The Principal Investigator (PI) will re-examine the basis for, and the limitations of, the "coherent flux tube" paradigm for coronal and interplanetary magnetic fields, in situations where magnetic fluctuations and turbulence are present. His research team will start from modeled fields in which Field Line Random Walk (FLRW) effects, as well as flux tube meandering and "shredding," cause perturbations of the standard flux tube picture, and eventually its breakdown. The PI will then progress through several models of turbulent fluctuations which give rise to flux tube irregularity, and investigate how the standard flux tube model breaks down. This work will impact our understanding of the propagation of solar energetic particles (SEPs). It will also provide valuable insights into the accuracy and limitations of standard smooth flux tube assumptions used in studying coronal structure, plasma heating models, models of the interplanetary magnetic field, and interplanetary coronal mass ejection (ICME) structure.
The PI's team will investigate the diffusive random walk limit of field line transport and the delay of full randomization associated with the topological complexity of magnetic fluctuations. The latter effect has been suggested as an explanation of "dropouts" or "channeling" of SEPs, and the PI will further evaluate this concept. The PI will examine the statistical transition from closed to open field line topology that is induced by addition of broadband fluctuations, as well as the formation of flux tubes and topological structure in dynamical simulations of magnetohydrodynamic (MHD) turbulence. His team will compare identified coherent flux tube properties with solar wind data. Quantitative estimates and fully analytical theories will be used, and numerical verification will be applied as feasible. The PI asserts that this effort will result in better recognition of the limitations of the smooth flux tube models that are used in a broad range of research fields, from astrophysics to space weather prediction. A PhD student will be a key participant in this work, which will involve partnerships with scientists in Thailand and Argentina.
Project Outcome Report NSF ATM-0752135 In this project we employed numerical simulation, spacecraft observations, and analytical theory, to improve understanding of the structure of the complex, turbulent magnetic fields and plasma flows that make up the interplanetary medium or solar wind. This is important because it strongly influences the way that energetic charged particles (such as Solar Energetic Particles, or SEPs and also Galactic Cosmic Rays, or GCRs). SEPs are a major contributor to phenomena associated with Space Weather, including effects on technological assets such as communication satellites, and effects on the human presence in space. The same magnetic field and plasma properties may influence the way that the plasma is heated in the solar atmosphere and therefore may help to understand the origin and nature of the solar wind, which in turn shapes the entire heliosphere. In the project we studied the nature of magnetic discontinuities, which are an important form of coherent structure produced by the strong nonlinear interactions that make up turbulence. The coherent structures mean that the plasma self organizes into cellular structures, and based on theory one would expect that the boundaries between cells would be places where heating and string dynamical effects occur. The rest of the plasma should relax towards low stress states as quickly as possible. But eth boundaries are numerous and reside at many scales, ranging over a factor of ten thousand in size in the solar wind. In our studies we showed that these coherent structures are spontaneously formed in simulations, and in a statistical distribution that is quite accurately reproduced in analysis of solar wind data. Furthermore we found, using a very large sample of data and conditional methods, that the solar wind plasma at discontinuities is hotter than the average solar wind, and that the nearby plasma has temperatures that smoothly fall off with distance from a known discontinuity. Very strong discontinuities are found to engage in a dynamic process known as magnet8c reconnection, that can accelerate particles, change magnetic connectivity, and release energy from the magnetic field. In this kind of complex magnetic environment, the paths followed by magnetic field lines, or by charged particles, can also become very complex or random. We also study how field lines and particles are distributed in a turbulent medium such as solar wind. This transport theory often involves the use of diffusion equations, and it is important to know the diffusion coefficients that are appropriate to use. We have continued to study diffusion in the heliosphere, through theory, numerical methods and observations. We applied these ideas to understand the phenomena of channeling or "dropouts" of SEPs and the observation of "moss" in the lower down, in the chromosphere. Very recently we have begun studying how turbulence influences the changes of connectivity between "open" and "closed" regions observed in the corona. This is ongoing. A very general conclusion that can be drawn from our research is that the usual assumptions that the interplanetary plasma is very uniform and in local equilibrium is not a good assumption when analyzing heating processes, diffusion, field line topology and connectivity, and related issues. All of these results are expected to be useful in further studies and applications of space physics, space weather, and in astrophysics.
