  We present a cascade model for turbulence in weakly collisional plasmas that
follows the nonlinear cascade of energy from the large scales of driving in the
MHD regime to the small scales of the kinetic Alfven wave regime where the
turbulence is dissipated by kinetic processes. Steady-state solutions of the
model for the slow solar wind yield three conclusions: (1) beyond the observed
break in the magnetic energy spectrum, one expects an exponential cut-off; (2)
the widely held interpretation that this dissipation range obeys power-law
behavior is an artifact of instrumental sensitivity limitations; and, (3) over
the range of parameters relevant to the solar wind, the observed variation of
dissipation range spectral indices from -2 to -4 is naturally explained by the
varying effectiveness of Landau damping, from an undamped prediction of -7/3 to
a strongly damped index around -4.

Astrophysics