  Recent (Bose \& Durbin, \textit{Phys. Rev. Fluids}, 1, 073602, 2016) direct
numerical simulations (DNS) of adverse- and zero-pressure-gradient boundary
layers beneath moderate levels of free stream turbulence ($Tu$ $\le 2\%$)
revealed a \textit{mixed mode} transition regime, intermediate between orderly
and bypass routes. In this regime, the amplitudes of the Klebanoff streaks and
instability waves are similar, and potentially can interact. Three-dimensional
visualizations of transitional eddies revealed a helical pattern, quite
distinct from the sinuous and varicose forms seen in pure bypass transition.
This raises the fundamental question of whether the helical pattern could be
attributed to a previously unknown instability mode.
  Two-dimensional stability analyses are performed herein for base flows
extracted from DNS flow fields. The three-dimensional structure of the
eigenfunction of the most unstable mode indeed reveals a helical pattern. These
instability modes are obtained significantly earlier than helical structures
are seen in flow visualizations, even for innocuous base flow structures. It is
inferred that the instability mode is the underlying cause of helical patterns
that emerge in the transition region. The streak configuration leading to the
formation of the helical mode instability is different from those leading to
sinuous and varicose modes in previous studies of pure bypass transition. Thus,
the mixed mode precursor is the distinctive cause.
