Using Twisted Filaments to Model the Inner Jet in M87
  Radio and optical images of the M87 jet show bright filaments, twisted into
an apparent double helix, extending from HST-1 to knot A. Proper motions within
the jet suggest a decelerating jet flow passing through a slower, accelerating
wave pattern. We use these observations to develop a mass and energy flux
conserving model describing the jet flow and conditions along the jet. We
determine the cocoon conditions required if the twisted filaments are the
result of the Kelvin-Helmholtz (KH) unstable elliptical mode. We find that the
cocoon must be cooler than the jet at HST-1 but must be about as hot as the jet
at knot A. Under these conditions we find that the observed filament wavelength
is near the elliptical mode maximum growth rate and growth is rapid enough for
the filaments to develop and saturate well before HST-1. We generate a
pseudo-synchrotron image of a model jet carrying a combination of normal modes
of the KH instability. The pseudo-synchrotron image of the jet reveals: (1)
that a slow decline in the model jet's surface brightness is still about five
times faster than the real jet; (2) that KH produced dual helically twisted
filaments can appear qualitatively similar to those on the real jet if any
helical perturbation to the jet is very small or nonexistent inside knot A; (3)
that the knots in the real jet cannot be associated with the twisted
filamentary features and are unlikely to be the result of a KH instability. The
existence of the knots in the real jet, the limb brightening of the real jet in
the radio, and the slower decline of the surface brightness of the real jet
indicate that additional processes --- such as unsteady jet flow and internal
particle acceleration --- are occurring within the jet. Disruption of the real
jet beyond knot A by KH instability is consistent with the jet and cocoon
conditions we find at knot A.
