Large amplitude (up to 70 mV m−1) whistler-mode waves at frequencies of ∼0.2–0.4 f ce (electron cyclotron frequency) are frequently observed in the solar wind. The waves are obliquely propagating at angles close to the resonance cone, resulting in significant electric fields parallel to the background magnetic field, enabling strong interactions with solar wind electrons. Very narrowband (sinusoidal waveforms) and less coherent waves (more irregular waveforms) occur, but do not have a bimodal distribution. Frequencies and/or propagation angles are distinctly different from whistler-mode waves usually observed in the solar wind, and amplitudes are 1–3 orders of magnitude larger. Waves occur most often in association with stream interaction regions, and are often “close-packed.” Wave occurrence as a function of normalized electron heat flux and beta is consistent with the whistler heat flux fan instability for both the narrowband coherent and the incoherent waves. The incoherent waves are associated with zero or near zero heat flux. This suggests that the less coherent waves may be more effective in regulating the electron heat flux, or that the scattering and energization of solar wind electrons by the narrowband waves results in broadening of the waves. The oblique propagation and large amplitudes of both the narrowband and less coherent whistlers enable resonant interactions with electrons over a broad energy range, and, unlike parallel whistlers, do not require that the electrons and waves counter-propagate. Therefore, they are much more effective in modifying solar wind electron distributions than parallel propagating waves.

Aims. Large amplitude narrowband obliquely propagating whistler-mode waves at frequencies of ~0.2 fce (electron cyclotron frequency) are commonly observed at 1 AU, and they are most consistent with the whistler heat flux fan instability. We want to determine whether similar whistler-mode waves occur inside 0.3 AU and how their properties compare to those at 1 AU. Methods. We utilized the waveform capture data from the Parker Solar Probe Fields instrument from Encounters 1 through 4 to develop a data base of narrowband whistler waves. The Solar Wind Electrons Alphas and Protons Investigation (SWEAP) instrument, in conjunction with the quasi-thermal noise measurement from Fields, provides the electron heat flux, beta, and other electron parameters. Results. Parker Solar Probe observations inside ~0.3 AU show that the waves are often more intermittent than at 1 AU, and they are interspersed with electrostatic whistler-Bernstein waves at higher-frequencies. This is likely due to the more variable solar wind observed closer to the Sun. The whistlers usually occur within regions when the magnetic field is more variable and often with small increases in the solar wind speed. The near-Sun whistler-mode waves are also narrowband and large amplitude, and they are associated with beta greater than 1. The association with heat flux and beta is generally consistent with the whistler fan instability. Strong scattering of strahl energy electrons is seen in association with the waves, providing evidence that the waves regulate the electron heat flux.

Observations of plasma waves by the Fields Suite and of electrons by the Solar Wind Electrons Alphas and Protons Investigation on the Parker Solar Probe provide strong evidence for pitch angle scattering of strahl-energy electrons by narrowband whistler-mode waves at radial distances less than ∼0.3 au. We present two example intervals of a few hours each that include eight waveform captures with whistler-mode waves and 26 representative electron distributions that are examined in detail. Two were narrow, seventeen were clearly broadened, and eight were very broad. The two with narrow strahl occurred when there were either no whistlers or very intermittent low amplitude waves. Six of the eight broadest distributions were associated with intense, long duration waves. Approximately half of the observed electron distributions have features consistent with an energy-dependent scattering mechanism, as would be expected from interactions with narrowband waves. A comparison of the wave power in the whistler-mode frequency band to pitch angle width and a measure of anisotropy provides additional evidence for electron scattering by whistler-mode waves. We estimate the range of resonances based on the wave properties and energies over which broadening is observed. These observations provide strong evidence that the narrowband whistler-mode waves scatter strahl-energy electrons to produce the halo and to reduce the electron heat flux.

In situ measurements in the near-Sun solar wind from the Parker Solar Probe have revealed the existence of quiescent solar wind regions: extended regions of solar wind with low-amplitude turbulent magnetic field fluctuations compared to adjacent regions. Identified through the study of harmonic waves near the electron cyclotron frequency (f ce), these quiescent regions are shown to host a variety of plasma waves. The near-f ce harmonic waves are observed exclusively in quiescent regions, and as such, they can be used as markers for quiescent regions. A blob-finding algorithm is applied to data from Encounters 1–6 in order to identify near-f ce harmonic wave intervals and thereby locate quiescent regions. We carry out a superposed epoch analysis on the identified quiescent regions, and compare their bulk solar wind properties with adjacent regions of solar wind. Quiescent regions are found to contain relatively weak magnetic field variation and are entirely devoid of magnetic switchbacks. In the quiescent solar wind, the magnetic field closely follows the Parker spiral, while adjacent regions prefer more radial orientations, providing a clear picture of the magnetic geometry of these regions. Quiescent regions show minimal differences in multiple particle plasma parameters relative to the non-quiescent solar wind. The quiescent solar wind regions, studied throughout this work, are thought to represent the underlying solar wind, through which Alfvénic fluctuations propagate. Quantifying the properties of these regions may help to understand the formation/origin of the solar wind, and furthermore, to constrain the role that low-frequency Alfvén waves play in the regulation of solar wind temperature.