Modelling magnetohydrodynamic waves in a helmet streamer: first comparison with Metis observations

Magnetohydrodynamic waves are often regarded as efficient pathways for magnetic energy transfer from the solar interior. Consequently, they are key not only to understanding the physical processes behind coronal heating and solar wind acceleration but also serve as useful diagnostic tools. While Alfvén wave modes can be difficult to detect directly by remote sensing – due to their incompressive nature – magnetoacoustic wave modes perturb the density and temperature as they propagate. Recently, observations from the Metis coronagraph on board Solar Orbiter have shown the ubiquitous presence of propagating density fluctuations in a helmet streamer with a 5-min period. Using 2.5D MHD simulations, we find that such density fluctuations can be generated either from non-linear Alfvén waves or directly from magnetoacoustic waves. Furthermore, density fluctuations generated from Alfvén waves have a doubled frequency with respect to the driven Alfvén wave frequency because of non-linear ponderomotive forces. In general, we find that in a low-β region, the dominant contribution to the density fluctuations is from slow waves propagating along the magnetic field. In a high-β region, the density fluctuations are likely associated with a mixture of both slow and fast waves.