Cascade and Dissipation of Solar Wind Turbulence at Electron Scales: Whistlers or Kinetic Alfv’en Waves? (Invited)

Over the past few decades, large-scales solar wind (SW) turbulence has been studied extensively, both theoretically and observationally. Observed power spectra of the low frequency turbulence, which can be described in the magnetohydrodynamic (MHD) limit, are shown to obey the Kolmogorov scaling, k^-5/3, down the local proton gyrofrequency (f_ci ∼ 0.1~Hz). Turbulence at frequencies above f_ci has not been thoroughly investigated and remains far less well understood. Above f_ci the spectrum steepens to ∼ f^-2.5 and a debate exists as to whether the turbulence has become dominated by dispersive kinetic Alfvén waves (KAW) or by whistler waves, before it is dissipated at small scales. In a case study Sahraoui et al., PRL (2009) have reported the first direct determination of the dissipation range of solar wind turbulence near the electron gyroscale using the high resolution Cluster magnetic and electric field data (up to 10²~Hz in the spacecraft reference frame). Above the Doppler-shifted proton scale f_{ρ i} a new inertial range with a scaling ∼ f^-2.3 has been evidenced and shown to remarkably agree with theoretical predictions of a quasi-two-dimensional cascade into KAW turbulence. Here, we use a wider sample of data sets of small scale SW turbulence under different plasma conditions, and investigate under which physical criteria the KAW (or the whistler) turbulence may be observed to carry out the cascade at small scales. These new observations/criteria are compared to the predictions on the cascade and the (kinetic) dissipation from the Vlasov theory. Implications of the results on the heating problem of the solar wind will be discussed.