On Alfvénic turbulence of solar wind streams observed by Solar Orbiter during March 2022 perihelion and their source regions

¹ Institute for National Astrophysics (INAF), Institute for Space Astrophysics and Planetology (IAPS), Via del Fosso del Cavaliere, 100, 00133 Rome, Italy
² Earth Planetary and Space Sciences, University of California, Los Angeles, USA
³ Advanced Heliophysics, Pasadena, CA, USA
⁴ Istituto per la Scienza e Tecnologia dei Plasmi, Consiglio Nazionale delle Ricerche, Bari, Italy
⁵ Space and Plasma Physics, School of Electrical Engineering and Computer Science, KTH Royal Institute of Technology, Stockholm, Sweden
⁶ ASI – Italian Space Agency, Rome, Italy
⁷ Naval Research Laboratory, Washington, USA
⁸ Institut de Recherche en Astrophysique et Planétologie, CNRS, Université de Toulouse, CNES, Toulouse, France
⁹ University College London, Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
¹⁰ Imperial College, London, UK
¹¹ Department of Mathematics, Physics and Electrical Engineering, Northumbria University, Newcastle Upon Tyne, UK
¹² Austrian Academy of Sciences, Graz, Austria
¹³ Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA
¹⁴ Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA
¹⁵ University of Texas at Austin, Austin, USA
¹⁶ University of Michigan, Ann Arbor, USA
¹⁷ Institut d’Astrophysique Spatiale, Paris, France
¹⁸ University of Florence, Florence, Italy
¹⁹ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 Place Jules Janssen, 92195 Meudon, France
²⁰ Laboratoire Cogitamus, Rue Descartes, 75005 Paris, France
²¹ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Konkoly Thege út 15-17, H-1121 Budapest, Hungary
²² Institute for National Astrophysics (INAF) – Osservatorio Astrofisico di Torino, Torino, Italy
²³ CNRS, École Centrale de Lyon, INSA de Lyon, Université Claude Bernard Lyon 1, Laboratoire de Mécanique des Fluides et d’Acoustique, F-69134 Écully, France
²⁴ Planetek Italia S.R.L., Via Massaua, 12, 70132 Bari, BA, Italy
²⁵ LEONARDO SpA, Grottaglie, 74023 Taranto, Italy
²⁶ TSD-Space, Via San Donato 23, 80126 Napoli, Italy

^⋆ Corresponding author; raffaella.damicis@inaf.it

Received: 28 July 2024
Accepted: 22 November 2024

Abstract

Context. It has been recently accepted that the standard classification of the solar wind solely according to flow speed is outdated, and particular interest has been devoted to the study of the origin and evolution of so-called Alfvénic slow solar wind streams and to what extent such streams resemble or differ from fast wind.

Aims. In March 2022, Solar Orbiter completed its first nominal phase perihelion passage. During this interval, it observed several Alfvénic streams, allowing for characterization of fluctuations in three slow wind intervals (AS1-AS3) and comparison with a fast wind stream (F) at almost the same heliocentric distance.

Methods. This work makes use of Solar Orbiter plasma parameters from the Solar Wind Analyzer (SWA) and magnetic field measurements from the magnetometer (MAG). The magnetic connectivity to the solar sources of selected solar wind intervals was reconstructed using a ballistic extrapolation based on measured solar wind speed down to the (spherical) source surface at 2.5 R_s below which a potential field extrapolation was used to map back to the Sun. The source regions were identified using SDO/AIA observations. A spectral analysis of in situ measured magnetic field and velocity fluctuations was performed to characterize correlations, Alfvénicity, normalized cross-helicity, and residual energy in the frequency domain as well as intermittency of the fluctuations and spectral energy transfer rate estimated via mixed third-order moments. A machine learning technique was used to separate proton core, proton beam, and alpha particles and to study v − b correlations for the different ion populations in order to evaluate the role played by each population in determining the Alfvénic content of solar wind fluctuations.

Results. The comparison between fast wind and Alfvénic slow wind intervals highlights the differences between the two solar wind regimes: The fast wind is characterized by larger amplitude fluctuations, and magnetic and velocity fluctuations are closer to equipartition of energy. In fact the Alfvénic slow wind streams appear to be on a spectrum of wind types, with AS1, originating from open field lines neighboring active regions and displaying similarities with the fast wind in terms of fluctuation amplitude and turbulence characteristics, but not with respect to the alpha particles and proton beams. The other two slow streams differed both in their sources as well as plasma characteristics, with AS2 coming from the expansion of a narrow coronal hole corridor and AS3 from a region straddling a pseudostreamer. The latter displayed the coldest and highest density but the slowest stream with the smallest fluctuation amplitude and greatest magnetic energy excess. It also showed the largest scatter in proton beam speeds and the greatest difference in speed between proton beam and alpha particles.

Conclusions. This study shows how the old fast–slow solar wind dichotomy, already called into question by the observations of slower Alfvénic solar wind streams, should further be refined, as the Alfvénic slow wind, originating in different solar wind regions, show significant differences in density, temperature, and proton and alpha-particle properties in the inner heliosphere. The observations presented here provide the starting point for a better understanding of the origin and evolution of different solar wind streams as well as the evolving turbulence contained within.