Exploring damping of Alfvén waves in a coronal hole through laboratory experiments
Abstract
We have performed laboratory experiments using scaled plasma parameters to understand the role of Alfvén waves in heating coronal holes. The experiments are conducted in the Large Plasma Device (LAPD) located at the University of California, Los Angeles. We have explored the effectiveness of longitudinal Alfvén speed gradients in reducing the energy of propagating Alfvén waves. Our results show that the energy of the transmitted Alfvén wave decreases as the inhomogeneity parameter, ?/L, increases. Here, ? is the wavelength of the Alfvén wave and L is the scale length of Alfvén speed gradient. The wave is observed to lose up to 91% of its energy while propagating through gradients similar to those in coronal holes. Contrary to theoretical expectations, the reduction in the energy of transmitted wave is not accompanied by observation of a reflected wave. We have considered other mechanisms that could reduce the energy of an Alfvén wave. Nonlinear effects associated with large amplitude waves 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. Because these effects are unimportant in coronal holes we have corrected for them using a model. After removing the effect of collisional and Landau damping we find a gradient-driven reduction in transmission of 86% for gradients similar to those in coronal holes. Since, the total energy must be conserved, the waves are most likely depositing their energy in the plasma. The results of the laboratory experiments will be discussed as they pertain to heating of coronal holes.
- Publication:
-
Solar Heliospheric and INterplanetary Environment (SHINE 2018)
- Pub Date:
- July 2018
- Bibcode:
- 2018shin.confE.250B