Abstract
Context. Solar wind evolution differs from a simple radial expansion, while wave-particle interactions are assumed to be the major cause for the observed dynamics of the electron distribution function. In particular, whistler waves are thought to inhibit the electron heat flux and ensure the diffusion of the field-aligned energetic electrons (Strahl electrons) to replenish the halo population. Aims. The goal of our study is to detect and characterize the electromagnetic waves that have the capacity to modify the electron distribution functions, with a special focus on whistler waves. Methods. We carried out a detailed analysis of the electric and magnetic field fluctuations observed by the Solar Orbiter spacecraft during its first orbit around the Sun, between 0.5 and 1 AU. Using data from the Search Coil Magnetometer and electric antenna, both part of the Radio and Plasma Waves (RPW) instrumental suite, we detected the electromagnetic waves with frequencies above 3 Hz and determined the statistical distribution of their amplitudes, frequencies, polarization, and k-vector as a function of distance. Here, we also discuss the relevant instrumental issues regarding the phase between the electric and magnetic measurements as well as the effective length of the electric antenna. Results. An overwhelming majority of the observed waves are right-handed circularly polarized in the solar wind frame and identified as outwardly propagating quasi-parallel whistler waves. Their occurrence rate increases by a least a factor of 2 from 1 AU to 0.5 AU. These results are consistent with the regulation of the heat flux by the whistler heat flux instability. Near 0.5 AU, whistler waves are found to be more field-aligned and to have a smaller normalized frequency (f/fce), larger amplitude, and greater bandwidth than at 1 AU.
Highlights
The properties of the solar wind are known to change along its propagation in the interplanetary space
Chust et al (2021) analyzed in detail three wave events observed by Solar Orbiter and found them to correspond to outwardly propagating whistler waves. We extend these results by presenting an overview of the waves observed above 3 Hz by the Solar Orbiter Radio and Plasma Waves (RPW) experiment between 0.5 AU and 1 AU during its first orbit
In this paper, we report the results of the study of the electromagnetic waves observed by Solar Orbiter in the frequency range between 3 Hz and 128 Hz during its first orbit and covering heliocentric distances between 0.5 AU and 1 AU
Summary
The properties of the solar wind are known to change along its propagation in the interplanetary space. During the first perihelion of Parker Solar Probe, Agapitov et al (2020) found the presence of numerous sunward whistlers whose propagation direction relative to the background magnetic field varies from aligned to oblique Oblique whistlers in their turn can very efficiently diffuse suprathermal electrons (Parail & Pogutse 1978; Vasko et al 2019). The properties of individual wave packet were determined both via a Fourier analysis to retrieve the wave amplitudes and phases as well as via a minimum variance analysis (MVA, Sonnerup & Scheible 1998) of the magnetic waveforms to retrieve the direction of the wave vector (±π) We estimated their planarity and ellipticity by using the ratio of the singular values (Santolík et al 2003). We ended up with 5035 spectra computed over 8 s and 17362 associated wave packets, most of which have a right-handed circular polarisation and are quasi-aligned with the magnetic field, as described
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