Reduction in the energy of an Alfvén wave propagating through a longitudinal inhomogeneity in Alfvén speed*

We have explored the effectiveness of longitudinal Alfvén speed gradients in reducing the energy of propagating Alfvén waves under conditions scaled to match a solar coronal hole. The experiments were conducted in the Large Plasma Device located at the University of California, Los Angeles. Our results show that the energy of the transmitted Alfvén wave decreases as the inhomogeneity parameter, λ/L_A , increases. Here, λ is the wavelength of the Alfvén wave and L_A is the scale length of Alfvén speed gradient. The waves are observed to lose up to ≈ 91% of their energy while propagating through gradients similar to those in coronal holes. Contrary to theoretical expectations, the reduction in the energy of the transmitted waves is not accompanied by a reflected wave. Nonlinear effects are ruled out as the amplitude of the initial wave is too small. However, collisional and Landau damping are found to contribute to wave damping in the experiment. After removing these effects, we find a gradient driven reduction in transmission of ≈ 86%.