Particle Acceleration during Collisionless Magnetic Reconnection

磁重联是自然界中重要现象的基本过程，这些现象包括太阳耀斑、磁层亚暴和实验室聚变实验的中断。经典的电阻率不足以解释在重联事件中观察到的粒子带电现象。最近的一个重要发现是，对于给定的一组等离子体参数，重新连接可以以两种不同的模式进行：一种是被称为Sweet-Parker重新连接的慢模式，另一种是涉及霍尔效应产生的电场和磁场的快速模式。这两种模式之间的灾难性转变可以解释在诸如日冕和实验室重联实验等阻性系统中快速重联的突然发生。该项目将研究磁性x线附近的重联区的结构，并确定该结构如何与磁重联率相关。它将专注于开发一种新的粒子加热模型，在这种模型中，在收缩的磁岛中循环的电子会经历类似费米的加速。该模型将电子的能量增益与重新连接过程中释放的磁能联系起来。离子的热速度通常与阿尔芬速度相当或远小于阿尔芬速度，它们的速度太慢，无法进行费米反射，必须调用其他过程来解释它们的能量。该项目将探索一种种子机制，在该机制中，进入重联废气的超阿尔文离子充当拾取粒子，并获得等于排气速度的热能。将使用动力学粒子、霍尔磁流体(MHD)和测试粒子模拟来解决与重新连接过程中的电子和离子加速有关的关键问题。将继续与从事实验室重新连接实验的科学家合作，并利用卫星数据将理论预测与观测进行基准比较。探索重新连接的动力学，特别是发展高能粒子谱的预测能力，对于评估宇航员的安全和我国的天基技术资产具有广泛的重要意义。本科生积极参与该计划下的研究，并支持女性研究生，将推动国家科学基金会的教育目标。

Magnetic reconnection is a fundamental process underlying important phenomena in nature, including solar flares, magnetospheric substorms and disruptions in laboratory fusion experiments. Classical resistivity is not adequate to explain the observed energization of particles during reconnection events. An important recent discovery is that for a given set of plasma parameters, reconnection can proceed in two distinct modes: a slow mode known as Sweet-Parker reconnection, and a fast mode that involves electric and magnetic fields produced by the Hall effect. The catastrophic transition between these two modes can explain the sudden onset of fast reconnection in resistive systems such as the solar corona and laboratory reconnection experiments. This project will examine the structure of the reconnection region near the magnetic x-line and determine how the structure is related to the rate of magnetic reconnection. It will focus on the development of a new model of particle heating in which electrons circulating in contracting magnetic islands undergo Fermi-like acceleration. The model will link the energy gain by electrons to the magnetic energy released during reconnection. Ions, whose thermal speeds are typically comparable to or much smaller than the Alfven speed, are too slow to undergo Fermi reflection and some other process must be invoked to explain their energization. This project will explore a seed mechanism in which super-Alfvenic ions entering in reconnection exhausts act like pickup particles and gain a thermal energy equal to the exhaust velocity. Kinetic particle, Hall magnetohydrodynamic (MHD) and test particle simulations will be used to address the key issues related to electron and ion acceleration during reconnection. Collaborations will be continued with scientists working on laboratory reconnection experiments and with satellite data to benchmark the theoretical predictions with observations. The exploration of the dynamics of reconnection and particularly the development of a predictive capability for energetic particle spectra has broad importance for assessing both the safety of astronauts and our country's space-based technological assets. The active involvement of undergraduate students in research under this program and support for female graduate students will further the educational goals of NSF.