Multiscale views of an Alfvénic slow solar wind: 3D velocity distribution functions observed by the Proton-Alpha Sensor of Solar Orbiter

¹ Institut de Recherche en Astrophysique et Planétologie, CNRS, Université de Toulouse, CNES, Toulouse, France
e-mail: philippe.louarn@irap.omp.eu
² Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles University, 18000 Prague 8, Czech Republic
³ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
⁴ INAF-Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Roma, Italy
⁵ Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA
⁶ AKA, Toulouse, France
⁷ Laboratoire de Physique des Plasmas, Ecole Polytechnique, Palaiseau, France
⁸ Department of Climate and Space Sciences and Engineering, The University of Michigan, Ann Harbour, MI, USA
⁹ Planetek Italia S.r.l., Via Massaua, 12, 70132 Bari BA, Italy
¹⁰ Leonardo, Viale del lavoro, 101, 74123 Taranto, Italy
¹¹ Space and Atmospheric Physics, The Blackett Laboratory, Imperial College London, London SW7 2AZ, UK
¹² LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
¹³ Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA
¹⁴ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, Pessac, France
¹⁵ National Research Council, Institute for the Science and Technology of Plasmas, Via Amendola 122/D, 70126 Bari, Italy
¹⁶ Space Science Center, University of New Hampshire, 8 College Road, Durham, NH 03824, USA
¹⁷ Swedish Institute for Space Physics, Ångström Laboratory, Lägerhyddsvägen 1, 751 21 Uppsala, Sweden

Received: 15 April 2021
Accepted: 11 October 2021

Abstract

Context. The Alfvénic slow solar wind is of particular interest, as it is often characterized by intense magnetic turbulence, complex proton 3D velocity distribution functions (VDF), and an ensuing richness of kinetic and dynamic processes.

Aims. We take advantage of the fast time cadence of measurements taken by the Proton-Alpha Sensor (PAS) on board Solar Orbiter to analyze the kinetic properties of the proton population, the variability of their VDFs, and the possible link with propagating magnetic structures. We also study the magnetic (B) and velocity (V) correlation that characterizes this type of wind down to the ion gyroperiod.

Methods. We analyzed the VDFs measured by PAS, a novelty that take advantages of the capability of 3D measurements at a 4 Hz cadence. In addition, we considered MAG observations.

Results. We first show that there is a remarkable correlation between the B and V components observed down to timescales approaching the ion gyrofrequency. This concerns a wide variety of fluctuations, such as waves, isolated peaks, and discontinuities. The great variability of the proton VDFs is also documented. The juxtaposition of a core and a field-aligned beam is the norm but the relative density of the beam, drift speed, and temperatures can considerably change on scales as short as as a few seconds. The characteristics of the core are comparatively more stable. These variations in the beam characteristics mostly explain the variations in the total parallel temperature and, therefore, in the total anisotropy of the proton VDFs. Two magnetic structures that are associated with significant changes in the shape of VDFs, one corresponding to relaxation of total anisotropy and the other to its strong increase, are analyzed here. Our statistical analysis shows a clear link between total anisotropy (and, thus, beam characteristics) and the direction of B with respect to the Parker spiral. In the present case, flux tubes aligned with Parker spiral contain an average proton VDF with a much more developed beam (thus, with larger total anisotropy) than those that are inclined, perpendicular, or even reverse with regard to the outward direction.

Conclusions. These observations document the variability of the proton VDF shape in relation to the propagation of magnetic structures. This is a key area of interest for understanding of the effect of turbulence on solar wind dynamics.