SPECTRAL ANISOTROPY OF ELSÄSSER VARIABLES IN TWO-DIMENSIONAL WAVE-VECTOR SPACE AS OBSERVED IN THE FAST SOLAR WIND TURBULENCE

Intensive studies have been conducted to understand the anisotropy of solar wind turbulence. However, the anisotropy of Elsässer variables ( ) in 2D wave-vector space has yet to be investigated. Here we first verify the transformation based on the projection-slice theorem between the power spectral density and the spatial correlation function Based on the application of the transformation to the magnetic field and the particle measurements from the WIND spacecraft, we investigate the spectral anisotropy of Elsässer variables ( ), and the distribution of residual energy Alfvén ratio , and Elsässer ratio in the space. The spectra of , , and (the larger of ) show a similar pattern that is mainly distributed along a ridge inclined toward the k⊥ axis. This is probably the signature of the oblique Alfvénic fluctuations propagating outwardly. Unlike those of , , and the spectrum of is distributed mainly along the k⊥ axis. Close to the k⊥ axis, becomes larger while becomes smaller, suggesting that the dominance of magnetic energy over kinetic energy becomes more significant at small k∥. is larger at small k∥, implying that of is more concentrated along the k⊥ direction as compared to that of The residual energy condensate at small k∥ is consistent with simulation results in which is spontaneously generated by Alfvén wave interaction.