Time evolution of magnetic field lines during onset of magnetic-flux eruption in prograde a = 0.9 case. Horizon-disconnected lines are colored by the magnetization parameter σ = b²/w, and doubly connected lines are shown in white. The magenta patches mark the relativistic temperature (log₁₀T ≥ 0.5). All images are seen from the latitude of θ_obs = 45° and the azimuth of ϕ_obs = 180°. Black spheres indicate the outer horizon at r_H = 1.436M, and black arcs seen in most panels mark the radius r = 2M in the equatorial plane θ = 90°. The upper sequence covers the time period of t = 19 592M − 19 596M every Δt = 1M (the middle frame corresponds to Fig. 6). The lower sequence covers the time period of t = 19 602M − 19 630M every Δt = 7M (the last frame corresponds to Fig. 8). An associated movie is available online.