First-year ion-acoustic wave observations in the solar wind by the RPW/TDS instrument on board Solar Orbiter

¹ Institute of Atmospheric Physics of Czech Academy of Sciences, Bocni II 1401, 141 00 Prague, Czech Republic
e-mail: dp@ufa.cas.cz
² Faculty of Mathematics and Physics, Charles University, V Holesovickach 2, 18000 Prague 8, Czech Republic
³ Southwest Research Institute, San Antonio, TX 78238, USA
⁴ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
⁵ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
⁶ Space Sciences Laboratory, University of California, Berkeley, CA, USA
⁷ Physics Department, University of California, Berkeley, CA, USA
⁸ LPP, CNRS, Ecole Polytechnique, Sorbonne Université, Observatoire de Paris, Université Paris-Saclay, Palaiseau, Paris, France
⁹ Swedish Institute of Space Physics (IRF), Uppsala, Sweden
¹⁰ LPC2E, CNRS, 3A avenue de la Recherche Scientifique, Orléans, France
¹¹ Université d’Orléans, Orléans, France
¹² CNES, 18 avenue Edouard Belin, 31400 Toulouse, France
¹³ Technische Universität Dresden, Würzburger Str. 35, 01187 Dresden, Germany
¹⁴ Space Research Institute, Austrian Academy of Sciences, Graz, Austria
¹⁵ Astronomical Institute of the Czech Academy of Sciences, Prague, Czech Republic
¹⁶ Department of Space and Plasma Physics, School of Electrical Engineering and Computer Science, Royal Institute of Technology, Stockholm, Sweden
¹⁷ Radboud Radio Lab., Department of Astrophysics, Radboud University, Nijmegen, The Netherlands
¹⁸ Department of Physics, Imperial College, SW7 2AZ London, UK
¹⁹ Institut de Recherche en Astrophysique et Planétologie, 9 avenue du Colonel Roche, BP 4346, 31028 Toulouse Cedex 4, France

Received: 30 March 2021
Accepted: 2 August 2021

Abstract

Context. Electric field measurements of the Time Domain Sampler (TDS) receiver, part of the Radio and Plasma Waves (RPW) instrument on board Solar Orbiter, often exhibit very intense broadband wave emissions at frequencies below 20 kHz in the spacecraft frame. During the first year of the mission, the RPW/TDS instrument was operating from the first perihelion in mid-June 2020 and through the first flyby of Venus in late December 2020.

Aims. In this paper, we present a year-long study of electrostatic fluctuations observed in the solar wind at an interval of heliocentric distances from 0.5 to 1 AU. The RPW/TDS observations provide a nearly continuous data set for a statistical study of intense waves below the local plasma frequency.

Methods. The on-board and continuously collected and processed properties of waveform snapshots allow for the mapping plasma waves at frequencies between 200 Hz and 20 kHz. We used the triggered waveform snapshots and a Doppler-shifted solution of the dispersion relation for wave mode identification in order to carry out a detailed spectral and polarization analysis.

Results. Electrostatic ion-acoustic waves are the most common wave emissions observed between the local electron and proton plasma frequency by the TDS receiver during the first year of the mission. The occurrence rate of ion-acoustic waves peaks around perihelion at distances of 0.5 AU and decreases with increasing distances, with only a few waves detected per day at 0.9 AU. Waves are more likely to be observed when the local proton moments and magnetic field are highly variable. A more detailed analysis of more than 10 000 triggered waveform snapshots shows the mean wave frequency at about 3 kHz and wave amplitude about 2.5 mV m⁻¹. The wave amplitude varies as R^−1.38 with the heliocentric distance. The relative phase distribution between two components of the E-field projected in the Y − Z Spacecraft Reference Frame (SRF) plane shows a mostly linear wave polarization. Electric field fluctuations are closely aligned with the directions of the ambient field lines. Only a small number (3%) of ion-acoustic waves are observed at larger magnetic discontinuities.