Black Hole Explorer (BHEX) is a space VLBI mission concept, which can probe the black hole spacetime and the plasma properties including the magnetic fields of the accretion flows and relativistic jets. We propose scientific inquiries anticipated to be addressed by EHE, primarily through the imaging of microarcsecond-scale signatures in target sources. An appearance of a crescent-shaped shadow in a bright state of the M87 will enable us to constrain the magnitude of the black hole spin (Kawashima et al. 2019). A possible appearance of the plasma injection region in the vicinity of the black hole results in the formation of the multiple ring structure and may enable us to understand the jet formation processes (Kawashima et al. 2021, and Ogihara et al. submitted to ApJ). In addition, the reversal of the sign of the circular polarization and the separation of linear and circular polarization flux peaks will constrain the magnetic field structure and the thermal properties of the electrons, respectively (Tsunetoe et al. 2021, 2022). Other topics, including potential scientific inquiries of luminous active galactic nuclei, will be also discussed.
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