SPECTRAL ANISOTROPY of ELSASSER 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 (Z±) 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 PSD_2D(k_∥, k_⊥) and the spatial correlation function CF_2D(r_∥, r_⊥). 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 Elssser variables (Z±), and the distribution of residual energy E_R, Alfvn ratio R_A, and Elsässer ratio R_E in the (k_∥, k_⊥) space. The spectra PSD_2D(k_∥, k_⊥) of B, V, and Z_major (the larger of Z±) show a similar pattern that PSD2D(k_∥, k_⊥) 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 B, V, and Zmajor, the spectrum PSD_2D(k_∥, k_⊥) of Z_minor is distributed mainly along the k? axis. Close to the k_⊥ axis, |E_R | becomes larger while R_A becomes smaller, suggesting that the dominance of magnetic energy over kinetic energy becomes more significant at small k_P. R_E is larger at small k_P, implying that PSD_2D(k_∥, k_⊥) of Z_minor is more concentrated along the k∥ direction as compared to that of Z_major. The residual energy condensate at small k_P is consistent with simulation results in which E_R is spontaneously generated by Alfvn wave interaction.