  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.
