  M87 is one of the best available source for studying the AGN jet-launching
region. To enrich our knowledge of this region, with quasi-simultaneous
observations using VLBA at 22, 43 and 86 GHz, we capture the images of the
radio jet in M87 on a scale within several thousand R s . Based on the images,
we analyze the transverse jet structure and obtain the most accurate
spectral-index maps of the jet in M87 so far, then for the first time, we
compare the results of the two analyses and find a spatial association between
the jet collimations and the local enhancement of the density of external
medium in the jet-launching region. We also find the external medium is not
uniform, and greatly contributes to the free-free absorption in this region. In
addition, we find for the jet in M87, its temporal morphology in the launching
region may be largely affected by the local, short-lived kink instability
growing in itself.

Astrophysics of Galaxies;High Energy Astrophysical Phenomena