Density fluctuations associated with turbulence and waves. First observations by Solar Orbiter

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 both has a high temporal resolution and is of high 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 use the plasma density based on measurements of the probe-to-spacecraft potential in combination with magnetic field measurements by MAG to study fields and density fluctuations in the solar wind. In particular, we use 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 (anti-correlated) with the compressible component of magnetic fluctuations for intervals of turbulence, while they are in phase for the circular-polarized waves. We analyze in detail two specific events with simultaneous presence of leftand right-handed waves at different frequencies. We compare 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 wavelength 7 × 106 > λ > 106 m. We conclude that most likely both the leftand right-handed waves correspond to the low-wavenumber part (close to the cut-off at ΩcHe++) proton-band electromagnetic ion cyclotron (left-handed wave in the plasma frame confined to the frequency range ΩcHe++ < ω < ΩcH+) 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.