Evolution of coronal hole solar wind in the inner heliosphere: Combined observations by Solar Orbiter and Parker Solar Probe

¹ ASI – Italian Space Agency, Via del Politecnico snc, 00133 Rome, Italy
e-mail: denise.perrone@asi.it
² Dipartimento di Fisica, Università della Calabria, Rende, Italy
³ National Institute for Astrophysics, Institute for Space Astrophysics and Planetology, Via del Fosso del Cavaliere 100, 00133 Rome, Italy
⁴ Mullard Space Science Laboratory, Holmbury St Mary RH5 6NT, UK
⁵ Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
⁶ Department of Physics, Imperial College London, London SW7 2AZ, UK
⁷ Department of Electromagnetism and Electronics, University of Murcia, Murcia, Spain
⁸ National Institute for Astrophysics, Astrophysical Observatory of Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁹ Department of Climate and Space Sciences and Engineering, University of Michigan, 2455 Hayward Street, Ann Arbor, MI 48109-2143, USA
¹⁰ Space Research Institute, Austrian Academy of Sciences, Graz, Austria
¹¹ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, 33615 Pessac, France
¹² Institut de Recherche en Astrophysique et Planétologie, CNRS, UPS, CNES, 31028 Toulouse, France
¹³ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 Place Jules Janssen, 92195 Meudon, France
¹⁴ KTH Royal Institute of Technology, SE-11428 Stockholm, Sweden
¹⁵ Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450, USA
¹⁶ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain

Received: 10 May 2022
Accepted: 29 July 2022

Abstract

We study the radial evolution, from 0.1 AU to the Earth, of a homogeneous recurrent fast wind, coming from the same source on the Sun, by means of new measurements by both Solar Orbiter and Parker Solar Probe. With respect to previous radial studies, we extend, for the first time, the analysis of a recurrent fast stream at distances never reached prior to the Parker Solar Probe mission. Confirming previous findings, the observations show: (i) a decrease in the radial trend of the proton density that is slower than the one expected for a radially expanding plasma, due to the possible presence of a secondary beam in the velocity distribution function; (ii) a deviation for the magnetic field from the Parker prediction, supported by the strong Alfvénicity of the stream at all distances; and (iii) a slower decrease in the proton temperature with respect to the adiabatic prediction, suggesting the local presence of external heating mechanisms. Focusing on the radial evolution of the turbulence, from the inertial to the kinetic range along the turbulent cascade, we find that the slopes, in both frequency ranges, strongly depend on the different turbulence observed by the two spacecraft, namely a mostly parallel turbulence in the Parker Solar Probe data and a mostly perpendicular turbulence in the Solar Orbiter intervals. Moreover, we observe a decrease in the level of intermittency for the magnetic field during the expansion of the stream. Furthermore, we perform, for the first time, a statistical analysis of coherent structures around proton scales at 0.1 AU and we study how some of their statistical properties change from the Sun to the Earth. As expected, we find a higher occurrence of events in the Parker Solar Probe measurements than in the Solar Orbiter data, considering the ratio between the intervals length and the proton characteristic scales at the two radial distances. Finally, we complement this statistical analysis with two case studies of current sheets and vortex-like structures detected at the two radial distances, and we find that structures that belong to the same family have similar characteristics at different radial distances. This work provides an insight into the radial evolution of the turbulent character of solar wind plasma coming from coronal holes.