2026ApJ...998..129E

Recent multiwavelength observations of M87* revealed a high-energy γ-ray flare without a millimeter counterpart. We present a theoretical study of potential millimeter flares in M87*, using general relativistic magnetohydrodynamical simulations with varying black hole (BH) spins and magnetic field configurations. The emergence of a millimeter flare is strongly influenced by both spin and magnetic structure, with limited sensitivity to the electron distribution. We model the intensity light curve with a damped random walk and compare the characteristic timescale (τ) with recent Submillimeter Array observations, finding that simulated τ exceeds observed values by over an order of magnitude. For BH spin a = +0.5, we identify a distinct flare followed by an order-of-magnitude flux drop. All Stokes parameters vary near the flare, including a sign reversal in the electric vector position angle. Most β_m modes remain stable, but the EB-correlation phase is sensitive to both the flare peak and decay. We examine polarimetric signatures in photon subrings, focusing on modes n_s = (0, 1). The n_s = 0 signal closely matches the full image, while n_s = 1 exhibits distinct behavior, highlighting the potential of space very long baseline interferometry to isolate subring features. Magnetic reconnection near the disk plane triggers a flux eruption that injects magnetized plasma into the jet funnel, enhancing outflow speeds and temporarily disrupting jet collimation before gradual recollimation. These results suggest transient variability near flares encodes key information about BH spin and magnetic structure, offering new probes of active galactic nuclei.