Short-wavelength turbulence in the solar wind: Linear theory of whistler and kinetic Alfvén fluctuations

There is a debate as to the identity of the fluctuations which constitute the relatively high‐frequency plasma turbulence observed in the solar wind. One school holds that these modes are kinetic Alfvén waves, whereas another opinion is that they are whistler modes. Here linear kinetic theory for electromagnetic fluctuations in homogeneous, collisionless, magnetized plasmas is used to compute two dimensionless transport ratios, the electron compressibility Ce and the magnetic compressibility C for these two modes. The former is a measure of the amplitude of density fluctuations, and the latter indicates the relative energy in magnetic fluctuations in the component parallel to the background magnetic field Bo. For βe ≪ 1, [C]Alfven ≪ [C]whistler, and the latter quantity is of order 0.5 at whistler propagation strongly oblique to Bo. Such values of C are sometimes measured at relatively high frequencies and βe ≪ 1 in the solar wind; thus, it is concluded that such observations correspond to whistler mode turbulence. But other solar wind observations indicate that kinetic Alfvén fluctuations also contribute to relatively high‐frequency solar wind turbulence.