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Turbulence in the Earth's cusp region: The k ‐filtering analysis
Author(s) -
Wang Tieyan,
Cao JinBin,
Fu Huishan,
Liu Wenlong,
Dunlop Malcolm
Publication year - 2014
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1002/2014ja019997
Subject(s) - physics , turbulence , computational physics , magnetic field , dispersion relation , polarization (electrochemistry) , solar wind , condensed matter physics , mechanics , quantum mechanics , chemistry
Abstract On 13 April 2002, four Cluster spacecraft with separations up to 127 km measured similar turbulence in the exterior cusp during northward interplanetary magnetic field (IMF) B z . Both the power spectra of magnetic and electric field fluctuations resemble the classical Kolmogorov power law, with the scaling f −1.7 under the proton gyrofrequency f cp (~0.3 Hz), breaks near f cp , and then steepens with the scalings f −2.8 and f −2.0 up to 10 Hz, respectively. The observed ratio of the electric to magnetic field is in agreement with the theoretical values of | δ E / δ B | for the quasi‐perpendicular kinetic Alfvén waves (KAWs), which reflects the features of Alfvén turbulence. The wave vector and dispersion relation of the turbulence are obtained using k ‐filtering technique. The results show that the waves propagate quasi‐perpendicularly to the background magnetic field. The similarity between the experimental and the theoretical dispersion relations indicates that the measured waves are kinetic Alfvén wave. The waves have right‐handed elliptical polarization in the plane perpendicular to k . The main axis of polarization ellipse is perpendicular to the average magnetic field. These features furthermore indicate that the turbulence properties agree well with those of KAW mode. The observed KAW is much possibly produced through resonance mode conversion. We calculate the density gradient vector using multipoint density data and found that the waves propagate basically toward high‐density region. The density gradient in the exterior cusp provides a favorable condition for the resonance converted KAW.