This phenomenon underwent a change 784,000 years ago named for Bernard Brunhes and Motonori Matuyama. Bauer plots compare this phenomenon’s dip angle with its declination, which is zero on agonic (“ay-gonic”) lines. (-5[1])Changes in this phenomenon’s orientation delineate chrons. This phenomenon’s strength reaches a minimum at the South Atlantic Anomaly, where the van Allen belts are closest to the surface. Dynamo theory posits this phenomenon is produced by the convection of iron and nickel. The interaction of solar wind with this phenomenon forms aurorae. For 10 points, the Earth’s core produces what phenomenon that makes compasses point north? ■END■ (10[1])