Astrophysical plasmas are rarely observed to be static. Instead they vary strongly in both time and space, a phenomena known as turbulence. Such plasmas are also typically endowed with magnetic fields, just like the Earth and the Sun. An understanding of turbulence in magnetized plasmas (known as MHD turbulence) is thus required to understand a wide variety of astrophysical phenomena, from the solar wind in our solar system (and how the solar wind impacts Earth) to rotating disks of gas falling into black holes (which produce some of the brightest sources of light in the Universe). Our proposed research is aimed at understanding the behavior of MHD turbulence on small scales where the energy is converted into heat.
We propose to carry out large-scale numerical calculations of the properties of MHD turbulence on small scales. These calculations will predict observable properties of the turbulence that can be directly compared to measurements in the solar wind, and ultimately to measurements in the laboratory on Earth. The same calculations will predict how plasmas are heated when the energy contained in the turbulence is dissipated -- one of the major unsolved problems in our understanding of turbulence. These results will be compared to measurements in the solar wind; they will also be used to predict the light we see from plasma falling into black holes -- our primary observational window onto black holes in nature. More broadly, the calculations proposed here will advance the state-of-the-art in numerical modeling of turbulence in magnetized plasmas and will have implications for a wide variety of laboratory, space, and astrophysical plasmas.
Turbulence involving magnetic fields and plasma (hot, ionized gas) is a key process in a wide variety of space and astrophysical systems. In the solar corona and solar wind, turbulence is one of the most promising mechanisms for heating and accelerating the solar wind, thus explaining why the sun has a wind in the first place. Much further away from us in the rapidly spinning plasmas around black holes, turbulence allows the plasma to spiral into the black hole and also heats the plasma, producing the tremendous sources of light we observe with astronomical telescopes. In the course of our work, we developed a detailed understanding of turbulence in magnetized plasmas, both in the context of the solar wind and in the context of plasma around black holes. We made the first first-principles predictions of the fluctuations in electric and magnetic fields on small scales in a turbulent plasma. These predictions are consistent with direct measurements in the solar wind. This puts models of turbulence in plasmas like the solar wind on much firmer footing. We also carried out the first study of the rotating plasmas around black holes explicitly following the individual motions of the protons and electrons that make up the plasma. We found dramatic and episodic production of high energy electrons and protons that can explain the flaring observed from the black hole at the center of our galaxy (these flares are qualitatively analogous to flares observed from the sun). In parallel to this study, we included electrons and protons spiraling around magnetic fields in our fluid calculations of turbulence and showed directly how the energy is transferred from the turbulent fluctuations to the constituent particles in the plasma. The research carried out in this study utilized both "pencil and paper" theory and numerical calculations on some of the world's largest supercomputers (run by the NSF). In the course of our work, we pioneered the application of techniques developed in the fusion community to the study of solar system and astrophysical plasmas. The NSF funding was primarily used to support young researchers early in their career.
