  Intensive studies have been conducted to understand the anisotropy of solar
wind turbulence. However, the anisotropy of Els\"asser variables
($\textbf{Z}^\pm$) 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$_{2D}(k_\parallel,k_\perp )$ and the spatial
correlation function CF$_{2D} (r_\parallel,r_\perp )$. 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 Els\"asser
variables ($\textbf{Z}^\pm$), and the distribution of residual energy E$_{R}$,
Alfv\'en ratio R$_{A}$ and Els\"asser ratio R$_{E}$ in the
$(k_\parallel,k_\perp)$ space. The spectra PSD$_{2D}(k_\parallel,k_\perp )$ of
$\textbf{B}$, $\textbf{V}$, and $\textbf{Z}_{major}$ (the larger of
$\textbf{Z}^\pm$) show a similar pattern that PSD$_{2D}(k_\parallel,k_\perp )$
is mainly distributed along a ridge inclined toward the $k_\perp$ axis. This is
probably the signature of the oblique Alfv\'enic fluctuations propagating
outwardly. Unlike those of $\textbf{B}$, $\textbf{V}$, and
$\textbf{Z}_{major}$, the spectrum PSD$_{2D}(k_\parallel,k_\perp )$ of
$\textbf{Z}_{minor}$ is distributed mainly along the $k_\perp$ axis. Close to
the $k_\perp$ axis, $\left| {E}_{R}\right|$ becomes larger while R$_{A}$
becomes smaller, suggesting that the dominance of magnetic energy over kinetic
energy becomes more significant at small $k_\parallel$. R$_{E}$ is larger at
small $k_\parallel$, implying that PSD$_{2D}(k_\parallel,k_\perp )$ of
$\textbf{Z}_{minor}$ is more concentrated along the $k_\perp$ direction as
compared to that of $\textbf{Z}_{major}$. The residual energy condensate at
small $k_\parallel$ is consistent with simulation results in which E$_{R}$ is
spontaneously generated by Alfv\'en wave interaction.
