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

California Institute Of Technology, Pasadena CA

Investigators

Abstract

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.