Weak kinetic Alfvén waves turbulence during the 14 November 2012 geomagnetic storm: Van Allen Probes observations

In the dawn sector, L ∼ 5.5 and MLT ∼ 4-7, from 01:30 to 06:00 UT during the 14 November 2012 geomagnetic storm, both Van Allen Probes observed an alternating sequence of locally quiet and disturbed intervals with two strikingly different power fluctuation levels and magnetic field orientations: either small (∼10^-2 nT²) total power with strong GSM B_x and weak B_y or large (∼10 nT²) total power with weak B_x and strong B_y and B_z components. During both kinds of intervals the fluctuations occur in the vicinity of the local ion gyrofrequencies (0.01-10 Hz) in the spacecraft frame, propagate oblique to the magnetic field, (θ ∼ 60^°), and have magnetic compressibility C=|δB_∥|/|δB_⊥|∼1, where δB_∥ (δB_⊥) are the average amplitudes of the fluctuations parallel (perpendicular) to the mean field. Electric field fluctuations are present whenever the magnetic field is disturbed, and large electric field fluctuations follow the same pattern for quiet and disturbed intervals. Magnetic frequency power spectra at both spacecraft correspond to steep power laws ∼f^-α with 4 < α < 5 for f≲2 Hz, and 1.1 < α < 1.7 for f≳ 2 Hz, spectral profiles that are consistent with weak kinetic Alfvén wave (KAW) turbulence. Electric power is larger than magnetic power for all frequencies above 0.1 Hz, and the ratio increases with increasing frequency. Vlasov linear analysis is consistent with the presence of compressive KAW with k⊥ρi≲1, right-handed polarization and positive magnetic helicity, in the plasma frame, considering a multiion plasma. All these results suggest the presence of weak KAW turbulence which dissipates the energy associated with the intermittent sudden changes in the magnetic field during the main phase of the storm.