First detection of acoustic-like flux in the middle solar corona

Context. Waves are thought to play a significant role in the heating of the solar atmosphere and the acceleration of the wind. Among the many types of waves observed in the Sun, the so-called p modes with a 3 mHz frequency peak dominate the lower atmosphere. In the presence of magnetic fields, these waves can be converted into magnetohydrodynamic modes, which then leak into the corona through magnetic conduits. High-resolution off-limb observations have revealed signatures of ubiquitous and global 3 mHz oscillations in the corona, although they are limited to low heights and to incompressible modes. Aims. We present high-cadence, high-resolution observations of the corona in the range 1.7–3.6 R⊙ taken in broad-band 580–640 nm visible light by the Metis coronagraph on board Solar Orbiter. These observations were designed to investigate density fluctuations in the middle corona. Methods. The data were acquired over several days in March 2022, October 2022, and for two days in April 2023. We selected representative regions of the corona on three sample dates. Analysis of the data in those regions revealed the presence of periodic density fluctuations. By examining several time-distance diagrams, we determined the main properties (apparent propagation speed, amplitude) of those fluctuations. We also show power spectra in selected locations in order to determine the dominant frequencies. Results. We found wave-like, compressible fluctuations of low amplitude – on the order of 0.1% of the background – in several large-scale regions in the corona at least up to 2.5 R⊙. We also found that the apparent propagation speeds of these perturbations typically fall in the range 150–450 km s−1. A power spectrum analysis of the time series revealed an excess power in the range 2–7 mHz, often with peaks at 3 or 5 mHz, i.e. in a range consistent with p-mode frequencies of the lower solar atmosphere.