Laboratory study of Alfvén wave reflection from an Alfvén-speed gradient relevant to coronal holes*

Coronal holes are regions of the Sun's atmosphere with open magnetic field lines that extend into interplanetary space. These regions are ≈200 times hotter than the underlying photosphere. Recent solar observations of strong damping of Alfvén waves at low heights in coronal holes suggest that wave-driven processes may be responsible for heating the plasma. The most promising wave-based model put forward to explain the damping of wave energy in coronal holes involves nonlinear interaction between outward and inward propagating waves, resulting in the development of turbulence, leading to heating. Most theories invoke partial reflection of outward Alfvén waves from gradients in the Alfvén-speed to explain the inward waves. We report the first experimental detection of a reflected Alfvén wave from an Alfvén-speed gradient under conditions similar to those in coronal holes. The experiments were conducted in the Large Plasma Device at the University of California, Los Angeles. We present the experimentally measured dependence of the coefficient of reflection versus the wave inhomogeneity parameter, i.e., the ratio of the wavelength of the incident wave to the length scale of the gradient. Two-fluid simulations using the Gkeyll code qualitatively agree with and support the experimental findings. Our experimental results support models of wave heating that rely on wave reflection at low heights from a smooth Alfven-speed gradient to drive turbulence.