Diagnostics for Wave Mode Identification in the Dissipation Range of Solar Wind Turbulence: Kinetic Alfven Waves versus Whistlers

We analyze the effect of Doppler shift on Kinetic Alfven Waves (KAW) as well as compressional proton Whistler Waves at frequencies above the local ion cyclotron frequency into the so-called dissipation range of solar wind turbulence (0.1 < f/f_ci < 10). Prior studies have shown that wave modes in this frequency range become dispersive and are consistent with Kinetic Alfven Waves (KAW) under the assumption that Taylor's hypothesis is still valid at those frequencies. However, the proton whistler is another possible wave mode in the solar wind in this frequency range. The temporal fluctuations of this mode combined with a slight Doppler shift can lead to the same apparent properties in the spacecraft (s/c) frame as strongly Doppler-shifted KAW. First, we present a different approach to resolve this long-standing question regarding the nature of the electromagnetic fluctuations in the dissipation range of solar wind turbulence. Specifically, we determine, both analytically and numerically, the dispersive properties of the KAW and the whistler wave modes and estimate the electric to magnetic field (E/B) ratio in the plasma and the s/c frame to make direct comparisons with s/c data. We discuss the predicted, observable, parameters of both KAW and whistlers in the plasma and s/c frames. We show that the properties of both KAW and whistlers appear to be similar in the s/c frame, yet there are quantifiable differences that one can use to distinguish between both wave modes. Those differences are discussed. Finally, we revisit Cluster electric field and magnetic field data in the solar wind using this technique. We focus our analysis on several low-beta (β < 1) ambient solar wind intervals. We compare the predicted parameters from our technique to the data directly in the s/c frame, without the use of Taylor's hypothesis. We propose this technique as an efficient diagnostics for wave-mode identification in the dissipation (or dispersion) range of solar wind turbulence.