First Solar Orbiter observation of the Alfvénic slow wind and identification of its solar source

¹ National Institute for Astrophysics (INAF) – Institute for Space Astrophysics and Planetology (IAPS), Via Fosso del Cavaliere, 100, 00133 Rome, Italy
e-mail: raffaella.damicis@inaf.it
² Advanced Heliophysics, Pasadena, CA, USA
³ National Institute for Astrophysics (INAF) – National Observatory of Turin (OATo), Via Osservatorio, Pino Torinese, Turin, Italy
⁴ Italian Space Agency (ASI), via del Politecnico snc, 00133 Rome, Italy
⁵ Department of Physics, Imperial College London, SW7 2AZ London, UK
⁶ UCLA Earth Planetary and Space Sciences Department, Los Angeles, CA, USA
⁷ Istituto Nazionale di Geofisica e Vulcanologia (INGV), Via di Vigna Murata 605, 00143 Rome, Italy
⁸ Mullard Space Science Laboratory, Holmbury, St Mary RH5 6NT, UK
⁹ Institut de Recherche en Astrophysique et Planétologie, Université Toulouse III–Paul Sabatier, CNRS, CNES, Toulouse, France
¹⁰ Southwest Research Institute, San Antonio, TX, USA
¹¹ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, Pessac, France
¹² European Space Agency, ESTEC (SCI-S), 299 2200 AG Noordwijk, The Netherlands
¹³ Gran Sasso Science Institute (GSSI), Viale F. Crispi 7, 67100 L’Aquila, Italy
¹⁴ INFN/Laboratori Nazionali del Gran Sasso, Via G. Acitelli 22, 67100 L’Aquila, Italy
¹⁵ Istituto per la Scienza e Tecnologia dei Plasmi, Consiglio Nazionale delle Ricerche, Via Amendola 112/D, 70126 Bari, Italy
¹⁶ Swedish Institute of Space Physics, Ångström Laboratory, Lägerhyddsvägen 1, 751 21 Uppsala, Sweden
¹⁷ The University of Texas at Austin, 2515 Speedway, 78712 Austin, TX, USA
¹⁸ Space Science Center, University of New Hampshire, 8 College Road, 03824 Durham, NH, USA
¹⁹ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain

Received: 30 March 2021
Accepted: 5 June 2021

Abstract

Context. Turbulence dominated by large-amplitude, nonlinear Alfvén-like fluctuations mainly propagating away from the Sun is ubiquitous in high-speed solar wind streams. Recent studies have demontrated that slow wind streams may also show strong Alfvénic signatures, especially in the inner heliosphere.

Aims. The present study focuses on the characterisation of an Alfvénic slow solar wind interval observed by Solar Orbiter between 14 and 18 July 2020 at a heliocentric distance of 0.64 AU.

Methods. Our analysis is based on plasma moments and magnetic field measurements from the Solar Wind Analyser (SWA) and Magnetometer (MAG) instruments, respectively. We compared the behaviour of different parameters to characterise the stream in terms of the Alfvénic content and magnetic properties. We also performed a spectral analysis to highlight spectral features and waves signature using power spectral density and magnetic helicity spectrograms, respectively. Moreover, we reconstruct the Solar Orbiter magnetic connectivity to the solar sources both via a ballistic and a potential field source surface (PFSS) model.

Results. The Alfvénic slow wind stream described in this paper resembles, in many respects, a fast wind stream. Indeed, at large scales, the time series of the speed profile shows a compression region, a main portion of the stream, and a rarefaction region, characterised by different features. Moreover, before the rarefaction region, we pinpoint several structures at different scales recalling the spaghetti-like flux-tube texture of the interplanetary magnetic field. Finally, we identify the connections between Solar Orbiter in situ measurements, tracing them down to coronal streamer and pseudostreamer configurations.

Conclusions. The characterisation of the Alfvénic slow wind stream observed by Solar Orbiter and the identification of its solar source are extremely important aspects for improving the understanding of future observations of the same solar wind regime, especially as solar activity is increasing toward a maximum, where a higher incidence of this solar wind regime is expected.