The Principal Investigator (PI) will test a long-standing hypothesis that solar coronal heating occurs in sub-resolution current sheets. The PI notes that theoretical estimates suggest that the thickness of such current sheets in the solar corona would be on the order of 100 meters or less, but there is no firm evidence that such sheets exist. These current sheets cannot be observed directly due to the inadequate spatial resolution of current observing technology. Since these sheets cannot yet be detected, the PI asserts that an indirect test for their existence would be to determine if magnetohydrodynamic (MHD) models can give rise to such phenomena from first principles physics, without in any way predisposing the MHD model to have current sheet solutions in the corona. The PI will create such a model and require that such solutions "arise naturally" from the intrinsic, nonlinear transport and radiation physics within the model. The PI's model will be designed to provide exact, self-consistent solutions for sub-resolution, heated, radiating current sheets under solar coronal conditions, and include realistic mathematical expressions for the radiative cooling rate and the anisotropic transport coefficients that determine solar coronal heating.
If the PI finds his anticipated current sheet solutions, then these would constitute the first theoretical evidence for the existence of current sheets in the solar corona and would lend support to the current sheet coronal heating hypothesis. Such solutions would contribute to our fundamental understanding of radiation generation and anisotropic transport processes in current sheets and would permit more accurate multi-dimensional MHD simulations for increasing our understanding of magnetic reconnection, a phenomenon that is pervasive in space physics, astrophysics, and plasma physics. Magnetic reconnection is also a key phenomenon in the most energetic space weather events.
Magnetic reconnection is a potentially important process in determining Earth's space weather. Reconnection converts the energy in the magnetic field into wave and particle energy. Reconnection is believed by many to occur in the atmosphere of the Sun, but has never been observed directly, ostensibly because, if it exists, it is predicted to occur in regions that are too small to be observed from Earth. According to theory, magnetic reconnection occurs in current sheets, which are thin layers of electric current. An important area of research is to determine what types of models have solutions for current sheets under solar atmospheric conditions, and to study the properties of these solutions to better determine the possible role of current sheets and magnetic reconnection in space weather phenomena. The ultimate goal is to understand the Earth-Sun system well enough to make accurate predictions of Earth's space weather, caused by flares, coronal mass ejections, and the solar wind, all of which come from the Sun. The objective of this project was to explore whether a certain class of mathematical models has solutions for current sheets in the solar corona, which is the outer atmoshpere of the Sun, where flares, coronal mass ejections, and the solar wind are generated. The first step in this project was to take an existing model that was shown to have current sheet solutions in a region of the atmosphere called the lower and middle chromosphere, which lies below the corona, and see if this model has current sheet solutions in an overlying region, called the upper chromosphere, that is just below the corona. This was done to try to gain insight into how to find current sheet solutions in the corona using a similar model. No solutions to the model were found under upper chromospheric conditions. This suggests that the model must be made more realistic in order to have current sheet solutions under upper chromospheric conditions. The next step is to develop such a more realistic model, and then use it as a guide and basis for developing a model that has current sheet solutions under coronal conditions.
