Whistler waves observed by Solar Orbiter/RPW between 0.5 AU and 1 AU

¹ LPC2E, UMR7328 CNRS, University of Orléans, 3A avenue de la recherche scientifique, Orléans, France
e-mail: matthieu.kretzschmar@cnrs-orleans.fr
² LPP, CNRS, Ecole Polytechnique, Sorbonne Université, Observatoire de Paris, Université Paris-Saclay, Palaiseau, Paris, France
³ Swedish Institute of Space Physics (IRF), Uppsala, Sweden
⁴ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, Meudon, France
⁵ Institute of Atmospheric Physics, Czech Academy of Sciences, Prague, Czech Republic
⁶ CNES, 18 Avenue Edouard Belin, 31400 Toulouse, France
⁷ Technische Universität Dresden, Würzburger Str. 35, 01187 Dresden, Germany
⁸ Space Research Institute, Austrian Academy of Sciences, Graz, Austria
⁹ Astronomical Institute of the Czech Academy of Sciences, Prague, Czech Republic
¹⁰ Radboud Radio Lab, Department of Astrophysics, Radboud University, Nijmegen, The Netherlands
¹¹ Space Sciences Laboratory, University of California, Berkeley, CA, USA
¹² Physics Department, University of California, Berkeley, CA, USA
¹³ Stellar Scientific, Berkeley, CA, USA
¹⁴ Department of Space and Plasma Physics, School of Electrical Engineering and Computer Science, Royal Institute of Technology, Stockholm, Sweden
¹⁵ Department of Physics, Imperial College, SW7 2AZ London, UK
¹⁶ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
¹⁷ Institut de Recherche en Astrophysique et Planétologie, 9, Avenue du Colonel ROCHE, BP 4346, 31028 Toulouse Cedex 4, France

Received: 31 March 2021
Accepted: 7 October 2021

Abstract

Context. Solar wind evolution differs from a simple radial expansion, while wave-particle interactions are assumed to be the major cause for the observed dynamics of the electron distribution function. In particular, whistler waves are thought to inhibit the electron heat flux and ensure the diffusion of the field-aligned energetic electrons (Strahl electrons) to replenish the halo population.

Aims. The goal of our study is to detect and characterize the electromagnetic waves that have the capacity to modify the electron distribution functions, with a special focus on whistler waves.

Methods. We carried out a detailed analysis of the electric and magnetic field fluctuations observed by the Solar Orbiter spacecraft during its first orbit around the Sun, between 0.5 and 1 AU. Using data from the Search Coil Magnetometer and electric antenna, both part of the Radio and Plasma Waves (RPW) instrumental suite, we detected the electromagnetic waves with frequencies above 3 Hz and determined the statistical distribution of their amplitudes, frequencies, polarization, and k-vector as a function of distance. Here, we also discuss the relevant instrumental issues regarding the phase between the electric and magnetic measurements as well as the effective length of the electric antenna.

Results. An overwhelming majority of the observed waves are right-handed circularly polarized in the solar wind frame and identified as outwardly propagating quasi-parallel whistler waves. Their occurrence rate increases by a least a factor of 2 from 1 AU to 0.5 AU. These results are consistent with the regulation of the heat flux by the whistler heat flux instability. Near 0.5 AU, whistler waves are found to be more field-aligned and to have a smaller normalized frequency (f/f_ce), larger amplitude, and greater bandwidth than at 1 AU.