We explore the degree of magnetization at the jet base of M87 by using the observational data of the event horizon telescope ( EHT ) at 230 GHz obtained by Doeleman et al .
By utilizing the method in Kino et al. , we derive the energy densities of magnetic fields ( U _ { B } ) and electrons and positrons ( U _ { \pm } ) in the compact region detected by EHT ( EHT-region ) with its full-width-half-maximum size 40 ~ { } { \mu as } .
First , we assume that an optically-thick region for synchrotron self absorption ( SSA ) exists in the EHT-region .
Then , we find that the SSA-thick region should not be too large not to overproduce the Poynting power at the EHT-region .
The allowed ranges of the angular size and the magnetic field strength of the SSA-thick region are 21 ~ { } { \mu as } \leq \theta _ { thick } \leq 26.3 ~ { } { \mu as } and 50 ~ { } { G } \leq B _ { tot } \leq 124 ~ { } { G } , respectively .
Correspondingly U _ { B } \gg U _ { \pm } is realized in this case .
We further examine the composition of plasma and energy density of protons by utilizing the Faraday rotation measurement ( RM ) at 230 GHz obtained by Kuo et al .
Then , we find that U _ { B } \gg U _ { \pm } + U _ { p } still holds in the SSA-thick region .
Second , we examine the case when EHT-region is fully SSA-thin .
Then we find that U _ { B } \gg U _ { \pm } still holds unless protons are relativistic .
Thus , we conclude that magnetically driven jet scenario in M87 is viable in terms of energetics close to ISCO scale unless the EHT-region is fully SSA-thin and relativistic protons dominated .