Activity on the Sun, such as solar flares and coronal mass ejections, hurl large numbers of charged particles into space. When the particles reach Earth, they can interfere with communications and the power grid by causing electromagnetic induction. In one example, a current of millions of amps (known as the auroral electrojet) that runs about 100 km above Earth's surface can be perturbed. The change in the current causes a change in the magnetic field it produces at Earth's surface, which induces a potential difference along Earth's surface and in the power grid (which is grounded). Induced electric fields as high as 6.0 V/km have been measured. We can model the circuit at Earth's surface as a rectangular loop made up of the power lines and completed by a path through the ground beneath, and we can treat the magnetic field as being uniform (but not constant). The height of the power lines is 5.0 m,they are 1 km long, and the potential induced in the loop is 6.0 V. At what rate must the magnetic field be changing to induce that emf?