Abstract
Abstract 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.
Highlights
The importance of whistler-mode waves in the evolution of solar wind electrons has long been a topic of interest
There are only a few observational studies focused on simultaneous measurements of whistler-mode waves and strahl electrons (Gurgiolo et al, 2012; Lacombe et al ., 2014; Kajdik et al 2016), many examined the consistency of the local or radial dependence of the electrons with theoretical predictions (Feldman et al, 1975; Scime et al.,1994; Graham et al.,2017). In contrast to these studies, Breneman et al (2010), using STEREO 3d electric field waveform capture data, discovered very narrowband, large amplitude whistler-mode waves with frequencies of ~0.2 to ~0.4 fce, that were most commonly observed in association with stream interactions regions(SIRs), and at much lower rates with interplanetary (IP) shocks
We present a study of STEREO 2.1s waveform captures, enabling us to, for the first time, determine the packet structure of these waves
Summary
The importance of whistler-mode waves in the evolution of solar wind electrons has long been a topic of interest. Given the limitations of the ISEE3 instrumentation, the properties of the waves, but not the amplitudes, are comparable to the narrowband whistlers reported by Breneman et al. Most theoretical studies of instability mechanisms for solar wind whistler-mode waves have focused on either temperature anisotropy (Kennel and Petscheck, 1966; Gary and Wang, 1996) or heat flux instabilities (Forslund, 1970; Feldman et al, 1975; Gary, 1978; Gary et al.,1975; Shaaban et al.,2018) They have concluded that only parallel propagating waves at low frequencies (~0.01fce) have significant growth rates.
Sign-in/Register to view highlights & summary 
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call