火星弓激波上游离子回旋波的观测研究

Observations of Ion Cyclotron Waves Upstream from the Martian Bow Shock

离子回旋波广泛分布于火星弓激波上游,在卫星坐标系下其频率接近当地质子回旋频率,准平行于背景磁场方向传播,与在火星外逸层太阳风拾起新生H离子有关。该文基于磁流体理论,对MAVEN探测器观测到的离子回旋波进行事例分析,通过对等离子体密度扰动、速度扰动以及磁场扰动的参数拟合,发现该离子回旋波事件可用斜传播的快磁声波与平行传播的阿尔文波的叠加来解释。该文有助于进一步理解火星上游所观测到的离子回旋波相关扰动的物理本质,对其中等离子体物理过程的重新建模和数值模拟具有指导意义。

Ion cyclotron waves are widely present upstream from the Martian bow shock,with frequencies near the local proton cyclotron frequency in the spacecraft frame.The waves propagate quasi-parallel to the background magnetic eld,and are associated with the Solar wind picking up newly-ionized hydrogen from the Martian exosphere.In this work,a typical ion cyclotron wave event was observed by MAVEN upstream from the Martian bow shock.The further MAV analysis results for this event show that the magnetic eld perturbations are left-handed polarized and propagate quasi-parallel(the propagation angleθ=12.37◦)to the background magnetic eld.The relationship between the plasma perturbations and magnetic eld perturbations is theoretically derived for slow magnetosonic,Alfven,and fast magnetosonic waves based on the magnetohydrodynamic theory.If we take the wave propagation angleθ=12.37◦estimated from the MVA analysis,the theoretically expected density perturbation and velocity perturbation along the background magnetic eld are in serious disagreement with the MAVEN data.Through tting the plasma density uctuation,velocity uctuation and magnetic eld uctuation,it is found that the ion cyclotron wave event can be explained by the superposition of oblique fast magnetosonic waves with a propagation angle of 63◦relative to the background magnetic eld and parallel propagating Alfven waves.The results are helpful to further understand the physical nature of the observed ion cyclotron wave-related perturbations upstream from the Martian bow shock and have guiding implications for the re-modeling and numerical simulation of the plasma physical processes therein.