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

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. Solar-wind electron density measured during 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. Selected intervals have been extracted to study and quantify the properties of turbulence. The Empirical Mode Decomposition has been used to obtain the generalized marginal Hilbert spectrum, equivalent to the structure functions analysis, and additionally reducing issues typical of non-stationary, short time series. The presence of waves was quantitatively determined introducing a parameter describing the time-dependent, frequency-filtered wave power. Results. A well defined inertial range with power-law scaling has been found almost everywhere. However, the Kolmogorov scaling and the typical intermittency effects are only present in part of the samples. Other intervals have shallower spectra and more irregular intermittency, 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 for the anomalous fluctuations.

Scaling Laws for the Energy Transfer in Space Plasma Turbulence

“…Some exceptions are represented by a series of works that revealed the possible passive nature of both those quantities (Consolini, 2012;, suggesting similarities with passive scalar turbulence in ordinary flows (see for example Shraiman and Siggia, 2000;Warhaft, 2000, and references therein). Such behaviour was not always confirmed by other studies of density fluctuations, which instead pointed to a broader variety of dynamical properties, not simply associated to active or passive role, particularly in the more compressive slow solar wind (Hnat et al, 2005;Bruno et al, 2014;Riazantseva et al, 2016;Sorriso-Valvo et al, 2017;Carbone et al, 2021). Nevertheless, at least in those cases where density and temperature actually behave like passive scalars, the analogous of the von Kármán-Howarth relation for passive scalars, better known as Yaglom law (Yaglom, 1949), can be recovered.…”

“…The enhanced turbulence might indeed be the cause for observation of the PCWs, or, more likely, both features could be associated with the same driver, possibly related with the seasonal changes in the Mars exosphere and magnetosphere connection with the solar wind. Previous observations of ion-scale cyclotron waves was typically associated with periods of reduced turbulence (Khotyaintsev et al, 2021;Carbone et al, 2021). This suggest that the MAVEN observations are in fact demonstrating a different feature, more strictly connected with the specific Mars environment, and which will deserve more accurate studies.…”

“…Given the typical plasma parameters, the large expansion velocity exceeds both the sound and the Alfvén speeds. In these conditions, the solar wind is mostly found in a state of highly developed turbulence (Tu and Marsch, 1995;Bruno and Carbone, 2013;Matthaeus, 2021), and at the same time it supports the propagation of various waves and instabilities (Alexandrova et al, 2013) that superpose and interact with the turbulence (Khotyaintsev et al, 2021;Carbone et al, 2021). Solar wind turbulence studies have been developed in depth for nearly 50 years, showing the ubiquitous presence of power-law trends in the power spectral density of velocity, density and magnetic field (Coleman, 1968;, with spectral exponents broadly compatible with the standard Kolmogorov phenomenology (Kolmogorov, 1941) or with the Iroshnikov-Kraichnan phenomenology (Iroshnikov, 1964;Kraichnan, 1965).…”

“…The complex interplay between sub-ion scale wave packets and local properties of turbulence remains an interesting open question (Khotyaintsev et al, 2021;Carbone et al, 2021). Such work demonstrated that the investigation of the details of the turbulent cascade, and in particular the local contributions to the scaling laws, may help answering one of the most crucial open questions in space plasma physics, namely, what are the mechanisms that dissipate the turbulent energy and provide the observed particle energization.…”