极尖区中氧离子O+能化过程研究

The cusp is a key region of the coupling of the solar wind-magnetosphere-ionosphere. Particles and energy from the solar wind can directly reach on the ionosphere throughout the cusp. Oxygen ions O+ upflow is very clear in the cusp during intense magnetic activities. Energization of O+ ions in the cusp is investigated in our project. It is one of effective methods to distinguish the coupling between the multiple-layer on the Earth outer space. Our project will focus on the conjunction observations of multiple satellites (e.g., Cluster、Polar、FAST、THEMIS、TC-1) to analyze the features of low frequency waves and variations of O+ ions number density, velocity and energy flux in the cusp during magnetic storms with different intensity. The kinetic theory and multiple distribution function of charged particles will be adopted to investigate the interaction between kinetic Alfvén waves (KAWs) and O+ ions, especially, O+ ions accelerated by the KAWs in the low to high beta, (beta is the ratio of plasma thermal pressure to the magnetic pressure) plasma. Our project will illustrate the effect of magnetic reconnection at the magnetopause on the variation of exterior cusp O+ ions energy flux under the different IMF. This project will reveal the rules of the acceleration and heating of O+ ions in the cusp during magnetic storms and provide the new path of O+ ions from the high-latitude entering the plasma sheet in the magnetotail and inner magnetosphere. It will also provide the new idea for the investigation of the coupling of the solar wind- magnetosphere-ionosphere.

极尖区是太阳风-磁层-电离层耦合的重要区域，太阳风中的粒子和能量通过该区域能直接到达电离层。强磁活动期间极尖区中存在明显的上行氧离子O+，探究该区域中O+能化过程是明晰地球外空间多圈层之间耦合机制的有效途径之一。本项目利用多卫星（Cluster、Polar、FAST、THEMIS、TC-1等） 协同观测，分析不同强度磁暴期间极尖区中O+上行过程中低频电磁场扰动和氧离子O+数密度、速度和能通量变化特征；采用多种分布函数（如Bi-Maxiwallian、Loss-cone）和动理学理论，研究不同beta (beta是等离子体热压与磁压之比值)等离子体中动理学Alfvén波对O+能化作用，阐明不同行星际磁场条件下磁层顶重联对外极尖区O+能通量变化的影响，揭示磁暴期间极尖区中O+加速和加热机制，为磁活动期间O+从磁层高纬进入低纬区、内磁层提供新途径，为太阳风-磁层-电离层耦合研究提供新思路。

本项目已完成相关研究内容，实现了预期目标。采用多卫星(Cluster、MMS、TC-1等)对强磁活动期间磁层不同活动区中的联合观测数据, 详细研究了不同高度极尖区内氧离子O+数密度、能通量和投掷角分布与电磁场扰动变化特征。研究结果表明磁暴期间氧离子O+数密度随极尖区高度的增加而呈下降趋势，而且在磁暴不同相位期间极尖区中氧离子O+数密度存在很大差异,变化范围在0.01-4.0 cm-3；磁暴初相期间氧离子O+数密度最大，中高度极尖区中O+离子数密度最大值可达4.0 cm-3; 氧离子O+能量随极尖区高度的增加有增加趋势，并且极尖区内氧离子O+能量分布在极尖区纬度上呈现明显差异，极尖区赤道侧氧离子O+能量较高,约几十 keV，而高纬侧氧离子O+能量偏低，约1 keV量级。极尖区中能量氧离子O+能化机制具有多样性。中高度极尖区中的低频电磁离子回旋波、动理学Alfven波与氧离子O+的发生波粒相互作用，这些低频波对氧离子的横向加热和场向加速有重要作用，促使氧离子O+逃逸；而行星际激波和磁层顶磁场重连电场对外极尖区中氧离子O+的垂直加速有重要作用，使其能量达到几十keV。在南向行星际磁场条件下外极尖区中的能量氧离子O+伴随磁场重联结构磁通量绳传输到外磁层区域并逃逸到行星际空间。本项目成果为太阳风-磁层-电离层耦合过程研究提供了新途径，为空间天气预报建模提供了新视角。