Statistical study of electron density turbulence and ion-cyclotron waves in the inner heliosphere: Solar Orbiter observations

¹ National Research Council – Institute of Atmospheric Pollution Research, C/o University of Calabria, 87036 Rende, Italy
² Swedish Institute of Space Physics (IRF), Ångström Laboratory, Lägerhyddsvägen 1, 75121 Uppsala, Sweden
e-mail: lucasorriso@gmail.com
³ CNR, Istituto per la Scienza e Tecnologia dei Plasmi, via Amendola 122/D, 70126 Bari, Italy
⁴ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne, Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
⁵ Research Institute for Mathematics, Astrophysics and Particle Physics, Radboud University, Nijmegen, The Netherlands
⁶ National Institute for Astrophysics – Astrophysical Observatory of Torino, Turin, Italy
⁷ Departamento de Física, Escuela Politécnica Nacional, Ladrón de Guevara E11-253, 170525 Quito, Ecuador
⁸ National Institute for Astrophysics (INAF) – Institute for Space Astrophysics and Planetology (IAPS), Via Fosso del Cavaliere, 100, 00133 Rome, Italy
⁹ Space Sciences Laboratory, University of California, Berkeley, CA, USA
¹⁰ Physics Department, University of California, Berkeley, CA, USA
¹¹ LPP, CNRS, Ecole Polytechnique, Sorbonne Université, Observatoire de Paris, Université Paris-Saclay, Palaiseau, Paris, France
¹² LPC2E, CNRS, 3A avenue de la Recherche Scientifique, Orléans, France
¹³ Université d’Orléans, Orléans, France
¹⁴ CNES, 18 avenue Edouard Belin, 31400 Toulouse, France
¹⁵ Technische Universität Dresden, Würzburger Str. 35, 01187 Dresden, Germany
¹⁶ Institute of Atmospheric Physics of the Czech Academy of Sciences, Prague, Czech Republic
¹⁷ Space Research Institute, Austrian Academy of Sciences, Graz, Austria
¹⁸ Astronomical Institute of the Czech Academy of Sciences, Prague, Czech Republic
¹⁹ Department of Space and Plasma Physics, School of Electrical Engineering and Computer Science, Royal Institute of Technology, Stockholm, Sweden
²⁰ Space and Atmospheric Physics, The Blackett Laboratory, Imperial College of London, London SW7 2AZ, UK

Received: 30 March 2021
Accepted: 31 May 2021

Abstract

Context. The recently released spacecraft potential measured by the RPW instrument on board Solar Orbiter has been used to estimate the solar wind electron density in the inner heliosphere.

Aims. The measurement of the solar wind’s electron density, taken in June 2020, has been analysed to obtain a thorough characterization of the turbulence and intermittency properties of the fluctuations. Magnetic field data have been used to describe the presence of ion-scale waves.

Methods. To study and quantify the properties of turbulence, we extracted selected intervals. We used empirical mode decomposition to obtain the generalized marginal Hilbert spectrum, equivalent to the structure functions analysis, which additionally reduced issues typical of non-stationary, short time series. The presence of waves was quantitatively determined by introducing a parameter describing the time-dependent, frequency-filtered wave power.

Results. A well-defined inertial range with power-law scalng was found almost everywhere in the sample studied. However, the Kolmogorov scaling and the typical intermittency effects are only present in fraction of the samples. Other intervals have shallower spectra and more irregular intermittency, which are not described by models of turbulence. These are observed predominantly during intervals of enhanced ion frequency wave activity. Comparisons with compressible magnetic field intermittency (from the MAG instrument) and with an estimate of the solar wind velocity (using electric and magnetic field) are also provided to give general context and help determine the cause of these anomalous fluctuations.