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

Abstract

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 nT 2 ) total power with strong GSM B x and weak B y or large (∼10 nT 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 (0.01-10 Hz) in the spacecraft frame, propagate oblique to the magnetic field, ( ∼60 ∘ ), and have magnetic compressibility C = | B ∥ |∕| B ⟂ | ∼1, where B ∥ ( 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 ∼ f − with 4 < < 5 for f ≲ 2 Hz, and 1.1 < < 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 ⟂ i ≲ 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.

Broadband low‐frequency electromagnetic waves in the inner magnetosphere

“…A similar result concerning the wave mode identification has recently been reported from an analysis of a storm time broadband electromagnetic wave event by Moya et al . []. These waves are the low‐frequency electromagnetic counterpart of the electric field variations examined by Malaspina et al .…”

Nonlinear effects associated with quasi‐electrostatic whistler waves relevant to that in radiation belts

“…The observations by Moya et al . [] show the electric power spectrum having a scaling of f −2 as observed by Radiation Belt Storm Probes (RBSP). Moya et al .…”

“…Moya et al . [] depicted the presence of KAW in radiation belts based on their observed scalings. The KAW is low‐frequency wave in which density localization takes place due to its interaction with high‐frequency pump quasi‐electrostatic whistler.…”

“…[] and Moya et al . []. For the further assistantship, readers can contact Ravinder Goyal (ravig.iitd@gmail.com).…”

Radial transport of radiation belt electrons in kinetic field‐line resonances

“…Recent observations from the Van Allen Probes spacecraft have revealed the prevalence at L ≳ 4 during storm times of broadband electromagnetic oscillations with some of the largest amplitude electric fields observed in the magnetosphere [Chaston et al, 2015]. These oscillations have scales transverse to the magnetic field of the order of ion gyroradii and locally have the dispersive properties of kinetic Alfvén waves [Chaston et al, 2014;Malaspina et al, 2015;Moya et al, 2015]. While kinetic Alfvén waves are narrow perpendicular to the geomagnetic field they have parallel wavelengths which in the inner magnetosphere may be of the order of the field-line length.…”