Narrowband large amplitude whistler-mode waves in the solar wind and their association with electrons: STEREO waveform capture observations

Large amplitude whistler waves at frequencies of 0.2 to 0.4 times electron cyclotron frequency are frequently observed in the solar wind. The waves are obliquely propagating close to the resonance cone, with significant electric fields parallel to the background magnetic field, enabling strong interactions with electrons. Propagation angles are distinctly different from whistlers usually observed in the solar wind, and amplitudes are significantly larger. Waves occur most often in association with stream interaction regions (SIRs), and are often close-packed. 68 percent of the 54 SIRs had narrowband whistler groups; 33 percent of the nine interplanetary coronal mass ejections had coherent groups. Although wave occurrence as a function of the electron temperature anisotropy and parallel beta is constrained by the thresholds for the whistler temperature anisotropy and firehose instabilities, neither is consistent with observed wave properties. We show for the first time that comparisons of wave data to thresholds for the electron beam driven instability (beam speed greater than twice the electron Alfven speed) and to the whistler heat flux fan instability indicate that either might destabilize the narrowband waves. In contrast, the less coherent waves, on average, are associated with zero or near zero heat flux and much higher electron Alfven speeds, without higher energy beams. 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 highly 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 does not require that the electrons and waves counter-propagate.