Recurrent release of coronal density structures during the rising phase of solar cycle 25

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Aims. We investigate the main physical characteristics and outward propagation in the solar corona of small-scale coronal transients detected during the rising phase of solar activity cycle 25. They were observed with the Metis coronagraph on Solar Orbiter when the spacecraft was at perihelion, 0.29 au, on October 12, 2022. We compare the results with the Metis observations obtained in February 2021, when Solar Orbiter was at 0.52 au, which revealed intermittent quasi-periodic small-scale structures propagating in a still relatively quiescent corona, between 3.10 and 5.70 R ⊙ . Methods. The method we adopted is based on the construction of height-time maps of total brightness detected in the Metis white-light channel for three selected angular sectors of the solar corona. We carried out a spectral analysis of the temporal series of the total brightness obtained as a function of heliocentric distance and solar latitude by applying the Lomb-Scargle procedure. Results. Propagating plasma density inhomogeneities are observed above the solar limb within the altitude range 1.76–3.10 R ⊙ , in fairly quiet coronal regions that correspond to segments of the warped streamer belt, which is highly inclined relative to the solar equator. This is expected in the ascending-maximum phase of the solar activity cycle. These structures are characterized by radial and transverse sizes of 30–75 ± 8 Mm (0.04–0.10 ± 0.01 R ⊙ ) and 30–50 ± 8 Mm (0.04–0.07 ± 0.01 R ⊙ ), respectively. Most of them are fast-evolving structures that rapidly change in shape, dimension, and brightness on a timescale even as short as the observation cadence. The enhanced density structures move into the ambient slow wind with a mean velocity of about 240 ± 70 km s −1 . They are intermingled with downflows that mainly originate in the range of heliocentric distances 2.0–2.5 R ⊙ and flow inward with speeds ranging over a wide interval, from about 35 ± 1 km s −1 to about 200 ± 15 km s −1 . The coronal emission integrated over specific latitude intervals of 10° – 20° is characterized by a well-defined periodicity of 42 min. The observations suggest that any dynamical process causing the presence and release of the density structures coherently acts in specific areas according to some periodic clock. Conclusions. The results show that propagating density structures are already present in the corona at 1.76 R ⊙ (the internal occulter edge of the coronagraph). Their concentration in the range of heliocentric distance between 2.0 and 2.4 R ⊙ , that is, inside well-defined regions corresponding to streamer belt segments running almost parallel to the limb, is higher. Numerous dark structures moving inward are also present, mainly at the interface between open and closed magnetic lines at the streamer borders. The coronal plasma ejecta is likely released by processes of magnetic reconnection that are triggered in specific locations along the streamer borders in a period of 42 minutes. The driver of the quasi-periodic release of the density structures might be related to the photospheric convective motions and associated emergence of magnetic flux tubes, and also to the propagation of shocks at the boundary of the convection cells.