Motivated by the measured velocity profile of the M87 jet using the KVN and VERA Array (KaVA) by Park et al. indicating that the starting position of the jet acceleration is farther from the central engine of the jet than predicted in general relativistic magnetohydrodynamic simulations, we explore how to mitigate the apparent discrepancy between the simulations and the KaVA observation. We use a semi-analytic jet model proposed by Tomimatsu and Takahashi. consistently solving the trans-magnetic field structure but neglecting any dissipation effects. By comparing the jet model with the observed M87 jet velocity profile, we find that the model can reproduce the logarithmic feature of the velocity profile, and can fit the observed data when choosing
is the gravitational radius. While a total specific energy ( ) of the
jet changes the terminal bulk Lorentz factor of the jet, a slower angular
velocity of the black hole magnetosphere (funnel region) ( ) makes a
light-cylinder radius ( ) larger and it consequently pushes out a
location of a starting point of the jet acceleration. Using the estimated
we further estimate the magnetic field strength on the event
horizon scale in M87 by assuming Blandford-Znajek (BZ) process is in action.
The corresponding magnetic flux threading the event horizon of M87 is in good
agreement with a magnetically arrested disc (MAD) regime.