Recurrent solar density transients in the slow wind observed with the Metis coronagraph^⋆

¹ INAF-Catania Astrophysical Observatory, Via S. Sofia 78, 95123 Catania, Italy
e-mail: rita.ventura@inaf.it
² INAF-Turin Astrophysical Observatory, Via Osservatorio 20, 10025 Pino Torinese (TO), Italy
³ Predictive Science Inc., 9990 Mesa Rim Rd., 92121 San Diego, CA, USA
⁴ INAF – Astronomical Observatory of Capodimonte, Salita Moiariello 16, 80131 Naples, Italy
⁵ INAF – Trieste Astronomical Observatory, Via Giambattista Tiepolo 11, 34149 Trieste, Italy
⁶ CNR – Institute for Photonics and Nanotechnologies, Via Trasea 7, 35131 Padua, Italy
⁷ University of Urbino Carlo Bo, Department of Pure and Applied Sciences, Via Santa Chiara 27, 61029 Urbino, Italy
⁸ National Institute for Nuclear Physics, Section in Florence, Via Bruno Rossi 1, 50019 Sesto Fiorentino, Italy
⁹ Czech Academy of Sciences, Astronomical Institute, Friova 298, 25165 Ondejov, Czech Republic
¹⁰ University of Wrocław, Centre of Scientific Excellence – Solar and Stellar Activity, ul. Kopernika 11, 51-622 Wrocław, Poland
¹¹ National Aeronautics and Space Administration, Headquarters, Washington, DC 20546, USA
¹² University of Padua, Department of Physics and Astronomy, Via Francesco Marzolo 8, 35131 Padova, Italy
¹³ University of Florence, Department of Physics and Astronomy, Via Giovanni Sansone 1, 50019 Sesto Fiorentino, Italy
¹⁴ Italian Space Agency, Via del Politecnico snc, 00133 Roma, Italy
¹⁵ Max Planck Institute for Solar System Research, Justus-von-LiebigWeg 3, 37077 Göttingen, Germany
¹⁶ National Institute for Astrophysics, Institute of Space Astrophysics and Cosmic Physics of Milan, Via Alfonso Corti 12, 20133 Milano, Italy

Received: 7 April 2023
Accepted: 25 May 2023

Abstract

Aims We aim to investigate and characterize the morphology and dynamics of small-scale coronal plasma density inhomogeneities detected as brighter, denser features propagating outward through the solar corona in the visible-light images of the Metis coronagraph on board Solar Orbiter on February 22, 2021. Our main focus is on investigating their possible origin and contribution to the slow wind variability and dynamics and their dependence on coronal magnetic field configurations and structure.

Methods. The method adopted is based on the computations of autocorrelation and cross-correlation functions applied to temporal and spatial series of total brightness as a function of the heliocentric distance and solar latitudes.

Results. We find that the plasma density inhomogeneities studied here are small-scale structures with typical radial and transverse sizes, as projected on the plane of sky, on the order of 500 Mm and 40 Mm, respectively, and that they are up to 24 times brighter than the ambient solar wind. The brighter density structures exhibit longer lifetime and more stable shape and dimensions as they travel toward the outer edge of the field of view. The enhanced density structures are ejected with a most probable cadence of about 80 min at or below the inner edge of the Metis field of view (within 3.1 R_⊙–5.7 R_⊙ at the time of observations) in a wide latitudinal region corresponding to the site of a complex web of separatrix and quasi-separatrix layers, as resulting from the simulated magnetohydrodynamic configuration of the west limb of the solar corona. Some of the moving density enhancements clearly show morphological characteristics compatible with the switchback phenomenon, supporting the results indicating that the switchbacks occur at the coronal level. The enhanced density structures were ejected into the ambient slow wind with a mean velocity of about 240 ± 40 km s⁻¹, which is significantly higher than that deduced for the ambient solar wind on the basis of previous Metis observations during the solar minimum of cycle 24. The absence of acceleration observed across the coronagraph field of view suggests that the ejected plasmoids are progressively reaching the expansion rate of the ambient wind.

Conclusions. The results suggest that the quasi-periodic enhanced-density plasmoids might be the consequence of reconnection phenomena occurring in the complex web of the separatrix and quasi-separatrix layers present in the solar corona. Moreover, the structural characteristics of some of the detected plasmoids are in favor of the presence of switchbacks that originate during interchange reconnection processes occurring at or below 3 R_⊙ in the S-web. The speed of the plasma ejected in the reconnection process is higher than that of the ambient slow solar wind and is likely to be related to the energy involved in the process generating the propagating structures.