  Probing magnetic fields in the interstellar medium (ISM) is notoriously
challenging. Motivated by the modern theories of magnetohydrodynamic (MHD)
turbulence and turbulence anisotropy, we introduce the Structure-Function
Analysis (SFA) as a new approach to measure the magnetic field orientation and
estimate the magnetization. We analyze the statistics of turbulent velocities
in three-dimensional compressible MHD simulations through the second-order
structure functions in both local and global reference frames. In the
sub-Alfvenic turbulence with the magnetic energy larger than the turbulent
energy, the SFA of turbulent velocities measured in the directions
perpendicular and parallel to the magnetic field can be significantly
different. Their ratio has a power-law dependence on the Alfven Mach number
$M_A$, which is inversely proportional to the magnetic field strength. We
demonstrate that the anisotropic structure functions of turbulent velocities
can be used to estimate both the orientation and strength of magnetic fields.
With turbulent velocities measured using different tracers, our approach can be
generally applied to probing the magnetic fields in the multi-phase
interstellar medium.
