Thermal evolution of the hyperactive magnetar. At early times, the temperature drops rapidly due to neutrino cooling (ambipolar heating is inefficient at high T because collisional coupling inhibits the plasma motion relative to neutrons). At 0.1 yr ≲t ≲ 400 yr, ambipolar heating balances neutrino cooling so that T ≈ const. At late times, the magnetic energy available for dissipation decreases, so ambipolar heating declines, and neutrino cooling again dominates.