Alfvén wave propagation in the partially ionized lower solar atmosphere: A test of the single-fluid approximation

Alfvén waves are widely believed to play an important role in the transport of energy from the solar photosphere to the corona through the partially ionized chromosphere. In previous work, the properties of torsional Alfvén waves were theoretically studied using a multi-fluid model. Here, we compare those multi-fluid results with results obtained using the single-fluid magnetohydrodynamic approximation, as a way to assess the performance of the latter in the context of Alfvénic waves in the lower solar atmosphere. We considered a broadband photospheric driver that excites torsional Alfvén waves with frequencies ranging from 0.1 mHz to 300 mHz. These waves propagate upward to the corona along a magnetic flux tube that expands with height. We compared the energy flux, chromospheric reflection, transmission and absorption coefficients, and associated heating rates yielded by the two models. In general, the results of the two models are almost identical, with two minor differences: (1) the net energy flux that reaches the corona is approximately 5% higher in the single-fluid model, mainly owing to the higher reflectivity found in the multi-fluid model for wave frequencies exceeding 10 mHz; and (2) in a narrow region around 500 km above the photosphere, the single-fluid model underestimates the plasma heating rate due to ion-neutral damping by about a factor of two compared with the multi-fluid model. Both discrepancies arise from the approximate treatment of the ion-neutral drift in the single-fluid model and are expected to have a very limited impact in practical applications.