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 o -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 largescale
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 s1. 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.