  Observations by the Parker Solar Probe mission of the solar wind at about
35.7 solar radii reveal the existence of whistler wave packets with frequencies
below 0.1 f/fce (20-80 Hz in the spacecraft frame). These waves often coincide
with local minima of the magnetic field magnitude or with sudden deflections of
the magnetic field that are called switchbacks. Their sunward propagation leads
to a significant Doppler frequency downshift from 200-300 Hz to 20-80 Hz (from
0.2 f/fce to 0.5 f/fce). The polarization of these waves varies from
quasi-parallel to significantly oblique with wave normal angles that are close
to the resonance cone. Their peak amplitude can be as large as 2 to 4 nT. Such
values represent approximately 10% of the background magnetic field, which is
considerably more than what is observed at 1 a.u. Recent numerical studies show
that such waves may potentially play a key role in breaking the heat flux and
scattering the Strahl population of suprathermal electrons into a halo
population.
