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We explore the degree of magnetization at the jet base of M87 by using theobservational data of the event horizon telescope (EHT) at 230~GHz obtained byDoeleman et al. By utilizing the method in Kino et al., we derive the energydensities of magnetic fields ($U_{B}$) and electrons and positrons ($U_{\pm}$)in the compact region detected by EHT (EHT-region) with itsfull-width-half-maximum size $40~{\rm \mu as}$. First, we assume that anoptically-thick region for synchrotron self absorption (SSA) exists in theEHT-region. Then, we find that the SSA-thick region should not be too large notto overproduce the Poynting power at the EHT-region. The allowed ranges of theangular size and the magnetic field strength of the SSA-thick region are$21~{\rm \mu as} \le \theta_{\rm thick}\le 26.3~{\rm \mu as}$ and $50~{\rmG}\le B_{\rm tot}\le 124~{\rm G}$, respectively. Correspondingly $U_{B}\ggU_{\pm} $ is realized in this case. We further examine the composition ofplasma and energy density of protons by utilizing the Faraday rotationmeasurement ($RM$) 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, weexamine the case when EHT-region is fully SSA-thin. Then we find that $U_{B}\ggU_{\pm}$ still holds unless protons are relativistic. Thus, we conclude thatmagnetically driven jet scenario in M87 is viable in terms of energetics closeto ISCO scale unless the EHT-region is fully SSA-thin and relativistic protonsdominated.

Magnetization degree at the jet base of M87 derived from the event horizon telescope data: Testing magnetically driven jet paradigm