Central supermassive black holes of active galactic nuclei (AGNs) host hot plasma with a temperature of $10^9$ K, namely coronae. Like the Sun, black hole coronae are theoretically believed to be heated by their magnetic activity. However, such activity has not been observed yet. In this talk, I will report the first clear detection of coronal magnetic activity in nearby AGNs using ALMA (radio telescopes). The coronal magnetic fields are typically $\sim 10$ G on scales of $\sim 40$ Schwarzschild radii. The measured magnetic field is weaker than the theoretical expectation, requiring a new corona heating mechanism. I will also discuss particle acceleration processes in the coronae of Seyferts, which may be the production sites of the high energy neutrinos seen by IceCube. Future MeV gamma-ray observations will provide critical tests to our scenario.
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