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

不断演化的、充满等离子体的磁拱覆盖了太阳的大部分表面，这些磁拱比地球大数千倍。这些拱门有时会突然爆发，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。这次喷发的碎屑会对地球磁场造成严重破坏，产生极光，损坏航天器，干扰无线电通信，在极端情况下，还会破坏电网。这些拱中压力、流量、磁场和电流的大规模演变可以用一组称为磁流体动力学（MHD）的方程很好地描述。然而，MHD并不能解释为什么这些喷发会发生，也不能解释为什么它们会产生高能粒子和x射线，因为MHD并没有描述导致这些关键现象的非常精细的物理现象。与太阳等离子体一样，受MHD物理控制的实验室等离子体将被安排以类似的方式进化和爆发，但以一种可复制的方式。从大规模MHD行为到产生x射线和波的精细尺度非MHD行为的转变将通过使用先进的诊断方法观察数千次受控喷发来研究。该研究项目将由PI在一名研究生和本科生的协助下完成，这些本科生作为暑期实习生或兼职学年研究员。从这项研究中获得的知识将促进国家健康，繁荣，并确保国防安全，因为高能太阳粒子和x射线可以损坏航天器并伤害宇航员，而地球磁场的破坏可以损坏电网并对通信系统产生不利影响。这个为期三年的研究项目将研究MHD爆发现象与产生x射线、高能粒子和波的精细尺度非MHD现象之间的跨尺度耦合。这项研究将确定x射线和哨声波产生的机制。初步证据表明，当瑞利-泰勒波纹将射流横截面阻塞到离子表皮深度的数量级时，就会发生这种情况，此时MHD失效，随之而来的是动力学不稳定。这种动力学不稳定性被认为大大提高了局部电阻率，从而中断了电流。这种电流中断被认为会引起一个大的感应电压尖峰，使一小部分电子加速到极高的能量。这些快速电子碰撞产生的轫致辐射被认为产生了观测到的x射线爆发。这一工作假设将通过扫描参数进行测试，以检查感应电压尖峰与阻塞电流截面有关的假设。将研究这一机制对日冕的影响。虽然MHD演化的尺度化很简单，但精细尺度的非MHD现象的尺度化需要解释MHD如何与日冕中的离子深度尺度耦合。这是具有挑战性的，因为在日冕中，MHD和离子层深度至少相差10万。假设耦合是由具有分形特征的MHD结构引起的，就像绳子的扭曲股由更小的扭曲股组成一样。等离子体填充的磁通管就像绳子一样，最细的尺度线就和离子皮的深度差不多。将在实验中生产成束的通量绳（股），并对其进行测试，以观察当整个束横向加速时，单个股是否变得瑞利-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，并用于模拟太阳爆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。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.