The Energy Content of Alfvén Waves in the Stratified Solar Atmosphere

Abstract

[eng] Alfvén waves propagating in a vertically stratified plasma, such as those traveling from the solar photosphere to the corona, are partially reflected due to the gradient in the Alfvén speed. Wave reflection naturally results in the superposition of upward- and downward-propagating waves. A simple analytic model demonstrates that this superposition leads to the nonequipartition of kinetic and magnetic energies in the Alfvén wave perturbations and slows down the net energy transport. A numerical model of Alfvén wave propagation in the lower solar atmosphere reveals significant wave reflection below the transition region, leading to highly variable kinetic and magnetic energy content in the lower chromosphere. At higher altitudes, the kinetic energy eventually dominates, depending on the wave frequency. The velocity at which net energy propagates upward is significantly smaller than the local Alfvén speed throughout the chromosphere. Consequently, the commonly used expression for unidirectional Alfvén waves in a uniform plasma, which relates the energy flux to the kinetic energy density, is not generally applicable in the stratified lower solar atmosphere and cannot be reliably used to estimate the energy content of observed waves. A generalized expression is given, incorporating correction factors that account for wave reflection and energy nonequipartition. The applicability of the expression is discussed.