Magnetic Reconnection — Questions & Challenges Xiaogang Wang School of Physics, Peking University IFTS, Zhejiang University Sept 23 2007, Suzhou, China
OUTLINE What we have known What we don’t know Possible answers Brief summary
What we have known Hall effect physics Alfven frequency whistler waves Quadruple field Fast reconnection rate 0.01 ~ 0.2VA Layer width of reconnecting field ~ di Electrical potential well on separatrix
What we have known In question Layer length of reconnecting field ~ (diL)1/2 Reconnecting rate ~ (di/L)1/2 Electron density dip on separatrix (numerically observed) Lower hybrid oscillations (observed) Electron Alfven speed disturbance (observed)
What we have known 3D reconnection features The existence of magnetic nulls (observed) Null-null line (Separator, observed) Spines and fans (observed) Lower hybrid waves on the separator (observed)
What we have known 3D reconnection features Electron density peak on separator (observed) Electrical potential well on separator (observed) Langmuir oscillation near separator (observed)
What we don’t know 电子惯性区中的波动，特别是低杂波的激发与传播； 电子惯性区（包括分形线）中的密度sh上升？下降？ —— 原因及后果； 重联过程中的电子加热机制？ 电子Alfven波的存在？ Electron hole 的产生原因以及电子声波是否存在？
What we don’t know 电子压强梯度张量非对角项的贡献； 重联率的Scaling 的继续研究； 电子惯性是否对重联率有贡献？ Whistler or 离子波的贡献、观测及其与其他模式（比如LHW）的耦合？ 剪切流对重联结构和重联率的影响
What we don’t know Hall四极分布是否就是Whistler or Alfven频率whistler的驻波表现？ 边界驱动对无碰撞磁重联的影响？
What we don’t know (3D) 3D重联中的Hall四极分布问题？ Separator上的电子加热机制及其与2D理论的对比？ Anti-parallel reconnection vs. Component reconnection (理论、观测上，对重联率的影响)?
What we don’t know (3D) Null以及Null-null line的产生机制（X-line不稳定导致null pair 产生是否是唯一机制）？ 3D重联的动力学理论（Theory of 3D reconnection dynamics） 单个Null与Null pair对重联发生和重联率大小的影响
Possible answers 应用回旋动力学理论研究电子压强梯度张量非对角项的贡献； 重联率Scaling 的实验研究； 双流体理论、回旋动力学理论中Whistler or Alfven频率whistler与其它模式（如LHW）的耦合； 剪切流及Alfven共振研究 边界驱动无碰撞磁重联研究（Hall MHD + Hybrid codes)
Possible answers Separator上的磁镜效应及电子加速与加热机制 Anti-parallel reconnection — Component reconnection：一个过程的两种表现形式；统计研究
SUMMARY Hall effects well understood Electron dynamics not 3D features getting attentions MHD & Magnetic reconnection FTP (PKU, ZJU, NAOC, USTC, DLUT)