Density fluctuations in pseudo-streamer and helmet streamer: Generation, propagation and comparison with Metis observations

Recent white-light observations from the Metis coronagraph on board Solar Orbiter have shown the presence of propagating density fluctuations with a period of ∼ 5 minutes, particularly in the observationally bright regions of helmet streamers and pseudo-streamers. These density fluctuations have been shown to originate from magnetohydrodynamic (MHD) wave modes – slow and fast modes can directly compress the plasma, whereas Alfvén waves generate density fluctuations via a non-linear coupling with the slow and fast modes. In general, in regions of low plasma 𝛽, slow modes contribute more towards the generation of density fluctuations, whereas in a high 𝛽 region, there are significant contributions from both slow and fast waves. In this work, we investigate the propagation of (non-linear) Alfvén waves and the associated density fluctuations using a 2.5D axisymmetric global solar wind model that includes two distinct magnetic field topologies – a helmet streamer and a pseudostreamer. Differences arise in their propagation due to, for example, the presence of a current sheet in the helmet streamer and an X-type null point in the pseudo-streamer, and the structure of the 𝛽 = 1 layer in both magnetic field configurations. This leads to the creation of prominent interference patterns along the helmet streamer axis, which do not appear along the pseudo-streamer axis. We also find that along the pseudo-streamer axis, the contribution to the density fluctuations is due to the fast waves only, despite being in a low 𝛽 plasma.