Intermittency of Fast MHD Modes and Regions of Anomalous Gradient
  Orientation in Low-beta Plasmas
  The strong alignment of small-scale turbulent Alfv\'enic motions with the
direction of the magnetic field that percolates the small-scale eddies and
imprints the direction of the magnetic field is a property that follows from
the MHD theory and the theory of turbulent reconnection. The Alfv\'enic eddies
mix magnetic fields perpendicular to the direction of the local magnetic field,
and this type of motion is used to trace magnetic fields with the velocity
gradient technique (VGT). The other type of turbulent motion, fast modes,
induces anisotropies orthogonal to Alfv\'enic eddies and interferes with the
tracing of the magnetic field with the VGT. We report a new effect, i.e., in a
magnetically dominated low-\beta subsonic medium, fast modes are very
intermittent, and in a volume, with a small filling factor the fast modes
dominate other turbulent motions. We identify these localized regions as the
cause of the occasional change of direction of gradients in our synthetic
observations. We show that the new technique of measuring the gradients of
gradient amplitudes suppresses the contribution from the fast-mode-dominated
regions, improving the magnetic field tracing. In addition, we show that the
distortion of the gradient measurements by fast modes is also applicable to the
synchrotron intensity gradients, but the effect is reduced compared to the VGT.
