of the observations of ion cyclotron waves upstream from Mars,including the characteristics of the associated plasma and magnetic field perturbations and the corresponding theoretical interpretations,and on the basis of these results in this study,points out the shortcomings of this work and looks forward to future observations of ion cyclotron waves upstream from Mars and related theoretical studies. |
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