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 × 10−6m−1, which corresponds to a wavelength of 7 × 106 > λ > 106m. 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 ΩcHe + +) of the proton-band electromagnetic ion cyclotron (left-handed wave in the plasma frame confined to the frequency range ΩcHe + + < ω < Ωcp) 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.
Highlights
The solar wind exhibits an abundance of plasma turbulence and waves (Belcher & Davis 1971; Tu & Marsch 1995; Bruno & Carbone 2013)
The main purpose of this paper is to demonstrate that the probe-to-spacecraft potential measured by Radio and Plasma Waves (RPW) (Maksimovic et al 2020) on Solar Orbiter can be used to derive the plasma density measurement, which exhibits both a high temporal resolution and a high level of accuracy
We present observations of plasma turbulence and quasicircularly polarized electromagnetic waves close to protoncyclotron frequency, fcp, near the first perihelion encounter by Solar Orbiter (∼0.5 AU from the Sun), which we analyze using the magnetic field and plasma density measurements
Summary
The solar wind exhibits an abundance of plasma turbulence and waves (Belcher & Davis 1971; Tu & Marsch 1995; Bruno & Carbone 2013). On the sole basis of the magnetic field measurement, it is not possible to determine the wave polarization in the plasma frame This presents a challenge since the wave polarization observed in the spacecraft frame may be modified due to the Doppler shift in a fast-flowing solar wind. When designing electric field measurements for the Solar Orbiter mission (Müller et al 2020), it was important to include the capability to measure high-quality electric fields and density fluctuations up to frequencies of at least about 100 Hz (Vaivads et al 2007) This is of vital importance for studying plasma processes and identifying plasma waves in the ion and electron kinetic range. We use the obtained density together with magnetic field measurements by MAG (Horbury et al 2020) to study density fluctuations associated with turbulence and waves in the solar wind
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