Magnetic reconnection as a mechanism to produce multiple thermal proton populations and beams locally in the solar wind

¹ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, Pessac, France
² Institut de Recherche en Astrophysique et Planétologie, CNRS, UPS, CNES, Toulouse, France
e-mail: benoit.lavraud@irap.omp.eu
³ AKKA, Toulouse, France
⁴ Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic
⁵ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, UK
⁶ INAF-Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Roma, Italy
⁷ Southwest Research Institute, San Antonio, USA
⁸ Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas, USA
⁹ Laboratoire de Physique des Plasmas, Ecole Polytechnique, Palaiseau, France
¹⁰ Department of Climate and Space Sciences and Engineering, The University of Michigan, Ann Arbor, USA
¹¹ Space and Atmospheric Physics, The Blackett Laboratory, Imperial College London, London, UK
¹² LPC2E, CNRS, University of Orléans, CNES, Orléans, France
¹³ LESIA, Meudon, France
¹⁴ Dipartimento di Fisica, Universita della Calabria, Rende, Italy
¹⁵ ASI – Italian Space Agency, Rome, Italy
¹⁶ Space Sciences Laboratory, University of California, Berkeley, USA
¹⁷ University of Murcia, Murcia, Spain
¹⁸ KTH, Stockholm, Sweden
¹⁹ Space Science Center, University of New Hampshire, Durham, NH 03824, USA
²⁰ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain

Received: 21 April 2021
Accepted: 18 September 2021

Abstract

Context. Spacecraft data revealed early on the frequent observation of multiple near-thermal proton populations in the solar wind. Decades of research on their origin have focused on processes such as magnetic reconnection in the low corona and wave-particle interactions in the corona and locally in the solar wind.

Aims. This study aims to highlight the fact that such multiple thermal proton populations and beams are also produced by magnetic reconnection occurring locally in the solar wind.

Methods. We used high-resolution Solar Orbiter proton velocity distribution function measurements, complemented by electron and magnetic field data, to analyze the association of multiple thermal proton populations and beams with magnetic reconnection during a period of slow Alfvénic solar wind on 16 July 2020.

Results. At least six reconnecting current sheets with associated multiple thermal proton populations and beams, including a case of magnetic reconnection at a switchback boundary, were found on this day. This represents 2% of the measured distribution functions. We discuss how this proportion may be underestimated, and how it may depend on solar wind type and distance from the Sun.

Conclusions. Although suggesting a likely small contribution, but which remains to be quantitatively assessed, Solar Orbiter observations show that magnetic reconnection must be considered as one of the mechanisms that produce multiple thermal proton populations and beams locally in the solar wind.