Weak kinetic Alfven waves turbulence during the 14November2012 geomagnetic storm: Van Allen Probes observations

Abstract

In the dawn sector, L approximate to 5.5 and MLT approximate to 4-7, from 01:30 to 06:00UT 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 (approximate to 10(-2)nT(2)) total power with strong GSM B-x and weak B-y or large (approximate to 10nT(2)) 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 (0.01-10Hz) in the spacecraft frame, propagate oblique to the magnetic field, ( approximate to 60 degrees), and have magnetic compressibility C=|B-vertical bar|/|B|approximate to 1, where B-vertical bar(B) 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 approximate to f(-) with 4 < < 5 for f less than or similar to 2Hz, and 1.1 < < 1.7 for f 2Hz, spectral profiles that are consistent with weak kinetic Alfven wave (KAW) turbulence. Electric power is larger than magnetic power for all frequencies above 0.1Hz, and the ratio increases with increasing frequency. Vlasov linear analysis is consistent with the presence of compressive KAW with ki less than or similar to 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.