Electromagnetic Fluctuations in the Dissipation Range of Solar Wind MHD Turbulence: Kinetic Alfven Waves or Whistlers?

Electromagnetic fluctuations in the inertial range of solar wind MHD turbulence and beyond (up to frequencies of 10Hz) have recently been studied for the first time using both magnetic field and electric field measurements on Cluster [Bale et al., PRL, 2005]. It has been shown that at frequencies above the spectral breakpoint at ~0.4Hz, in the so-called dissipation range, the wave modes become dispersive and are consistent with Kinetic Alfven Waves (KAW). This interpretation is based on the simple assumption that the measured frequency spectrum is actually a Doppler shifted wave number spectrum (ω ≈ k Vsw), commonly used in the solar wind and known as Taylor's hypothesis. While Taylor's hypothesis is valid in the inertial range of solar wind turbulence, it may break down in the dissipation range where temporal fluctuations can become important. In this work, we analyze the effect of Doppler shift on KAW as well as compressional proton whistler waves, and revisit Cluster solar wind data using this approach. We focus our analysis on low-beta (β < 1) ambient solar wind intervals. We first 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 spacecraft frame. Finally, we compare those estimates with the data directly in the spacecraft frame.