From the topology of a synthetic aurora map,we propose a mechanism for the magnetic anomalies on the southe martian hemisphere,i.e.,impacts by asteroids when the dynamo is active.The quasi concentric circles of aurora suggest that there are two-to-three convectional cells for each impact.The whole synthetic aurora is induced by three major impacts of asteroids.The east-west lineation features of crust magnetizations are due to the east-west trending locations of three impacts.The alteatively changed sign of crust magnetization originates from the alteatively changed flow direction on the tops of adjacent convectional cells.