Understanding Wave Energy Transport Through the Complex Chromosphere and Transition Region

Plasma waves appear to be responsible for much of the heating of the chromosphere, transition region, and corona of the Sun. Fluid motions in the photosphere provide power for the waves, but the precise excitation mechanism is yet unknown. Once excited, the wave energy must be transmitted to the corona. However, recent observations have shown that the interface region between the photosphere and corona (i.e., the chromosphere and transition region) is complex. Thus, it is known how wave energy propagates through the interface region. These issues hinder the development of coronal heating models, which are limited by our poor understanding of the wave properties at the base of the corona. To tackle these important science problems for solar physics, this three-year project will use data from the Interface Region Imaging Spectrometer (IRIS) to measure the properties of waves in the interface region of the solar atmosphere. The main science objectives of the project are to: (1) identify where plasma waves are generated; (2) determine how wave energy propagates from the photosphere to the corona; and, (3) specify the wave boundary conditions at the base of the corona.

During this three-year research project, archival data from IRIS will be analyzed to address the following three major issues. First, determine where MHD waves are generated. It is currently unknown whether waves are generated mainly in the photosphere and propagate up through the interface region or whether the waves are generated within the interface region. The photosphere exhibits various fluid motions that can generate waves, such as the buffeting of magnetic field lines by granular motions. But, the waves can be reflected by the strong density gradients at the transition region and so not reach the corona. Global acoustic p-modes at the photosphere could launch acoustic waves, but they must undergo mode conversion if they are the source of the Alfvenic waves observed in the corona. Alternatively, waves may be generated in the chromosphere, for example, by reconnection. The IRIS data will be used to determine the wave modes and sources and sinks of wave power through varying heights in the interface region and thereby identify signatures of these processes and other possible ones. Second, measure the propagation of waves from the photosphere into the corona. Wave reflection and damping may prevent much of the wave power from reaching the corona. This project will determine how waves are transmitted through the interface region by observing the propagation of waves along structures and by measuring the power spectrum of the waves. These measurements will then be compared to existing observations and spectra of lower lying photospheric fluctuations and of Alfvenic waves in the higher lying corona. The analysis will determine where Alfvenic waves are reflected, if there is conversion of wave power from longitudinal to transverse modes or vice versa, and where wave energy is dissipated. Results will be compared to theories for the propagation, reflection, and damping of waves throughout this complex region. Third and final, characterize the wave modes and power at the base of the corona in order provide the critical boundary conditions needed for models of coronal heating. By comparing the amplitudes and phases of velocity and intensity fluctuations one can constrain the relative contribution of compressible versus incompressible waves. IRIS has sufficient spatial resolution to see torsional oscillations and thereby estimate the Alfvenic wave energy content of torsional versus kink waves. By studying the power spectrum of the waves, it will be determined whether the fluctuations are already turbulent at the base of the corona or whether the turbulence develops in the corona at larger heights.

This research project will have broader impacts through postdoctoral training and public outreach. The project will train a postdoctoral research scientist in the field of solar physics and in techniques for the analysis of spectroscopic observations. The research and EPO agenda of this project supports the Strategic Goals of the AGS Division in discovery, learning, diversity, and interdisciplinary research.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.