  The origins and properties of large amplitude whistler wave packets in the
solar wind are still unclear. In this Letter we utilise single spacecraft
electric and magnetic field waveform measurements from the ARTEMIS mission to
calculate the plasma frame frequency and wavevector of individual wave packets
over multiple intervals. This allows direct comparison of experimental
measurements with theoretical dispersion relations to identify the observed
waves as whistler waves. The whistlers are right-hand circularly polarised,
travel anti-sunward and are aligned with the background magnetic field. Their
dispersion is strongly affected by the local electron parallel beta in
agreement with linear theory. The properties measured are consistent with the
electron heat flux instability acting in the solar wind to generate these
waves.
