Evolution of Alfvénic slow wind parcels coming from the same solar source: observations by Parker Solar Probe, Solar Orbiter, and Wind

¹ Institute for National Astrophysics, Institute for Space Astrophysics and Planetology, Via del Fosso del Cavaliere, 100 00133 Rome Italy
² Istituto per la Scienza e la Tecnologia dei Plasmi, Consiglio Nazionale delle Ricerche Via Amendola 122/D 70126 Bari, Italy
³ Space and Plasma Physics, School of Electrical Engineering and Computer Science, KTH Royal Institute of Technology Teknikringen 31 114 28 Stockholm, Sweden
⁴ Advanced Heliophysics Inc, Pasadena CA 91106, USA
⁵ ASI – Italian Space Agency, Via del Politecnico snc 00133 Roma, Italy
⁶ Space Sciences Laboratory, University of California Berkeley CA 94720, USA
⁷ Earth, Planetary, and Space Sciences, University of California Los Angeles CA 90095, USA
⁸ National Institute for Astrophysics, Astrophysical Observatory of Torino Via Osservatorio 20 10025 Pino Torinese, Italy
⁹ Planetek Italia S.R.L., Via Massaua 12 70132 Bari BA, Italy
¹⁰ LEONARDO SpA Grottaglie 74023 Taranto, Italy
¹¹ TSD-Space Via San Donato 23 80126 Napoli, Italy
¹² University College London, Mullard Space Science Laboratory, Holmbury St. Mary Dorking Surrey RH5 6NT, UK
¹³ Institut de Recherche en Astrophysique et Planétologie, CNRS, Université de Toulouse CNES Toulouse, France
¹⁴ Southwest Research Institute 6220 Culebra Road San Antonio TX 78238, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 1 August 2025
Accepted: 24 November 2025

Abstract

Context. At the end of April 2021, Parker Solar Probe (PSP) was in alignment with Solar Orbiter and one week later with Earth, observing solar wind streams originating from the same region of the Sun. During the first conjunction, PSP was at 0.10 au and Solar Orbiter at 0.89 au while, during the second conjunction with Earth, PSP was at 0.33 au.

Aims. During the two conjunctions, PSP, Solar Orbiter and Earth were connected to the same solar source, an open field region in the neighborhood of a pseudostreamer configuration that is a typical source of Alfvénic slow wind streams. This particular orbital configuration allows us to study not only the evolution of Alfvénic turbulence in the slow wind using multi-s/c measurements at different heliocentric distances, but also the magnetic evolution of the solar source region.

Methods. In this work, we reconstructed the solar source using a potential field source surface model and performed a spectral analysis by using magnetic field observations and plasma parameters from spacecraft located at different heliocentric distances to characterize solar wind fluctuations. We then investigated the radial evolution of the turbulent energy transfer rate through the Politano-Pouquet law.

Results. The open field region identified as the solar source of the different plasma parcels is characterized by a well-developed pseudostreamer configuration with strong non-monotonic expansion of the open magnetic field that gradually decayed one week later. In turn, in situ observations, although showing a general radial evolution of Alfvénicity due to solar wind expansion, may also reflect the changes experienced by the solar source. The radial weakening of the v-b alignment and the predominance of magnetic energy are associated with an increase in the intermittency of the fluctuations at MHD scales, providing insight on the interaction between Alfvénic fluctuations and turbulence in the heliosphere. The energy transfer rate shows a rapid decay as a function of the radial distance, indicating possible fast energy dissipation for the sample analyzed in this work.

Conclusions. Results show a strong radial dependence of Alfvénicity, energy equipartition, homogeneities of the fluctuations and turbulent energy transfer, highlighting how both the solar source evolution and the interaction with local inhomogeneities can shape the properties of solar wind turbulence in different points of the heliosphere.