Determining the Cross-Scale Coupling whereby Magnetohydrodynamics (MHD) Solar Eruptions Produce Energetic Electrons and X-Rays

不断进化的、充满等离子体的磁拱覆盖着太阳表面的大部分，这些磁拱比地球大数千倍。这些拱门有时会突然喷发，并向太空喷射出等离子体、磁场、高能电子/离子、X射线以及丰富的波的混合物。火山喷发的碎屑会对地球磁场造成严重破坏，造成极光，破坏航天器，扰乱无线电通信，在极端情况下，还会摧毁电网。磁流体动力学(MHD)方程很好地描述了这些拱门中压力、流动、磁场和电流的大规模演化。然而，MHD既不能解释这些喷发发生的原因，也不能解释它们产生高能粒子和X射线的原因，因为MHD没有描述导致这些临界现象的非常精细的物理现象。受与太阳等离子体相同的MHD物理支配的实验室等离子体将被安排成类似的演化和喷发，但以一种可重现的方式。将通过使用先进的诊断技术观察数千次受控喷发来研究从大规模MHD行为到产生X射线和波的精细非MHD行为的转变。研究项目将由PI在一名研究生和本科生作为暑期实习生或兼职学年研究人员的协助下完成。从这项研究中获得的知识将促进国家的健康、繁荣和国防安全，因为高能太阳粒子和X射线会损坏航天器和伤害宇航员，而地球磁场的破坏会破坏电网和不利的通信系统。这项为期三年的研究项目将研究MHD喷发现象和产生X射线、高能粒子和波的细尺度非MHD现象之间的跨尺度耦合。这项研究将确定X射线和哨声波的产生机制。初步证据表明，当瑞利-泰勒涟漪扼杀喷流横截面到离子皮肤深度的数量级时，就会发生这种情况，在这一点上，MHD失效，动力学不稳定性随之而来。这种动力学不稳定性被认为会极大地提高局部电阻率，从而中断电流。这种电流中断被认为会导致大的感应电压尖峰，从而将一小部分电子加速到极高的能量。据推测，这些快电子碰撞产生的韧致辐射会产生观测到的X射线爆发。这一工作假设将通过扫描参数来检验，以验证感应电压尖峰与阻塞电流横截面相关的假设。这一机制在日冕中的比例将被研究。虽然MHD演化的尺度是直截了当的，但精细尺度的非MHD现象的尺度需要解释MHD如何耦合到日冕中的离子深度尺度。这是具有挑战性的，因为在日冕中，MHD和离子皮肤深度的尺度至少相差10万。假设耦合是由具有分形特征的磁流体结构产生的，就像缠绕在一起的绳索由更小的缠绕的绳索组成。填充等离子体的磁通管就像绳索一样，最精细的鳞片将是离子皮肤深度的数量级。在实验中将产生成束的通量绳(股)，并对其进行测试，以确定当整个束被横向加速时，个别束是否会变得瑞利-泰勒不稳定。MHD尺度(喷流、扭结、瑞利-泰勒)和非MHD尺度(高能粒子、X射线、哨声波)之间的耦合将被确定并用于模拟太阳喷发如何产生高能粒子、X射线和波。该项目的研究和EPO议程支持AGS部门在发现、学习、多样性和跨学科研究方面的战略目标。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Constantly-evolving, plasma-filled magnetic arches that are thousands of times bigger than the Earth cover much of the surface of the Sun. These arches sometimes suddenly erupt and eject into space a mix of plasma, magnetic field, energetic electrons/ions, X-rays, as well as copious waves. The detritus of this eruption can wreak havoc on Earth's magnetic field creating aurora, damaging spacecraft, disrupting radio communications, and in extreme circumstances, knocking out electric power grids. The large-scale evolution of the pressure, flow, magnetic fields and electric currents in these arches is described quite well by a set of equations called magnetohydrodynamics (MHD). However, MHD cannot explain why these eruptions occur nor why they generate energetic particles and X-rays because MHD does not describe the very fine-scale physics responsible for these critical phenomena. Laboratory plasmas governed by the same MHD physics as solar plasmas will be arranged to similarly evolve and erupt but in a reproducible way. The transition from large-scale MHD behavior to fine-scale non-MHD behavior producing X-rays and waves will be investigated by observing thousands of controlled eruptions using advanced diagnostics. The research project will be done by the PI assisted by a graduate student and undergraduates working as summer interns or part-time academic-year researchers. The knowledge gained from this research will advance the national health, prosperity, and secure the national defense because energetic solar particles and X-rays can damage spacecraft and harm astronauts while the disruption of Earth's magnetic field can damage electric power grids and adversely affect communications systems.This three-year research project will investigate the cross-scale coupling between MHD eruptive phenomena and fine-scale non-MHD phenomena that produce X-rays, energetic particles, and waves. The research will determine the mechanism by which the X-rays and whistler waves are generated. Preliminary evidence suggests this happens when the Rayleigh-Taylor ripples choke the jet cross-section to be of the order of the ion skin depth at which point MHD fails and a kinetic instability ensues. This kinetic instability is presumed to greatly enhance the local resistivity and thus interrupt the electric current. This current interruption is presumed to cause a large inductive voltage spike that accelerates a small fraction of the electrons to extremely high energy. Brehmsstrahlung from the collision of these fast electrons is then presumed to produce the observed X-ray burst. This working hypothesis will be tested by scanning parameters to check the presumption that the inductive voltage spike is associated with choking the current cross-section. The scaling of this mechanism to the solar corona will be investigated. While scaling of the MHD evolution is straightforward, scaling of the fine-scale non-MHD phenomena requires explaining how MHD couples to the ion depth scale in the corona. This is challenging because in the solar corona the MHD and ion skin depth scales differ by at least 100,000. The hypothesis is that coupling results from the MHD structures having a fractal character like the twisted strands of a rope being composed of still smaller twisted strands. Plasma-filled magnetic flux tubes would be like the rope strands and the finest scale strands would be of the order of the ion-skin depth. Bundles of flux ropes (strands) will be produced in the experiment and tested to see if individual strands become Rayleigh-Taylor unstable when the entire bundle is laterally accelerated. Coupling between the MHD scale (jet, kinks, Rayleigh-Taylor) and the non-MHD scale (energetic particles, X-rays, whistler waves) will be determined and used to model how solar eruptions generate energetic particles, X-rays, and waves. The research and EPO agenda of this project supports the Strategic Goals of the AGS Division in discovery, learning, diversity, and interdisciplinary research.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.