Identification of Widespread Turbulence of Dispersive Alfvén Waves

A revised analysis of Freja observations shows that events interpreted previously as broadband turbulence in the frequency domain, f ∼ 1 - 500 Hz represent in fact turbulence of dispersive Alfvén waves (DAW) covering a wide range of wave vectors k_⊥. A satellite's motion through broad Δk_⊥, low frequency, ω < ω_ci, Alfvénic structures makes them appear as broadband waves at (Doppler) frequencies Δω = v_s · Δk_⊥ in the satellite reference frame. The identification is made with the measured ratio δE/δB fluctuations which is consistent with the theoretical prediction for inertial Alfvén wave spectrum at spatial scales λ_⊥ ∼ 30 - 7000 m. In addition, we demonstrate that the multipoint measurements with double density probes provide independent confirmation of the spatial nature of the low frequency fluctuations.