We will determine if density fluctuations along open magnetic field lines can enhance alfven wave reflection and dissipation enough to heat coronal holes and drive the fast solar wind. Alfven wave…

This is a grant awarded by the National Aeronautics and Space Administration (NASA) to the Smithsonian Astrophysical Observatory (SAO), a division of the Smithsonian Institution, to conduct research on Alfvén wave reflection and dissipation in coronal holes and its impact on heating the solar corona and driving the fast solar wind. The $582,518 grant will fund the use of models and observations to quantify the heating effects of realistic density fluctuations along open magnetic field lines. The research aims to determine if density variations can enhance Alfvén wave reflection enough to provide the necessary heating to match observations. The work supports NASA's goal of understanding the origins of solar activity and space environment variations. As a non-profit organization, the Smithsonian Institution serves as a prime contractor for NASA and NSF, providing specialized instrumentation, observatory systems, and scientific research.

We will determine if density fluctuations along open magnetic field lines can enhance alfven wave reflection and dissipation enough to heat coronal holes and drive the fast solar wind. Alfven wave turbulence is one of the major theories proposed to heat the corona. In coronal holes, alfven waves excited at the base of the corona travel outward along the magnetic field lines. Inward propagating waves are generated by reflection off radial gradients in the alfven speed. The reflected inward propagating waves interact with the outward waves, driving turbulence and heating. A major problem for wave-driven models is that the large scale alfven speed gradients in coronal holes are not steep enough to reflect waves efficiently. Such models cannot reach the required heating rates. A possible solution is that alfven wave reflection is enhanced by density fluctuation. Such fluctuations are observed in coronal holes. Our preliminary modeling shows that they can greatly increase the reflection and heating rate. We will use a complementary approach of models and observations to quantify heating in the presence of realistic density fluctuations and determine if IT is sufficient to heat open field regions. From observations, we will infer the magnitudes, periods, and length scales of density fluctuations in coronal holes. These empirical parameters will be input into our model of wave turbulence in order to predict the heating rate and the properties of the alfven waves. We will compare these predictions to observations. We will simulate alfven wave turbulence using our 3D reduced magnetohydrodynamic (rmhd) model. A strength of our model is that we include the effects of the solar wind outflow, unlike most other models where this is neglected. Alfven waves are launched at the base of the model, propagate into the corona, and reflect. We will use the model to study the dependence of the heating rate on various parameters that describe density fluctuations, such as their magnitude and length scales. We will then incorporate empirically constrained density fluctuations into the model. We will use data from the atmospheric imaging assembly (AIA) and the extreme ultraviolet imaging spectrometer (EIS) to study density fluctuations in coronal holes. There are a number of open observational questions about these fluctuations including how their amplitudes vary with height, how they differ in plume versus interplume regions, and how they vary from the center to the edge of coronal holes (variations across a coronal hole is expected due to the changes in the geometric expansion of the magnetic field lines). In observations, density fluctuations appear as intensity changes. We will analyze time series data of AIA and EIS images using techniques such as height-time plots and fourier methods. From the data we will infer the amplitudes, propagation speeds, periods, and length scales of the density fluctuations. We will incorporate the observational results into our model and determine how density variations affect turbulence in coronal holes and evaluate if the observed fluctuations drive heating sufficient to match what is required to heat the corona and accelerate the fast solar wind. We will also explore the influence on wave reflections of other model parameters, such as temperature and alfven wave amplitude, correlation length, and time scales. Lastly, we will compare the predictions of the model to measurements of alfven waves. Wave reflection and dissipation is a prime candidate for heating coronal holes and driving the fast solar wind. Thus, our work is directly relevant to the decadal survey science goal of determining the origins of the sun's activity and predict the variations in the space environment. Moreover, wave reflection and dissipation are fundamental processes so that our work also addresses the goal to discover and characterize fundamental processes that occur both within the heliosphere and throughout the universe.