Statistical Analysis of Whistler Waves in Pristine Solar Wind at 1 AU

We present statistical analysis of whistler waves observed in pristine solar wind aboard ARTEMIS spacecraft. Three-component magnetic field measurements provided by the search-coil magnetometer and fluxgate magnetometer are used to determine whistler wave frequencies, amplitudes and obliqueness with respect to local background magnetic field. We demonstrate that whistler waves in pristine solar wind at 1 au have relatively small amplitudes (less than a few percent of local magnetic field and typically around 0.1%) and propagate quasi-parallel (within about 30 degrees) to local magnetic field. The electric field measurements were examined to determine the propagation direction (sunward / anti-sunward) of the whistler waves and we found that more than 90% of whistler waves propagate anti-sunward. The analysis of particle measurements showed that the classical whistler heat flux instability is the source of the anti-sunward whistler waves. The source of sunward propagating whistler waves is most likely temperature anisotropy, but it is still under examination. The results are discussed in the light of heat flux regulation problem in the solar wind.