  We present a novel method to evaluate the Alfven speed and the magnetic field
strength along the streamer plasma sheet in the outer corona. The method is
based on recent observations of streamer waves, which are regarded as the fast
kink body mode carried by the plasma sheet structure and generated upon the
impact of a fast CME (coronal mass ejection) on a nearby streamer. The mode
propagates outwards with a phase speed consisting of two components. One is the
phase speed of the mode in the plasma rest frame, the other is the speed of the
solar wind streaming along the plasma sheet. The former can be well represented
by the Alfven speed outside the plasma sheet, according to a linear wave
dispersion analysis with a simplified slab model of magnetized plasmas. The
radial profiles of the Alfven speed can be deduced with constraints put on the
speed of the solar wind, which is done by making use of the measurements of
streamer blobs flowing passively in the wind. The radial profiles of the
strength of the coronal magnetic field can be depicted once the electron
density distribution is specified, this is done by inverting the observed
polarized brightness data. Comparing the diagnostic results corresponding to
the first wave trough and the following crest, we find that both the Alfven
speed and magnetic field strength at a fixed distance decline with time. This
is suggestive of the recovering process of the CME-disturbed corona.
