The Principal Investigator (PI) will study the process of turbulent magnetic reconnection through the use of large-scale numerical simulations of magnetohydrodynamic (MHD) models in regimes relevant to the solar corona. Specifically, this research team will investigate how random magnetic field footpoint motions on the Sun produce instabilities, large-scale fluid motions, Alfvén waves, and turbulence; determine how the magnetic reconnection rate correlates with the level of self-consistently produced MHD turbulence; and obtain the overall Lundquist number scaling of the reconnection time-scale and energy dissipation rate. The researchers will also study the process that generates Alfvén wave turbulence dynamically, instead of imposing turbulence externally.
This problem of magnetic reconnection and turbulence is highly relevant to solar research. The science results obtained from this project will increase our understanding of many fundamental phenomena in solar physics, such as coronal heating, solar flares, and coronal mass ejections. The process of magnetic reconnection not only applies to the physics of the solar corona and heliosphere, but is also fundamental in many different physical systems, such as planetary magnetospheres, astrophysical accretion disks, and fusion experiments. This project will fully support a graduate student and provide partial support for a Post Doctoral Research Fellow. The PI plans to actively participate in education and outreach activities in the University of Alaska Fairbanks (UAF) community, such as those organized through the 'Public Information and Education Outreach Office' of UAF's Geophysical Institute, as well as those coordinated by UAF's 'Science Education Outreach Network.' The PI will also teach university courses in plasma and space physics at UAF. The simulation codes developed in this project will be made available to the scientific community.
This project is to study the physics of the solar corona. Despite many years of research in observations and theory, there are still a number of important unresolved problems in understanding the solar corona. One major problem is how the solar corona can be heated to millions of degrees Celsius while the solar surface is only about six thousands degrees. Other outstanding problems include understanding the generation mechanisms for solar flares, solar coronal mass ejections, and how the solar wind gets accelerated to high speed. Gaining understandings on these problems are essential in the study and forecast of space weather, which can have negative effects on important industrial systems such as the power grids, communications, and satellite operations. One major theoretical concept in trying to explain the physics in all these problems is the process of magnetic reconnection, which can release stored magnetic energy into kinetic energy in an explosive way, if the reconnection rate is fast enough. However, how to explain and understand fast magnetic reconnection has been a challenge despite many years of research. One mechanism, among many others, to achieve fast reconnection is the conjecture that magnetic turbulence might significantly increase the reconnection rate. Our research for this project has contributed to a number of new understandings of magnetic reconnection and magnetic turbulence, within the context of coronal heating. We have computed by large-scale numerical simulations the heating rate due to the twisting of magnetic field lines (the Parker model) by random motions of the photosphere, which then sporadically produces strong current layers that heat the corona (Fig. 1). We have confirmed the Parker model and showed that the heating rate is enough to explain the high temperature of the corona, even when we scale our results to realistic parameters of the corona (Fig. 2). Such strong current layers in small scales have been regarded as one possible mechanism for the so-called nano-flares, i.e., heating events that are two small to be observed directly but contributed significantly to the overall coronal heating. Since our simulations were run for long periods of time, we have computed the statistical distributions of heating events based on our simulations. Such distributions are found to be consistent with many observations of flare distribution, as well as analytical theories based on the concept of complex system. We have also studied the effect of enhancement of the reconnection rate due to magnetic turbulence. Using two-dimensional numerical simulations, we have shown that the reconnection rate becomes fast when magnetic turbulence is imposed. We have studied a possible mechanism to produce magnetic turbulence (through the excitation of Alfven waves) due to the oscillations of magnetic field lines during magnetic reconnection. We have also developed new simulation codes that include other physical effects (such as the Hall effect) that can possibly increase the reconnection rate. To understand further the effects due to turbulence on coronal heating, we have extended our simulations beyond the Parker model, which is valid for shorter coronal loop length (L), to cases with intermediate to long L. We have found that the heating rate for different L is best described by the sum of the Parker heating rate and the heating rate due to turbulence heating (Fig. 3). The physics behind this effect is that for longer L, Alfven waves are launched from both boundaries of the corona (i.e. from the photosphere). These waves are then travelling to opposite directions. When they collide, magnetic turbulence is produced and thus heats the corona (Fig. 4). A main outcome of our research results for this project is the demonstration that a unified theoretical approach based on both reconnection and turbulence heating is needed to describe the heating of the solar corona. This is important in the incorporation of essential physical processes of both magnetic reconnection and turbulence in more realistic solar models for the purpose of accurate space weather forecasting. This project has contributed to the support of a postdoctoral position. Three persons have been hired successively for this position, with the latest one hired for over two years. These postdoctoral fellows have received training for the numerical and theoretical tools for this project, as well as chances to develop presentation and communication skills by given presentations in local and national meetings. A graduate student, who has hearing disability, has also been supported for about six months. Another PhD student, at another institution and thus was not supported financially by this project, has worked for some part of this project with his PhD thesis partially based on results from such research. He has since graduated and is continuing his research within space physics as a postdoctoral researcher.
