Density fluctuations associated with turbulence and waves

First observations by Solar Orbiter

¹ Swedish Institute of Space Physics (IRF), Uppsala 75121, Sweden
e-mail: yuri@irfu.se
² Division of Space and Plasma Physics, School of Electrical Engineering and Computer Science, KTH Royal Institute of Technology, Stockholm 11428, Sweden
³ Space and Plasma Physics, Department of Physics and Astronomy, Uppsala University, Uppsala 75120, Sweden
⁴ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
⁵ Space Sciences Laboratory, University of California, Berkeley, CA, USA
⁶ Physics Department, University of California, Berkeley, CA, USA
⁷ LPP, CNRS, Ecole Polytechnique, Sorbonne Université, Observatoire de Paris, Université Paris-Saclay, Palaiseau, Paris, France
⁸ LPC2E, CNRS, 3A avenue de la Recherche Scientifique, Orléans, France
⁹ Université d’Orléans, Orléans, France
¹⁰ CNES, 18 Avenue Edouard Belin, 31400 Toulouse, France
¹¹ Technische Universität Dresden, Helmholtz Str. 10, 01187 Dresden, Germany
¹² Institute of Atmospheric Physics of the Czech Academy of Sciences, Prague, Czech Republic
¹³ Space Research Institute, Austrian Academy of Sciences, Graz, Austria
¹⁴ Astronomical Institute of the Czech Academy of Sciences, Prague, Czech Republic
¹⁵ Radboud Radio Lab, Department of Astrophysics, Radboud University, Nijmegen, The Netherlands
¹⁶ Imperial College London, South Kensington Campus, London SW7 2AZ, UK

Received: 30 March 2021
Accepted: 10 June 2021

Abstract

Aims. The aim of this work is to demonstrate that the probe-to-spacecraft potential measured by RPW on Solar Orbiter can be used to derive the plasma (electron) density measurement, which exhibits both a high temporal resolution and a high level of accuracy. To investigate the physical nature of the solar wind turbulence and waves, we analyze the density and magnetic field fluctuations around the proton cyclotron frequency observed by Solar Orbiter during the first perihelion encounter (∼0.5 AU away from the Sun).

Methods. We used the plasma density based on measurements of the probe-to-spacecraft potential in combination with magnetic field measurements by MAG to study the fields and density fluctuations in the solar wind. In particular, we used the polarization of the wave magnetic field, the phase between the compressible magnetic field and density fluctuations, and the compressibility ratio (the ratio of the normalized density fluctuations to the normalized compressible fluctuations of B) to characterize the observed waves and turbulence.

Results. We find that the density fluctuations are 180° out of phase (anticorrelated) with the compressible component of magnetic fluctuations for intervals of turbulence, whereas they are in phase for the circular-polarized waves. We analyze, in detail, two specific events with a simultaneous presence of left- and right-handed waves at different frequencies. We compare the observed wave properties to a prediction of the three-fluid (electrons, protons, and alphas) model. We find a limit on the observed wavenumbers, 10⁻⁶ < k < 7 × 10⁻⁶ m⁻¹, which corresponds to a wavelength of 7 × 10⁶ > λ > 10⁶ m. We conclude that it is most likely that both the left- and right-handed waves correspond to the low-wavenumber part (close to the cut-off at Ω_cHe + +) of the proton-band electromagnetic ion cyclotron (left-handed wave in the plasma frame confined to the frequency range Ω_cHe + + < ω < Ω_cp) waves propagating in the outwards and inwards directions, respectively. The fact that both wave polarizations are observed at the same time and the identified wave mode has a low group velocity suggests that the double-banded events occur in the source regions of the waves.