Improving the Accuracy of Magnetic Field Tracing by Velocity Gradients:
  Principal Component Analysis
  Tracing of the magnetic field with Velocity Gradient Technique (VGT) allows
observers to probe magnetic field directions with spectroscopic data. In this
paper, we employ the method of Principal Component Analysis (PCA) to extract
the spectroscopic information most valuable for VGT. By using synthetic
observation data from numerical simulations, we show that PCA acts in a way
similar to spatial filtering along the velocity axis. We study both subsonic
and supersonic simulations and show that with the PCA filtering the tracing of
magnetic fields by the VGT is significantly improved. Using 21 cm GALFA data,
we demonstrate that the PCA filtering improves the alignment of the velocity
gradients and the Planck dust polarization.
