  We present an analysis of simultaneous particle and field measurements from
the ARTEMIS spacecraft which demonstrate that quasi-parallel whistler waves in
the solar wind can be generated locally by a bulk flow of halo electrons
(whistler heat flux instability). ARTEMIS observes quasi-parallel whistler
waves in the frequency range $\sim 0.05 - 0.2 f_{ce}$ simultaneously with
electron velocity distribution functions that are a combination of
counter-streaming core and halo populations. A linear stability analysis shows
that the plasma is stable when there are no whistler waves, and unstable in the
presence of whistler waves. In the latter case, the stability analysis shows
that the whistler wave growth time is from a few to ten seconds at frequencies
and wavenumbers that match the observations. The observations clearly
demonstrate that the temperature anisotropy of halo electrons crucially affects
the heat flux instability onset: a slight anisotropy
$T_{\parallel}/T_{\perp}>1$ may quench the instability, while a slight
anisotropy $T_{\parallel} / T_{\perp}<1$ may significantly increase the growth
rate. These results demonstrate that heat flux inhibition is strongly dependent
on the microscopic plasma properties.
